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Ch5.1 - Centripetal Force INERTIA (velocity) Top View Centripetal Force.

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1 Ch5.1 - Centripetal Force INERTIA (velocity) Top View Centripetal Force

2 There is no force pushing outward! Inertia wants object to fly off tangent to circle. (Centrifugal Force is a fake force we think we feel throwing us outward.) Centripetal Force – real force acting towards center of circle. INERTIA (velocity) Top View Centripetal Force

3 Ex 1) What agent exerts the centripetal force in each? earth m

4 Ex 1) What agent exerts the centripetal force in each? earth m FgFg F net = F g

5 Ex 1) What agent exerts the centripetal force in each? earth m FgFg F net = F g m. a c = F g

6 Ex 1) What agent exerts the centripetal force in each?

7 FTFT F net = F T

8 Ex 1) what agent exerts the centripetal force in each? FTFT F net = F T m. a c = F T

9 Ex 1) What agent exerts the centripetal force in each? (Top view of record player).. (coin)

10 Ex 1) What agent exerts the centripetal force in each? (Top view of record player).. (coin) F f,s F net = F f,s

11 Ex 1) What agent exerts the centripetal force in each? (Top view of record player).. (coin) F f,s F net = F f,s m. a c = F f,s

12 Centripetal Acceleration

13 Ex 2) The ride “Spin- Out” is a circular room, 5m in diameter. Once it gets to speed, its linear speed is 10 m/s. What is the centripetal acceleration?

14 Centripetal Acceleration Ex 2) The ride “Spin- Out” is a circular room, 5m in diameter. Once it gets to speed, its linear speed is 10 m/s. What is the centripetal acceleration?

15 CENTRIPETAL FORCE

16

17 Ex 3) A 60 kg astronaut stands on a bathroom scale in a 2 km diameter rotating space station that spins every 62.8 sec. What does the scale read?

18 CENTRIPETAL FORCE Ex 3) A 60 kg astronaut stands on a bathroom scale in a 2 km diameter rotating space station that spins ever 62.8 sec. What does the scale read? FnFn F net = F N

19 CENTRIPETAL FORCE Ex 3) A 60 kg astronaut stands on a bathroom scale in a 2 km diameter rotating space station that spins ever 62.8 sec. What does the scale read? FnFn F net = F N m. a c = F N

20 CENTRIPETAL FORCE Ex 3) A 60 kg astronaut stands on a bathroom scale in a 2 km diameter rotating space station that spins ever 62.8 sec. What does the scale read? F net = F N m. a c = F N

21 Ex 4) A sport car on a flat track rounds a corner with a radius of 100m, at a maximum speed of 25 m/s before sliding out. What is the coefficient of static friction? Ch5 HW#1 1 – 5

22 Ex 4) A sport car on a flat track rounds a corner with a radius of 100m, at a maximum speed of 25 m/s before sliding out. What is the coefficient of static friction? F f,s F net = F f,s

23 Ch5 HW#1 1 – 5 1. A youngster on a carousel horse 5.0 m from the center revolves at a constant rate, once around in 15.0 s. What is her acceleration? 2. A beetle standing on the edge of a 12-inch record (r =.152m) whirls around at 33.3 rotations per minute. What is its centripetal accl? What agent exerts the force? 3. A ‘foreign-made’ space station only has a radius of 3 meters. If it rotates around once every 1.15 sec, what is the centripetal accl at the floor? If an astronaut is 1.5 m tall, what is the centripetal accl at his head? Does anyone see any physiological implications of this? How could we remedy this?

24 At head:At feet:

25 3.A ‘foreign-made’ space station only has a radius of 3 meters. If it rotates around once every 1.15 sec, what is the centripetal accl at the floor? If an astronaut is 1.5 m tall, what is the centripetal accl at his head? Does anyone see any physiological implications of this? How could we remedy this? At head:At feet:

26 4. The Talladega Speedway …1500kg cars complete turns of radius 100m at a speed of 80m/s. What is net force on car? What agent exerts it? F net = F n m. a c = F n

27 5. 700kg flat track car rounds a corner of radius 30m at a speed of 20m/s, without slipping. What is the minimum coeffiecient of static friction that can accomplish this? F net = F f,s m. a c = μ. F n

28 Ch5.2 - Gravity All matter is attracted to all other matter. - More matter = larger force - Distance apart is important (Inverse square law) Newton’s Law of Universal Gravitation

29 Ch5.2 - Gravity All matter is attracted to all other matter. - More matter = larger force - Distance apart is important (Inverse square law) Newton’s Law of Universal Gravitation - Universal gravitational constant, G = 6.67x Ex1) Compute the gravitational attraction between 2 100kg uniform spheres by 1.00m. (Roughly lb football players) Ex2) The mass of the moon is 7.35x kg and its distance from the Earth is 3.84x 10 3 km. Taking the Earth’s mass to be 5.98x10 24 kg, what force keeps the moon in her orbit?

30 Ch5.2 - Gravity All matter is attracted to all other matter. - More matter = larger force - Distance apart is important (Inverse square law) Newton’s Law of Universal Gravitation - Universal gravitational constant, G = 6.67x Ex2) The mass of the moon is 7.35x kg and its distance from the Earth is 3.84x 10 3 km. Taking the Earth’s mass to be 5.98x10 24 kg, what force keeps the moon in her orbit?

31 Ex3) 2 objects both of mass m are a distance r apart and exert a force F on each other. How does the force change if: a) Both masses are doubled. b) Instead the distance is doubled? c) Distance made 3X smaller?

32 Ex4) How fast does an object have to travel, to stay in circle? - what is the direction of the instantaneous velocity? - what would happen if it traveled slower? - what would happen if it traveled faster? Ch5 HW#2 1 – 4 M m

33 1. Using our equation for the universal law of gravitation: Explain how the force will change if: a) One mass doubles b) Both masses cut in half and the distance between them cut in half. c) The masses stay the same and the distance triples. d) The masses double and the distance between is cut in half.

34 Ch5 HW#2 1. Using our equation for the universal law of gravitation: Explain how the force will change if: a) Both masses double b) Both masses cut in half and the distance between them cut in half. c) The masses stay the same and the distance quadruples. d) The masses triple and the distance between is cut to one third.

