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Announcements EXAM: October 13 (through Chapter 9)  calculator (no stored functions, etc)  #2 pencil.

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Presentation on theme: "Announcements EXAM: October 13 (through Chapter 9)  calculator (no stored functions, etc)  #2 pencil."— Presentation transcript:

1 Announcements EXAM: October 13 (through Chapter 9)  calculator (no stored functions, etc)  #2 pencil

2 Reading and Review

3 Elastic Collisions v v m M Carefully place a small rubber ball (mass m) on top of a much bigger basketball (mass M) and drop these from the same height h so they arrive at the ground with the speed v. What is the velocity of the smaller ball after the basketball hits the ground, reverses direction, and then collides with the small rubber ball? a) zero b) v c) 2v d)3v e) 4v

4 Remember that relative velocity has to be equal before and after collision! Before the collision, the basketball bounces up with v and the rubber ball is coming down with v, so their relative velocity is –2v. After the collision, it therefore has to be +2v!! Elastic Collisions v v v v 3v v (a)(b)(c) m M Carefully place a small rubber ball (mass m) on top of a much bigger basketball (mass M) and drop these from the same height h so they arrive at the ground with the speed v. What is the velocity of the smaller ball after the basketball hits the ground, reverses direction, and then collides with the small rubber ball? a) zero b) v c) 2v d)3v e) 4v

5 Recoil Speed a) 0 m/s b) 0.5 m/s to the right c) 1 m/s to the right d) 20 m/s to the right e) 50 m/s to the right A cannon sits on a stationary railroad flatcar with a total mass of 1000 kg. When a 10-kg cannonball is fired to the left at a speed of 50 m/s, what is the recoil speed of the flatcar?

6 Recoil Speed Because the initial momentum of the system was zero, the final total momentum must also be zero. Thus, the final momenta of the cannonball and the flatcar must be equal and opposite. p cannonball = (10 kg)(50 m/s) = 500 kg-m/s p flatcar = 500 kg-m/s = (1000 kg)(0.5 m/s) a) 0 m/s b) 0.5 m/s to the right c) 1 m/s to the right d) 20 m/s to the right e) 50 m/s to the right A cannon sits on a stationary railroad flatcar with a total mass of 1000 kg. When a 10-kg cannonball is fired to the left at a speed of 50 m/s, what is the recoil speed of the flatcar?

7 Elastic Collisions in 2-D Two-dimensional collisions can only be solved if some of the final information is known, such as the final velocity of one object

8 CEBAF at JLAB Precision nuclear microscopy CEBAF at Jefferson Lab 3 experimental Halls recirculation through continuous- wave superconducting RF linacs simultaneous beam delivery to 3 experimental halls with large complementary spectrometers cold RF = stable, clean, quiet up to 200 microAmps per hall, E ~ GeV, >80% polarization An ideal machine for precision nuclear microscopy! -Project started in First experiments begun in User community 1200 members strong

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10 A proton collides elastically with another proton that is initially at rest. The incoming proton has an initial speed of 3.5x10 5 m/s and makes a glancing collision with the second proton. After the collision one proton moves at an angle of 37 o to the original direction of motion, the other recoils at 53 o to that same axis. Find the final speeds of the two protons. v 0 = 3.5x10 5 m/s initial 37 o 53 o v2v2 v1v1 final

11 A proton collides with another proton that is initially at rest. The incoming proton has an initial speed of 3.5x10 5 m/s and makes a glancing collision with the second proton. After the collision one proton moves at an angle of 37 o to the original direction of motion, the other recoils at 53 o to that same axis. Find the final speeds of the two protons. v 0 = 3.5x10 5 m/s initial 37 o 53 o v2v2 v1v1 final Momentum conservation: if we’d been given only 1 angle, would have needed the other angle or conservation of energy also!

12 Is energy conserved in this collision? In an elastic collision between a moving mass incident on a stationary identical mass, the angle between the final momenta is always 90 degrees.

13 Center of Mass Treat extended mass as a bunch of small masses: In a uniform gravitational field you can treat gravitational force as if it acts at the “Center of Mass”

14 Center of Mass The center of mass of a system is the point where the system can be balanced in a uniform gravitational field. For two objects: The center of mass is closer to the more massive object.

15 Center of Mass In general: Symmetry often very useful in determining the Center of Mass

16 The center of mass need not be within the object

17 Motion about the Center of Mass The center of mass of a complex or composite object follows a trajectory as if it were a single particle - with mass equal to the complex object, and experiencing a force equal to the sum of all external forces on that complex object

18 Motion of the center of mass Action/Reaction pairs inside the system cancel out

19 The total mass multiplied by the acceleration of the center of mass is equal to the net external force The center of mass accelerates just as though it were a point particle of mass M acted on by

20 Momentum of a composite object

21 Recoil Speed II a) 0 m/s b) 0.5 m/s to the right c) 1 m/s to the right d) 20 m/s to the right e) 50 m/s to the right A cannon sits on a stationary railroad flatcar with a total mass of 1000 kg. When a 10-kg cannonball is fired to the left at a speed of 50 m/s, what is the speed of the center of mass (of the flatcar + cannonball)?