35 2.I want you to calculate my weight 2 ways: (My mass = 75 kg; Earth’s mass = 6x10 24 kg; Earth’s radius = 6x10 6 m) 1) F g = m. g 2) (Roughly the same with rounding error.) 3.What is the force of gravity on me (75 kg) on the surface of Jupiter? M J = 2x10 27 kgR J = 7x10 7 m 4.What is the gravitational attraction between the earth and the Sun? M sun = 2x10 30 kg D ES = 1.5x10 11 m

36 2.I want you to calculate my weight 2 ways: (My mass = 75 kg; Earth’s mass = 6x10 24 kg; Earth’s radius = 6x10 6 m) 1) F g = m. g= (75kg)(9.8m/s 2 ) = 748.5N 2) (Roughly the same with rounding error.) 3.What is the force of gravity on me (75 kg) on the surface of Jupiter? M J = 2x10 27 kgR J = 7x10 7 m 4.What is the gravitational attraction between the earth and the Sun? M sun = 2x10 30 kg D ES = 1.5x10 11 m

37 2.I want you to calculate my weight 2 ways: (My mass = 75 kg; Earth’s mass = 6x10 24 kg; Earth’s radius = 6x10 6 m) 1) F g = m. g= (75kg)(9.8m/s 2 ) = 748.5N 2) (Roughly the same with rounding error.) 3.What is the force of gravity on me (75 kg) on the surface of Jupiter? M J = 2x10 27 kgR J = 7x10 7 m 4.What is the gravitational attraction between the earth and the Sun? M sun = 2x10 30 kg D ES = 1.5x10 11 m

38 Ch5.3 - Gravity Applications Acceleration of gravity at surface of any object:

39 Apparent weightlessness -What does the bathroom scale read in each accelerating elevator? (scale reads F N ) (assume m= 100kg) a = 0 m/s 2 a = 10 m/s 2

40 Satellite Orbits Orbital Speed:

41 Satellite Orbits Orbital Speed:

42 Ex1) A satellite in geostationary orbit must be at a height of 3.6x10 7 m above the earth’s surface. What speed must it travel at to stay directly above good ol’ AV?

43 Kepler’s 3 Laws of Planetary Motion: 1) The planets move in elliptical orbits with the sun as 1 focus. 2) Each planet sweeps out equal (areas) in equal time intervals. 3) The ratio of the average distance from the Sun cubed to the period squared is the same constant value. Ch5 HW#3

44 Ch5 HW#3 p172 37,44,46,47, + Hard Bonus Problem Hard Bonus Problem. (Year 2000 AP Test FRQ #1) Prove that the acceleration on the surface of Mars is 38% the acceleration on the surface of the Earth, given M Mars = M Earth, R Mars = R Earth.

45 37. The acceleration due to gravity on the surface of mars is 3.7 m/s 2. If the planet’s diameter is 6.8x10 6 m, determine the mass of the planet and compare it to Earth. F net = F G

46 37. The acceleration due to gravity on the surface of mars is 3.7 m/s 2. If the planet’s diameter is 6.8x10 6 m, determine the mass of the planet and compare it to Earth. F net = F G M m = 6.4x10 23 kg

47 44. Locate the position of a spaceship on the Earth-Moon center line, such that at that point, the tug of each celestial body exerted on it will cancel and the craft would literally be weightless. F net = F ER – F RM E m

48 E m 44. Locate the position of a spaceship on the Earth-Moon center line, such that at that point, the tug of each celestial body exerted on it will cancel and the craft would literally be weightless.

49 F net = F ER – F RM E m R ER = 3.46x10 8 m (346,000km) 44. Locate the position of a spaceship on the Earth-Moon center line, such that at that point, the tug of each celestial body exerted on it will cancel and the craft would literally be weightless.

50 46. Three very small spheres of mass 2.50 kg, 5.00 kg, and 6.00 kg are located on a strait line in space awayf rom everything else. The first one is a point between the other two, 10.0 cm to the right of the second and 20.0 cm to the left of the third. Compute the net gravitational force on it

51 5 6 2

52 47. It is believed that during the gravitational collapse of certain stars, such great densities and pressures will be reached that the atoms themselves will be crushed, leaving only a residual core of neutrons. Such a neutron star is, in some respects, very much like a giant atomic nucleus with a tremendous density of roughly about 3 x kg/m 3. DON’T compute the surface acceleration due to gravity for a one-solar-mass neutron star, just trip out on this unfathomable fact!

53 At all points on the circle, F net points to the center of the circle. F N also always points to the center, but its value changes. F g only plays a role top and bottom. (At the sides F g is not in the direction of F net so we disregard it.) (The scale reads F N ) (The scale reads more on the bottom, less on the top.) v = 14 m/s v = 11 m/s v = 20 m/s Ch5.5 – Roller Coasters Ex1) 60kg person sits on bathroom scale on roller coaster that does a loop of radius 10m. What does the scale read at 1-4?

54 Ch5.4 – Roller Coasters Ex1) 60kg person sits on bathroom scale on roller coaster that does a loop of radius 10m. What does the scale read at 1-4? v = 14 m/s v = 11 m/s v = 20 m/s

55 v = 14 m/s v = 11 m/s v = 20 m/s v = 14 m/s F net = F N F net = F N + F g F net = F N – F g

56 Ex2) A 50kg student sits on a bathroom scale while riding the Revolution. As the coaster enters the loop it begins slowing. At the top of the loop it has a speed of 14 m/s. By the bottom of the loop it has sped up to 20 m/s. If the loop has a radius of 7m, what does the scale read at the top and bottom? v = 14 m/s v = 20 m/s Ch5 HW#4

57 Ex2) A 50kg student sits on a bathroom scale while riding the Revolution. As the coaster enters the loop it begins slowing. At the top of the loop it has a speed of 14 m/s. By the bottom of the loop it has sped up to 20 m/s. If the loop has a radius of 7m, what does the scale read at the top and bottom? v = 14 m/s v = 20 m/s v = 14 m/s v = 20 m/s Ch5 HW#4

58 Lab5.1 Circular Motion - due at end of period - Go over Ch5 HW#4 - Ch5 HW#5 due tomorrow

59 Ch5 HW#4 1 – 4 1. A front-loading clothes washer has a horizontal drum that is thoroughly perforated with small holes. Assuming it to spin dry at 1 rotation per second, have a radius of 40 cm, and contain a 4.5-kg wet teddy bear, what maximum force is exerted by the wall of the bear? What happens to the water?

60 F net = F N – F g m. a c = F N – mg F N F N = mv 2 /r + mg = 71N + 44N = 115N F g

61 2. While whirling around in a vertical circle with a radius of 1.50 m, a 2.00 kg mass is held on a rope attached to a very light spring scale. What does the scale read when the mass is moving at 4.00 m/s at the lowest point in its orbit? What does it read at the top when moving at 2.50 m/s? What does it read on the way down when moving at 3.00 m/s? Top: F net = F N + F g Side: F net = F N Bottom: F net = F N – F g

62 2. While whirling around in a vertical circle with a radius of 1.50 m, a 2.00 kg mass is held on a rope attached to a very light spring scale. What does the scale read when the mass is moving at 4.00 m/s at the lowest point in its orbit? What does it read at the top when moving at 2.50 m/s? What does it read on the way down when moving at 3.00 m/s? Top: F net = F N + F g ma c = F N + mg F N = mv 2 /r – mg = 1.7N Side: F net = F N ma c = F N F N = mv 2 /r= 21.3N Bottom: F net = F N – F g ma c = F N – mg F N = mv 2 /r + mg = 41N v

63 3. A scale is fitted in the seat of a roller coaster car and a person weighing 800 N sits down on it. The car then descends along a path that has the shape of a m radius vertical circle with its lowest point at the bottom where the car reaches its greatest speed of 40.0 m/s. What is the maximum reading of the scale? v = 40 m/s F net = F N + F g

64 v = 40 m/s F net = F N – F g ma c = F N – 800N v = 40 m/s 3. A scale is fitted in the seat of a roller coaster car and a person weighing 800 N sits down on it. The car then descends along a path that has the shape of a m radius vertical circle with its lowest point at the bottom where the car reaches its greatest speed of 40.0 m/s. What is the maximum reading of the scale? F N F g

65 Ch5 HW#5 p ,27, What would happen to the weight of an object if its mass were doubled and its distance to the center of the earth were also doubled? 27. Suppose 2 identical spheres separated by 1.00 m experience a force of 1.00N on each other. Compute masses. 30. Compute g-force between earth n moon, sun n moon.

66 Ch 5.5 Conservation of Energy and Roller Coasters FRQ#1) Part of the track of an amusement park roller coaster is shaped as shown. A safety bar is oriented lengthwise along the top of each car. In one roller coaster car, a small 0.10kg ball is suspended from this bar by a short length of light, inextensible string. a) Initially, the car is at rest in point A. i. On the diagram to the right, draw and label all the forces acting on the 0.10kg ball. ii. Calculate the tension in the string.

67 The car is then accelerated horizontally, goes up a 30° incline, and then goes around a vertical circular loop of radius 25 meters. For each of the four situations described in parts (b) to (e), do all three of the following: In each situation, assume that the ball has stopped swinging back and forth. Determine the horizontal component T h of the tension in the string in newtons and record your answer in the space provided. Determine the vertical component T v of the tension in the string in newtons and record your answer in the space provided. Show the adjacent diagram the approximate direction of the string with respect to the vertical. The dashed line shows the vertical line in each situation. b) The car at point B moving horizontally to the right with an acceleration of 5.0 m/s 2. T h = ____________N T v = _____________N

68 c) The car is at point C and is being pulled up the 30° incline with a constant speed of 30 m/s. T h =____________N T v =_____________N d) The car is at point D moving down the 30° incline with an acceleration of 5.0 m/s 2. T h =____________N T v =_____________N e) The car is at point E moving upside down with an instantaneous speed of 25 m/s and no tangential acceleration at the top of the vertical loop of radius 25 meters. T h =____________N T v =_____________N Ch5 HW#6 RC FRQ

69 Ch5 HW#6 Roller Coaster FRQ A roller coaster ride at an amusement park lifts a car of mass 700kg to pt A at a height of 90m above the lowest point on the track, as shown. The car starts from rest at point A, rolls with negligible friction down the incline and follows the track around a loop of radius 20m. Point B, the highest point on the loop, is at a height of 50m above the lowest point on the track. a) i. indicate on the figure the point P at which the maximum speed of the car is attained. ii. Calculate the value v max of this maximum speed. b) Calculate the speed v B of the car at point B. c) i. On the figure of the car, draw and label vectors to represent the forces acting on the car when it is upside down at point B. ii. Calculate the magnitude of all the forces identified in (c-i). d) Now suppose that friction is not negligible. How could the loop be modified to maintain the same speed at the top of the loop as found in (b)? Justify your answer.

70 Ch5 HW#6 Roller Coaster FRQ A roller coaster ride at an amusement park lifts a car of mass 700kg to pt A at a height of 90m above the lowest point on the track, as shown. The car starts from rest at point A, rolls with negligible friction down the incline and follows the track around a loop of radius 20m. Point B, the highest point on the loop, is at a height of 50m above the lowest point on the track. a) i. indicate on the figure the point P at which PE  KE the maximum speed of the car is attained. mgh = ½mv 2 ii. Calculate the value v max of this maximum speed. v = 42 m/s b) Calculate the speed v B of the car at point B. c) i. On the figure of the car, draw and label vectors to represent the forces acting on the car when it is upside down at point B. ii. Calculate the magnitude of all the forces identified in (c-i). d) Now suppose that friction is not negligible. How could the loop be modified to maintain the same speed at the top of the loop as found in (b)? Justify your answer.

71 a) i. indicate on the figure the point P at which PE  KE the maximum speed of the car is attained. mgh = ½mv 2 ii. Calculate the value v max of this maximum speed. v = 42 m/s b) Calculate the speed v B of the car at point B. mgh = ½mv 2 c) i. On the figure of the car, draw and label vectors v = 28 m/s to represent the forces acting on the car when it is upside down at point B. ii. Calculate the magnitude of all the forces identified in (c-i). d) Now suppose that friction is not negligible. How could the loop be modified to maintain the same speed at the top of the loop as found in (b)? Justify your answer.

72 b) Calculate the speed v B of the car at point B. mgh = ½mv 2 v = 28 m/s c) i. On the figure of the car, draw and label vectors to represent the forces acting on the car when it is upside down at point B. ii. Calculate the magnitude of all the forces identified in (c-i). d) Now suppose that friction is not negligible. F N How could the loop be modified to maintain the same speed at the top of the loop as found in (b)? Justify your answer. F g c-ii) F net = F g + F N

73 b) Calculate the speed v B of the car at point B. mgh = ½mv 2 v = 28 m/s c) i. On the figure of the car, draw and label vectors to represent the forces acting on the car when it is upside down at point B. ii. Calculate the magnitude of all the forces identified in (c-i). d) Now suppose that friction is not negligible. F N How could the loop be modified to maintain the same speed at the top of the loop as found in (b)? Justify your answer. F g c-ii) F net = F g + F N d) Drop the loop lower. More PE needed  PE = KE + W f

74 Ch 8.1 – Rotational Motion Angular Displacement- distance around in circles r

75 Ch 8.1 – Rotational Motion Angular Displacement- distance around in circles or r 1 radian when s=r or 6.28 rad around circle Ex1) A pendulum of length.40m sweeps out an angle of 30° to cover half its period. What is displacement of the pendulum bob?