22 Recoil Speed II Because the initial momentum of the system was zero, the final total momentum must also be zero, regardless of the release of internal energy, internal forces, etc. If no external forces act, the motion of the center of mass does not change a) 0 m/s b) 0.5 m/s to the right c) 1 m/s to the right d) 20 m/s to the right e) 50 m/s to the right A cannon sits on a stationary railroad flatcar with a total mass of 1000 kg. When a 10-kg cannonball is fired to the left at a speed of 50 m/s, what is the speed of the center of mass (of the flatcar + cannonball)?

23 Center of Mass (1) X CM (2) a) higher b) lower c) at the same place d) there is no definable CM in this case The disk shown below in (1) clearly has its center of mass at the center. Suppose the disk is cut in half and the pieces arranged as shown in (2). Where is the center of mass of (2) as compared to (1) ?

24 (1) X CM (2) CM Center of Mass The CM of each half is closer to the top of the semicircle than the bottom. The CM of the whole system is located at the midpoint of the two semicircle CMs, which is higher than the yellow line. a) higher b) lower c) at the same place d) there is no definable CM in this case The disk shown below in (1) clearly has its center of mass at the center. Suppose the disk is cut in half and the pieces arranged as shown in (2). Where is the center of mass of (2) as compared to (1) ?

25 A charging bull elephant with a mass of 5240 kg comes directly toward you with a speed of 4.55 m/s. You toss a kg rubber ball at the elephant with a speed of 7.81 m/s. When the ball bounces back toward you, what is its speed?

26 Our simplest formulas for speed after an elastic collision relied on one body being initially at rest. So lets try a frame where one body (the ball) is at rest! What is the speed of the elephant relative to the ball? In the frame where this is the elephant’s speed, what is the final speed of the ball? Back in the frame of the ground:

27 A charging bull elephant with a mass of 5240 kg comes directly toward you with a speed of 4.55 m/s. You toss a kg rubber ball at the elephant with a speed of 7.81 m/s. When the ball bounces back toward you, what is its speed? Our simplest formulas for speed after an elastic collision relied on one body being initially at rest. So lets try a frame where one body (the ball) is at rest! What is the speed of the elephant relative to the ball? In the frame where this is the elephant’s speed, what is the final speed of the ball? Back in the frame of the ground: NOTE: Formulas for 1-D elastic scattering with non-zero initial velocities are given in end-of-chapter problem 88.

28 Rotational Kinematics

29 Angular Position

30 Degrees and revolutions: Angular Position θ > 0 θ < 0

31 Arc Length Arc length s, from angle measured in radians: s = rθ - Arc length for a full rotation (360 o ) of a radius=1m circle? - What is the relationship between the circumference of a circle and its diameter? C / D = π C = 2 π r s = 2 π (1 m) = 2 π meters 1 complete revolution = 2 π radians 1 rad = 360 o / (2π) = 57.3 o

32 Angular Velocity

33 Instantaneous Angular Velocity Period = How long it takes to go 1 full revolution Period T: SI unit: second, s

34 Linear and Angular Velocity

35 Greater translation for same rotation

36 Bonnie and Klyde  Bonnie Klyde a) Klyde b) Bonnie c) both the same d) linear velocity is zero for both of them Bonnie sits on the outer rim of a merry-go-round, and Klyde sits midway between the center and the rim. The merry-go-round makes one revolution every 2 seconds. Who has the larger linear (tangential) velocity?

37 a) Klyde b) Bonnie c) both the same d) linear velocity is zero for both of them linear speeds v v = r  Bonnie is located farther out Their linear speeds v will be different because v = r  and Bonnie is located farther out (larger radius r) than Klyde. Bonnie Klyde Bonnie and Klyde Bonnie sits on the outer rim of a merry- go-round, and Klyde sits midway between the center and the rim. The merry-go-round makes one revolution every 2 seconds. Who has the larger linear (tangential) velocity?