76 Angular speed - how fast you go in circles Linear speed (v) relates to angular speed (ω) omega radians sec

77 Angular speed - how fast you go in circles Ex p243) A horse completed a race around a 1 mile track in 92.2 seconds, at an average speed of 17.4 m/s. What was the angular speed? Linear speed (v) relates to angular speed (ω) omega rad sec

78 Angular acceleration: Tangential acceleration: Ex p246) At one moment in a race, a race car moving around a turn of radius 50m had an angular speed of 0.60 rad/s and an angular accerastion of 0.20 rad/s 2. a) Linear speed: b) Centripetal accl: c) Tangential accl: Ch8 HW#1 p278 7,13,14,19,25,27,35,42

79 7. A model plane at the end of a control line circles at a constant speed 10.6 times around in 50.0 s. Through how many radians does it fly in 25.0 s? In 25 sec, completes 5.3 revs. 13. If a ball 30 cm in diam rolls 65 m without slipping, how many revs did it make in the process? s = 65mC = 2πr r =.15m = 2π(.15m) = 14. The bob at the end of pendulum 100 cm long swings out an arc 15.0 cm in length. Find the angle in radians and degrees through which it moves. Check your answer by determining the fraction of the complete circle to which this corresponds and then taking that fraction of 2 pi.

80 Ch8 HW#1 p278 7,13,14,19,25,27,35,42 7. A model plane at the end of a control line circles at a constant speed 10.6 times around in 50.0 s. Through how many radians does it fly in 25.0 s? In 25 sec, completes 5.3 revs. 5.3 rev2π rad= 33.3 rad 1 rev 13. If a ball 30 cm in diam rolls 65 m without slipping, how many revs did it make in the process? s = 65mC = 2πr r =.15m = 2π(.15m) = 14. The bob at the end of pendulum 100 cm long swings out an arc 15.0 cm in length. Find the angle in radians and degrees through which it moves. Check your answer by determining the fraction of the complete circle to which this corresponds and then taking that fraction of 2 pi.

81 Ch8 HW#1 p278 7,13,14,19,25,27,35,42 7. A model plane at the end of a control line circles at a constant speed 10.6 times around in 50.0 s. Through how many radians does it fly in 25.0 s? In 25 sec, completes 5.3 revs. 5.3 rev2π rad= 33.3 rad 1 rev 13. If a ball 30 cm in diam rolls 65 m without slipping, how many revs did it make in the process? s = 65mC = 2πr65 m 1 rev= 69 rev r =.15m = 2π(.15m).94 m =.94m/rev 14. The bob at the end of pendulum 100 cm long swings out an arc 15.0 cm in length. Find the angle in radians and degrees through which it moves. Check your answer by determining the fraction of the complete circle to which this corresponds and then taking that fraction of 2 pi.

82 Ch8 HW#1 p278 7,13,14,19,25,27,35,42 7. A model plane at the end of a control line circles at a constant speed 10.6 times around in 50.0 s. Through how many radians does it fly in 25.0 s? In 25 sec, completes 5.3 revs. 5.3 rev2π rad= 33.3 rad 1 rev 13. If a ball 30 cm in diam rolls 65 m without slipping, how many revs did it make in the process? s = 65mC = 2 πr65 m 1 rev= 69 rev r =.15m = 2 π(.15m).94 m =.94m/rev 14. The bob at the end of pendulum 100 cm long swings out an arc 15.0 cm in length. Find the angle in radians and degrees through which it moves. Check your answer by determining the fraction of the complete circle to which this corresponds and then taking that fraction of 2 pi. θ = s/r.15rad180° = 8.6° =.15m/1.00m π rad =.15 rad

83 19. The angular speed of a wheel is to be determined by affixing a tiny mirror to the circumference and recording the returning light bounced off it as it spins past a laser beam. If 100 return pulses are detected in s, what is the angular speed of the wheel? ω = ? ω = 25. Thirty seconds after the start button is pressed on a big electric motor, its shaft is whirling around at 500 rev/s. Determine its average acceleration. ω i = 0 ω f = 500 rev/sω = 100 rev 2π rad = 3142 rad/s t = 30 sec 0.02 s 1 rev α = ? α = ∆ω = ∆t 27. A steam engine is running at 200 rpm when the engineer shuts it off. The friction of its various parts produces torques that combine to decelerate the machine at 5.0 rad/s. How long will it take to come to rest? ω f = 0 ω i = 200 rpm = 21 rad/s α = - 5 rad/s 2

84 19. The angular speed of a wheel is to be determined by affixing a tiny mirror to the circumference and recording the returning light bounced off it as it spins past a laser beam. If 100 return pulses are detected in s, what is the angular speed of the wheel? ω = ? ω = 100 rev 2π rad = 3.14x10 4 rad/s 0.02 s 1 rev 25. Thirty seconds after the start button is pressed on a big electric motor, its shaft is whirling around at 500 rev/s. Determine its average acceleration. ω i = 0 ω f = 500 rev/sω = 100 rev 2π rad = 3142 rad/s t = 30 sec 0.02 s 1 rev α = ? α = ∆ω = ∆t 27. A steam engine is running at 200 rpm when the engineer shuts it off. The friction of its various parts produces torques that combine to decelerate the machine at 5.0 rad/s. How long will it take to come to rest? ω f = 0 ω i = 200 rpm = 21 rad/s α = - 5 rad/s 2

85 19. The angular speed of a wheel is to be determined by affixing a tiny mirror to the circumference and recording the returning light bounced off it as it spins past a laser beam. If 100 return pulses are detected in s, what is the angular speed of the wheel? ω = ? ω = 100 rev 2π rad = 3.14x10 4 rad/s 0.02 s 1 rev 25. Thirty seconds after the start button is pressed on a big electric motor, its shaft is whirling around at 500 rev/s. Determine its average acceleration. ω i = 0 ω f = 500 rev/sω = 100 rev 2π rad = 3142 rad/s t = 30 sec 0.02 s 1 rev α = ? α = ∆ω = 3142 rad/s – 0 rad/s ∆t 30 sec = rad/s A steam engine is running at 200 rpm when the engineer shuts it off. The friction of its various parts produces torques that combine to decelerate the machine at 5.0 rad/s. How long will it take to come to rest? ω f = 0 ω i = 200 rpm = 21 rad/s α = - 5 rad/s 2

86 19. The angular speed of a wheel is to be determined by affixing a tiny mirror to the circumference and recording the returning light bounced off it as it spins past a laser beam. If 100 return pulses are detected in s, what is the angular speed of the wheel? ω = ? ω = 100 rev 2π rad = 3.14x10 4 rad/s 0.02 s 1 rev 25. Thirty seconds after the start button is pressed on a big electric motor, its shaft is whirling around at 500 rev/s. Determine its average acceleration. ω i = 0 ω f = 500 rev/sω = 100 rev 2π rad = 3142 rad/s t = 30 sec 0.02 s 1 rev α = ? α = ∆ω = 3142 rad/s – 0 rad/s ∆t 30 sec = rad/s A steam engine is running at 200 rpm when the engineer shuts it off. The friction of its various parts produces torques that combine to decelerate the machine at 5.0 rad/s. How long will it take to come to rest? ω f = 0∆t = ∆ω = 0 rad/s – 21 rad/s= 4.2 sec ω i = 200 rpm = 21 rad/s α -5 rad/s 2 α = - 5 rad/s 2

87 35. New York City is traveling around at a tangential speed of about 353 m/s (790 mph) as the Earth spins. Assuming the planet is a sphere of radius 6371km, how long is the perpendicular from the city to the spin axis? Compute the city’s latitude (the angle measured above the equator). v NYC = 353 m/sradius to NYC t = 86,400 s ω =7.27x10 -5 rad/s r = 6.37x10 6 m θ = 40°

88 Ch8.2 Angular Equations Linear EquationAngular Equivalent 1. v f = v i + at1.

89 Ch8.2 Angular Equations Linear EquationAngular Equivalent 1. v f = v i + at1. ω f = ω i + αt 2. s = ½(v i + v f )t2

90 Ch8.2 Angular Equations Linear EquationAngular Equivalent 1. v f = v i + at1. ω f = ω i + αt 2. s = ½(v i + v f )t2. θ = ½(ω i + ω f )t 3. s = v i t + ½at 2 3.

91 Ch8.2 Angular Equations Linear EquationAngular Equivalent 1. v f = v i + at1. ω f = ω i + αt 2. s = ½(v i + v f )t2. θ = ½(ω i + ω f )t 3. s = v i t + ½at 2 3. θ = ω i t + ½αt 2 4. v f 2 = v i 2 + 2as4.

92 Ch8.2 Angular Equations Linear EquationAngular Equivalent 1. v f = v i + at1. ω f = ω i + αt 2. s = ½(v i + v f )t2. θ = ½(ω i + ω f )t 3. s = v i t + ½at 2 3. θ = ω i t + ½αt 2 4. v f 2 = v i 2 + 2as4. ω f 2 = ω i 2 + 2αθ Ex1) Mounted on a bus engine is a 2.0m diameter flywheel, a massive disk used to store rotational energy. If it is accl at a const rate of 2rpm per sec, what will be the angular speed of a point on its rim after 5 sec? Thru what angle will it have rotated?

93 Ex2) A cyclist traveling at 5.0 m/s uniformly accl up to 10 m/s in 2.0 sec. A small pebble is caught in the tread of one tire, that has a radius of 35cm. a. What is the angular accl of the pebble in those 2 secs? b. Thru what angle does the pebble revolve? c. How far does the pebble travel? Ch8 HW#2 p280 43,44,49,53,55

94 43. A videodisc revolves at 1800rpm beneath a laser read-out head. If the beam is 12cm from the center of the disc, how many meters of data pass beneath it in 0.10sec?

95 Ch8 HW#2 p280 43,44,49,53, A videodisc revolves at 1800rpm beneath a laser read-out head. If the beam is 12cm from the center of the disc, how many meters of data pass beneath it in 0.10sec? ω = 1800 rpm r = 0.12mθ = ω. t t = 0.10s = (188.5 rad/s)(0.10s) s = ? = rad s = r. θ = (0.12m)(18.85 rad) = 2.26m

96 44. A variable speed electric drill motor turning at 100 rev/s is uniformly accl at 50 rev/s 2 up to 200 rev/s. How many turns does it make in the process?

97 44. A variable speed electric drill motor turning at 100 rev/s is uniformly accl at 50 rev/s 2 up to 200 rev/s. How many turns does it make in the process? ω i = 100 rev/s ω f 2 = ω i 2 + 2αθ ω f = 200 rev/s200 2 = (50) θ α = 50 rev/s 2 θ = ? θ = 300 rev

98 49. A wheel is revolving at 20 rad/s when a brake is engaged and the wheel is brought to a stop in rev. How much time elapsed and what was the angular deceleration? ω i = 20 rad/s ω f = 0 rad/s θ = rev  100 rad α = ? t = ?

99 53. A chimp sitting on a unicycle with a wheel diameter of 20in (.508m) is pedaling away at 100 rpm. How fast does he travel? r = 0.254m ω = 100 rad/s  10.5 rad/s v i = ?

100 53. A chimp sitting on a unicycle with a wheel diameter of 20in (.508m) is pedaling away at 100 rpm. How fast does he travel? r = 0.254m ω = 100 rad/s  10.5 rad/s v i = ? v = r. ω = (0.254m)(10.5 rad/s) = 2.6 m/s

101 55. An electric circular saw reaches an operating speed of 1500rpm in the process of revolving thru 200 turns. Find the angular accl and time elapsed. ω i = 0 rpm  0 rad/s ω f 2 = ω i 2 + 2αθ ω f = 1500 rpm  157 rad/s θ = 200 revs  1257 rads α =? t = ? θ = ½(ω i + ω f )t

102 Ch8.2B – More Rotation Equations Ex1) A record player is turned on and reaches a speed of 33 1/3 rpm in 6.0 sec. a. What is the ave ang accl? b. Thru what angle is the record displaced?

103 Ex2) A bicycle is most efficient at a wheel rotation of 50rpm. If the wheel has a diameter of 700mm, a. How fast is the bike moving? b. If the bike’s brakes can decelerate it at 0.5 rad/s 2, thru what angle does it spin? c. What linear distance does it move?

104 HW#30) The motor is mounted with a 20cm diameter pulley and revolves at 100rpm. It is used to drive an attic fan with 4 1m long blades. the ends of the blades cannot exceed 7.0 m/s. What is the size of the pulley? Gear ratio equation: r. ω = r. ω Ch8 HW#3 p ,45,46,47,50

105 45. A wheel is released from rest on an incline and rolls for 30.0s until it reaches a speed of 10.0 rad/s. If accl is const, thru what angle did it rotate?

106 46. A large motor-driven grindstone is spinning at 4 rad/s, when the power is turned off. It rotates thru 100 rad as it uniformly comes to rest. What was its ang accl?

107 47. A ring-shaped space satellite is revolving at 100 rpm when its despin rockets are fired and it decelerates at a constant 2.0 rad/s 2. How much time elapses to bring it to 0 rpm’s? Thru what angle does it spin in this process? ω i = 100 rpm  10.5 rad/s ω f = ω i + αt ω f = 0 rpm  0 rad/s α = –2 rad/s 2 t = ? θ = ?θ = ½(ω i + ω f )t

108 50. A bicycle with a 24in (0.6m) diameter wheel is traveling at 10 mph (4.47 m/s). What is the angular speed? How much time elapses to complete 1 revolution? r = 0.12in = 0.30m v = 4.47 m/s v = r. ω ω = ? t = ?

109 Ch8.3 Torque and Rotational Equilibrium Torque - a force that causes an object to rotate. (must be ) 2 nd Condition of Equilibrium: Ex1) A hand exerts a force of 200 N at the end of a lever 1.0 m long. A return spring attached at the midpoint of the lever pulls back with what force, if the lever isn’t moving?

110 Ex2) A 30kg boy wishes to play on a seesaw with his 10kg dog. When the dog sits 3m, from the pivot, where should the boy sit?

111 Center of Mass (Center of Gravity) - point on an object where we can pretend all the mass is located (there is no net torque from that point) Where is the center of mass on a broom? on a human male? female? irregular chalkboard? earth-moon system?

112 Ex3) The bicep muscle is attached 5cm from elbow (pivot). If the fore arm and hand have a mass of 3.85 kg, and a 2 kg ball is in the hand, what kind of force does the bicep exert to keep the 34 cm fore arm horizontal?

113 Ex4) What does the scale read? m meterstick = 100g, m 1 = 1000g, m 2 = 500g m1m1 m2m2 Ch8 HW#4 p282 63,64,67,68

114 63. While working with precision components, especially out in space, it is often necessary to use a torque wrench, a devise that allows the user to exert only a preset amount of torque. Having dialed in a value of 35.0 Nm, what maximum perpendicular force should be exerted on the handle of a wrench 25.0 cm from the bolt? Τ = 35 Nm Τ = F ┴. r F ┴ = ? r =.25m 64. Harry, who weighs 320 N, and 200 N Gretchen are about to play on a 5.00 m long seesaw. He sits at one end and she sits at the other. Where should the pivot be located if they are to be balanced? Neglecting the weight of the seesaw beam, what is the reaction force exerted by the force on it? T net = T H – T G 0 = F. r – F. r r 5 – r F┴F┴

115 Ch8 HW#4 p282 63,64,67, While working with precision components, especially out in space, it is often necessary to use a torque wrench, a devise that allows the user to exert only a preset amount of torque. Having dialed in a value of 35.0 Nm, what maximum perpendicular force should be exerted on the handle of a wrench 25.0 cm from the bolt? Τ = 35 Nm Τ = F ┴. r F ┴ = ? r =.25m F ┴ = Τ= 35Nm = 140N r.25m 64. Harry, who weighs 320 N, and 200 N Gretchen are about to play on a 5.00 m long seesaw. He sits at one end and she sits at the other. Where should the pivot be located if they are to be balanced? Neglecting the weight of the seesaw beam, what is the reaction force exerted by the force on it? T net = T H – T G 0 = F. r – F. r r 5 – r F┴F┴

116 Ch8 HW#4 p282 63,64,67, While working with precision components, especially out in space, it is often necessary to use a torque wrench, a devise that allows the user to exert only a preset amount of torque. Having dialed in a value of 35.0 Nm, what maximum perpendicular force should be exerted on the handle of a wrench 25.0 cm from the bolt? Τ = 35 Nm Τ = F ┴. r F ┴ = ? r =.25m F ┴ = Τ= 35Nm = 140N r.25m 64. Harry, who weighs 320 N, and 200 N Gretchen are about to play on a 5.00 m long seesaw. He sits at one end and she sits at the other. Where should the pivot be located if they are to be balanced? Neglecting the weight of the seesaw beam, what is the reaction force exerted by the force on it? T net = T H – T G 0 = F. r – F. r = (320N)(r) – (200N)(5 – r) r = 1.9m from Harry r 5 – r F┴F┴

117 67. Two campers carry their gear (90.72 kg) on a light, rigid horizontal pole whose ends they support on their shoulders m apart. If Selma experiences a compressive force of N, where is the load hung on the pole, and what will Rocko feel? T net = T S – T R 0 = F. r – F. r = (533.8N)(r) – (F)(1.829 – r) r = r r

118 67. Two campers carry their gear (90.72 kg) on a light, rigid horizontal pole whose ends they support on their shoulders m apart. If Selma experiences a compressive force of N, where is the load hung on the pole, and what will Rocko feel? r T net = T S – T R 0 = F. r – F. r = (533.8N)(r) – (F)(1.829 – r) 68. (next slide)

119 68. The bridge has a uniform weight of 20.0 kN. Calculate forces a, b. F g20000 ab F g m 35m 15m 20m

120 68. The bridge has a uniform weight of 20.0 kN. Calculate forces a, b. F gB = ? 15m 35m 15m 20m Pick a fulcrum, I picked A F g8000 F g8000 F g20000 T net = F g8000. r + F gB. r + (-F g r) + (-F g8000. r) 0 = (8000N)(15m) + F gB. (70m) – (20000)(35) – (8000N)(50m) F gB = 14,000N F gA = 36,000N – 14,000N = 22,000N ab

121 Ch8.3b – Net Torque Equations Ex1) What does the scale read? (mass of meterstick= 75g) 2kg

122 Ex2) What do both scales read? (mass of meterstick 100g) 0.5kg1kg

123 Ex3) A symmetric bridge weighs 30,000N is 200M long. A truck weighing 7500N is 50m from the west side, a 3500N car is 80m from the west side. What is the force on each landing? Ch8 HW#5 p282 69,72, +2 more problems

124 Ch8 HW#5 p282 69,72, +2 Bonus Questions 69. The beam is of negligible mass, what value does the scale read? 2 1

125 72-a. Find the tension in the rope. The beam is 2m and the rope is attached at 1.50m. Beam is negligible mass. T net = F Ty. r + (-F g. r) 400N 50°

126 72-a. Find the tension in the rope. The beam is 2m and the rope is attached at 1.50m. Beam is negligible mass. T net = F Ty. r + (-F g. r) 0 = (F Ty. sinθ )(1.50m) - (400N)(2m) F T F T = 686N 50° F g 400N

127 72-b. Find the tension in the rope. The beam is 2m and the rope is attached at 1.50m. Beam is 200N. T net = F Ty. r + (-F g. r) + (-F gmeterstick. r) F T 50° F gmeterstick F g 400N

128 1. Find the reading on scale 2 and the unknown mass m 2. m 1 = 15N, mass of meterstick = 2N, Scale 1 reads 20N m2m2 m1m1 F s1 = 20N F S2 = ?

129 2. What do the scales read? m 1 = 10N, m 2 = 5N, mass meterstick = 2N T net = F S1. r + F S2. r + (-F g1. r) + (-F g2. r) + (-F ms. r) 5N10N F s1 = ? F S2 = ?

130 Lab8.1 Mass of a Meterstick - Ch8 HW#5 p282 69,72, +2 Bonus Questions due at beginning of period. Go over it before lab. - Ch8 HW#6 p278 12,48,66 due tomorrow - Lab8.1 end of period

131 Ch8 HW#6 p278 12,48, An ant positioned on the very edge of a Beatles record that is 26cm in diam revolves around 100° as the disk turns. What distance traveled? 48. Record player at 78rpm. Brake brings to stop in 1 sec. How many radians does it turn? 66. Weightless ruler, find mass M and scale 1. 2 M1kg 1 2kg

132 Ch8 HW#6 p278 12,48, An ant positioned on the very edge of a Beatles record that is 26cm in diam revolves around 100° as the disk turns. What distance traveled? θ = 100° π rads = rθ 180° = 1.75 rad = (.13m)(1.75rad) =.23m 48. Record player at 78rpm. Brake brings to stop in 1 sec. How many radians does it turn? 66. Weightless ruler, find mass M and scale 1. 2 M1kg 1 2kg

133 Ch8 HW#6 p278 12,48, An ant positioned on the very edge of a Beatles record that is 26cm in diam revolves around 100° as the disk turns. What distance traveled? θ = 100° π rads = rθ 180° = 1.75 rad = (.13m)(1.75rad) =.23m 48. Record player at 78rpm. Brake brings to stop in 1 sec. How many radians does it turn? ω i = 78rpm = 8.2rad/sθ= ½(ω i +ω f )t ω f = 0 = 4.1rad t = 1s θ=?

134 66. Weightless ruler, find mass M and scale 1. scale2 = 4.17kg 2 M1kg 1 2kg Pivot at mass M: netT = T 2 + T 1 – T s1 Pivot at scale 1: netT = T 1 + T 2 + T M – T s2

135 Ch8.4 Rotational Motion To keep an object from moving, there must be no net __________.

136 Ch8.4 Rotational Motion To keep an object from moving, there must be no net _Force_. (It can be moving at const speed w no net force.) To keep an object from spinning, there must be no net __________.

137 Ch8.4 Rotational Motion To keep an object from moving, there must be no net _Force_. (It can be moving at const speed w no net force.) To keep an object from spinning, there must be no net _torque_. (It can be spinning at const speed w no net torque.) If you apple a force to an object’s center of mass, it will accelerate linearly. FF net = m. a.

138 Ch8.4 Rotational Motion To keep an object from moving, there must be no net _Force_. (It can be moving at const speed w no net force.) To keep an object from spinning, there must be no net _torque_. (It can be spinning at const speed w no net torque.) If you apple a force to an object’s center of mass, it will accelerate linearly. FF net = m. a If you apply a force to an object outside its CoM, it will rotate (angular accl) F moment angular of inertia accl..

139 What about gravity? To keep something from spinning, there must be no net torque.

140 What about gravity? To keep something from spinning, there must be no net torque. The center of gravity must be over the support.

141 1st Law of Rotation - A body at rest stays at rest, a body in rotation tends to keep that rotation unless acted upon by a net torque. - Spinning objects have rotational inertia (movement of inertia) - just like moving objects have inertia, which depends on mass, spinning objects have rotational inertia, which depends on distribution of mass (distance from the CoM) - each type of structure has its own value for. (examples on ppts, p261, and internet) - more mass, further from center=larger I - masses spin around their CoM (aka center of gravity)

142

143 Ex1) A lab group sets up a torque experiment as shown. When they let go of the 100g meterstick, will it fall? If so, what direction and with what angular acceleration? 1N 2.5N

144 Ex1) A lab group sets up a torque experiment as shown. When they let go of the 100g meterstick, will it fall? If so, what direction and with what angular acceleration? 1N 2.5N T net = T 1 + T ms – T 2.5 I. α = F 1 r + F ms r – F 2.5 r ((1/12)ml 2 ). α = (1N)(.4m) + (1N)(0m) – (2.5N)(.3m) ((1/12)(.1)(1) 2. α = -.35 α = 42 rad/s 2 (Not completely true, doesn’t rotate around total CoM)

145 Ex2) A lab group sets up a torque experiment as shown. When they let go of the 100g meterstick, will it fall? If so, what direction and with what angular acceleration? 1N 2.5N Ch8 HW#7 1 – 9

146 1. Inertia depends on mass, rotational inertia depends on mass and what? 2. Compare the effects of a force exerted on an object and torque exerted on an object. 3. How do clockwise and counterclockwise torques compare when a system is balanced?

147 Ch8 HW#7 1 – 9 1. Inertia depends on mass, rotational inertia depends on mass and what? 2. Compare the effects of a force exerted on an object and torque exerted on an object. 3. How do clockwise and counterclockwise torques compare when a system is balanced? 4. A rock has a mass of 1 kg. What is the mass of the measuring stick if it is balanced by a support force at the one-quarter mark?

148 5. Find the rotational inertia (moment of inertia) of a solid cylinder, with a mass of 5 kg and a radius of.25 meters. I = ½mR 2 6. Find the moment of inertia of a ring rotating about its normal axis, with a mass of 150g and a diameter of 10cm. I = mr 2

149 7. A bicycle wheel has a radius of 0.33m. The bike is flipped over and the wheel is spun by a torque of 68 N. m. If the rim and tire together have a mass of 1.46 kg, determine the angular acceleration of the wheel. Ignore the contribution of the spokes. 8. Determine the angular acceleration that would result when a torque of 0.60 N. m is applied about the central spin axis of a hoop of mass 2.0 kg and radius 0.50 m.

150 7. A bicycle wheel has a radius of 0.33m. The bike is flipped over and the wheel is spun by a torque of 68 N. m. If the rim and tire together have a mass of 1.46 kg, determine the angular acceleration of the wheel. Ignore the contribution of the spokes. T net = 68 N. m I. α = 68 N. m α = 423 rad/s 2 mr 2. α = 68 N. m (1.46kg)(.33m) 2. α = 68 N. m 8. Determine the angular acceleration that would result when a torque of 0.60 N. m is applied about the central spin axis of a hoop of mass 2.0 kg and radius 0.50 m.

151 9. A 10kg solid steel cylinder with a 10cm radius is mounted on bearings so that it rotates freely about a horizontal axis. Around the cylinder is wound a number of turns of a fine gold thread. A 1.0kg monkey named Fred holds on to the loose end and descends on the unwinding thread as the cylinder turns. Compute Fred’s acceleration.

152 Ch8.4b - Rotational Motion and Energy Ex1) A solid ball with a radius of 10cm and a mass of 1kg starts rolling from a height of 150 cm down a 30° incline. Find its velocity at the bottom. h = 150cm 30°

153 Ex2) A 250g toy wind-up motorcycle is rolled backward, until the spring is completely wound. The bike is released, and after 3 sec is moving at 4 m/s. If the spring is attached to the rear wheel, with a diameter of 5cm, how much potential energy was stored in the spring initially? Ch8 HW#8 p287 Bonus #1,119,120,125,126,129

154 #1) A 0.5kg mass hangs on a rope wrapped around a freely rotating 2kg cylinder with a radius of.20m. What is the acceleration of the.5kg mass and the tension in the rope?

155 119. Determine the kinetic energy of a nontranslating disk that is spinning around its central symmetry axis at 300 rpm. The disk has a moment-of-inertia of 1.00 kg·m 2. KE r = I. ω 2 = 120. A spherical space satellite having a moment-of-inertia of 250 kg·m 2 is to be spun up from rest to a speed of 12 rpm. How much energy must be imparted to the satellite? ∆KE = KE rf – KE ri

156 119. Determine the kinetic energy of a nontranslating disk that is spinning around its central symmetry axis at 300 rpm. The disk has a moment-of-inertia of 1.00 kg·m 2. KE r = I. ω 2 = (1)(~30) 2 = 900J 120. A spherical space satellite having a moment-of-inertia of 250 kg·m 2 is to be spun up from rest to a speed of 12 rpm. How much energy must be imparted to the satellite? ∆KE = KE rf – KE ri

157 125. A hollow cylinder, or hoop, of mass m rolls down an inclined plane from a height h. If it begins at rest, show that its final speed is given by v=√gh PE = KE t + KE r mgh = ½mv 2 + I. ω 2 (mR 2 ) 126. A solid cylinder of mass 2.0 kg rolls without slipping down a long curved track from a height of 10.0 m. Calculate the linear speed with which it leaves the track.PE = KE t + KE r mgh = ½mv 2 + I. ω 2

158 129. A long pencil is balanced straight up on its point on a horizontal surface. Without slipping, the pencil topples over. Show that the speed at which the eraser end strikes the surface is v=√3gL PE = KE r mgh = I. ω 2 L mg(½L) = (1/3ml 2 )(v/r) 2

159 Ex1) Taking the Earth to be a uniform sphere of radius 6.37x10 6 m/s and a mass 5.98x10 24 kg, compute its angular momentum about its spin axis. Ch8.5 - Angular Momentum

160 Ex2) The moon’s velocity around the earth is 1000 m/s and is currently at a distance of 3.85x10 8 m away. 1 billion years from now the moon will be 10 million meters further. What will be the new velocity? Conservation of Angular Momentum

161 Ex3) By pushing on the ground, a skater with arms extended manages to whirl around at a max speed of 1.0 rev/s. In that configuration, her moment of inertia about the spin axis is 3.5 kg. m 2. The point-masses making up her outstretched arms and leg are fairly far from the vertical axis, making her moment of inertia fairly large. What will happen to her spin rate as she draws her arms and leg in, making her moment of inertia only 1.0 kg. m 2 ? mr 2. (v/r) Ch8 HW#9 p ,136,138, + 1 Bonus Question mvr Alternate formula:Can we derive?

162 Ch8 HW#9 p ,136,138, + 1 Bonus Question 130. A uniform disk of mass 800 kg and radius 0.5 m is rotating around its central symmetry axis at a rate of 60 rpm. Determine its angular momentum.

163 136.Compute the orbital angular momentum of Jupiter (m=1.9 x kg, r = 7.8 x m, and v av = 13.1 x 10 3 m / s ) and then compare it to the spin angular momentum of the sun (m = 1.99 x kg, r= 6.96 x 10 8 m). Assume the sun whose equator rotates once in about 26 days, is a rigid sphere of uniform density. It would seem that most of the angular momentum of the solar system is out there with the giant planets that also rotate quite rapidly. Jupiter: L = (1.9x10 27 kg)(13.1x10 3 m / s )(7.8x10 11 m) ω of Sun: 2π/t = 2π/(2.2x10 6 s) = 2.86x10 -6 rad/s Sun: L = I. ω = 2/5mR 2. ω

164 138. A small mass m is tied to a string and swung in a horizontal plane. The string winds around a vertical rod as the mass revolves, like a length of jewelry chain wrapping around an outstretched finger. Given that the initial speed and length are v i and r i, compute v f when r f = r i /10. or

165 Bonus #1) A rather unruly child hits a 2.5 kg tether ball so that it swings around the center pole at its maximum radius of 1.2m with a speed of 10 m/s. As the ball winds its way around the pole it speeds up in order to conserve angular momentum, but of course you knew that. What is its speed when it is just about to hit the pole, with a radius of 0.2 m?

166 Lab8.2 – The Torque Experiment - end of period - Ch8 HW#9 due at beginning of period (Go over before lab.) - Ch5,8 Rev p171 18,46, + 2 Bonus ?’s, p ,88 10 cm 50g 90cm 100g 85cm 200g

167 Lab8.3 – Rotation Stations - due tomorrow - Ch5,8 Rev p171 18,46, + 2 Bonus ?’s, p ,88

168 Ch5,8 Rev p171 18,46, + 2 Bonus ?’s, Ch8 p , A 1000 kg car traveling on a road that runs straight up a hill reaches the rounded crest at 10.0 m/s. If the hill at that point has a radius of curvature (in a vertical plane) of 50 m, what is the net downward force acting on the car at the instant it is horizontal at the very peak? What is the apparent weight of the car?

169 F net = F g – F N ma c = mg – F N F N = mg – mv 2 /r F N = (1000kg)(9.8m/s 2 ) – (1000kg)(10m/s) 2 /(50m) F N = 9800N – 2000N = 7800N

170 46. Three very small spheres of mass 2.50 kg, 5.00 kg, and 6.00 kg are located on a strait line in space away from everything else. The first one is a point between the other two, 10.0 cm to the right of the second and 20.0 cm to the left of the third. Compute the net gravitational force on it. 5kg 2.5kg 6kg.1m.2m 1 2 3

171 46. Three very small spheres of mass 2.50 kg, 5.00 kg, and 6.00 kg are located on a strait line in space away from everything else. The first one is a point between the other two, 10.0 cm to the right of the second and 20.0 cm to the left of the third. Compute the net gravitational force on it. 5kg 2.5kg 6kg.1m.2m 1 2 3

172 Bonus #1) On which planet would you weigh the least? a.b.c.d. 1M 1R 1M 2R 2M 1R 4M 2R

173 Bonus #2) 60 kg person sits on bathroom scale on roller coaster that does a loop of radius 10m. What does the scale read at 1-4? v = 14 m/s v = 11 m/s v = 20 m/s

174 51. A 1.00 m diameter disk is made to accelerate from rest up to 20 rpm at a rate of 5 rad/s2. Through how many turns will it revolve in the process? How far will a point on its rim travel while all this is happening? r = 0.5m ω f = 20 rpm  2.1 rad/s ω i = 0 rpm  0 rad/s θ = ? s = ? α = 5 rad/s 2

175 51. A 1.00 m diameter disk is made to accelerate from rest up to 20 rpm at a rate of 5 rad/s2. Through how many turns will it revolve in the process? How far will a point on its rim travel while all this is happening? r = 0.5m ω f 2 = ω i 2 + 2αθ ω f = 20 rpm  2.1 rad/s ω i = 0 rpm  0 rad/s ω f =.44 rad/s θ = ? s = ? s = r θ α = 5 rad/s 2 s =.22 m

176 88. An essentially weightless 10.0m long beam is supported at both ends. A 300N child stands 2.0m from the left end. A 6.0m long stack of newspapers weighing 100N per linear meter is uniformly distributed at the other end. Determine the 2 reaction forces supporting the beam.


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