38 Angular Acceleration

39 Instantaneous Angular Acceleration

40 Rotational Kinematics, Constant Acceleration If the acceleration is constant: If the angular acceleration is constant: v = v 0 + at

41 Analogies between linear and rotational kinematics:

42 An object at rest begins to rotate with a constant angular acceleration. If this object rotates through an angle  in the time t, through what angle did it rotate in the time t? Angular Displacement I ½ a)  b)  c)  d) 2  e) 4  ½ ¼ ¾

43 An object at rest begins to rotate with a constant angular acceleration. If this object rotates through an angle  in the time t, through what angle did it rotate in the time t ? a)  b)  c)  d) 2  e) 4  half the time one-quarter the angle The angular displacement is  =  t 2 (starting from rest), and there is a quadratic dependence on time. Therefore, in half the time, the object has rotated through one-quarter the angle. Angular Displacement I ½ ¼ ½ ¾

44 Which child experiences a greater acceleration? (assume constant angular speed)

45 Larger r: - larger v for same ω - larger a c for same ω a c is required for circular motion. An object may have a t as well, which implies angular acceleration

46 Angular acceleration and total linear acceleration

47 Angular and linear acceleration

48 Rolling Motion If a round object rolls without slipping, there is a fixed relationship between the translational and rotational speeds:

49 Rolling Motion We may also consider rolling motion to be a combination of pure rotational and pure translational motion: + =

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51 Suppose that the speedometer of a truck is set to read the linear speed of the truck but uses a device that actually measures the angular speed of the tires. If larger diameter tires are mounted on the truck instead, how will that affect the speedometer reading as compared to the true linear speed of the truck? a) speedometer reads a higher speed than the true linear speed b) speedometer reads a lower speed than the true linear speed c) speedometer still reads the true linear speed Truck Speedometer

52 Suppose that the speedometer of a truck is set to read the linear speed of the truck but uses a device that actually measures the angular speed of the tires. If larger diameter tires are mounted on the truck instead, how will that affect the speedometer reading as compared to the true linear speed of the truck? a) speedometer reads a higher speed than the true linear speed b) speedometer reads a lower speed than the true linear speed c) speedometer still reads the true linear speed The linear speed is v =  R. So when the speedometer measures the same angular speed  as before, the linear speed v is actually higher, because the tire radius is larger than before. Truck Speedometer

53 Jeff of the Jungle swings on a vine that is 7.20 m long. At the bottom of the swing, just before hitting the tree, Jeff’s linear speed is 8.50 m/s. (a) Find Jeff’s angular speed at this time. (b) What centripetal acceleration does Jeff experience at the bottom of his swing? (c) What exerts the force that is responsible for Jeff’s centripetal acceleration?

54 Jeff of the Jungle swings on a vine that is 7.20 m long. At the bottom of the swing, just before hitting the tree, Jeff’s linear speed is 8.50 m/s. (a) Find Jeff’s angular speed at this time. (b) What centripetal acceleration does Jeff experience at the bottom of his swing? (c) What exerts the force that is responsible for Jeff’s centripetal acceleration? a) b) c) This is the force that is responsible for keeping Jeff in circular motion: the vine.

55 Rotational Kinetic Energy For this mass m, and a massless rod

56 PHYS2010 midterm 1, Fall ) If the net work done on an object is zero, then the object's kinetic energy A) is zero. B) decreases. C) increases. D) remains the same. E) cannot be determined without knowing the object mass. suggested time: 1 minute Please do not ask questions about this problem at discussion sessions before 10/7 Provided in lecture notes on: 10/6

57 PHYS2010 midterm 1, Fall ) A horizontal force of 40 N pushes an object of mass 5.0 kg up an inclined plane through a distance of 1.6 m measured along the plane. The plane is inclined at an angle of 30 o to the horizontal. Neglect friction and use g=10 m/s 2, What is the work done by the normal force on the object? A) 53 J B) 15 J C) 0 J D) -25 J E) 37 J suggested time: 1 minute Please do not ask questions about this problem at discussion sessions before 10/7 Provided in lecture notes on: 10/6

58 PHYS2010 midterm 2, Fall ) In a particle accelerator, protons are fired at a stationary target and collide elastically with the nuclei of the atoms in the target. In one such experiment, a beam of protons is fired with a speed of 2.10 x 10 7 m/s. The protons that undergo a collision rebound with a speed of 1.68 x 10 7 m/s. What is the mass of the nuclei that the protons are colliding against? A) 9.00 proton masses B) 10.0 proton masses C) 7.00 proton masses D) 8.00 proton masses E) 6.00 proton masses suggested time: 3-4 minutes Please do not ask questions about this problem at discussion sessions before 10/7 Provided in lecture notes on: 10/6

59 PHYS2010 midterm 2, Fall ) A Ferris wheel with a radius of 8.00 m rotates at a constant rate, completing one revolution in 30.0 s. What is the apparent weight of a 60.0-kg passenger when she is at the top of the wheel? A) 615 N B) 589 N C) 568 N D) 325 N E) 432 N suggested time: 4-5 minutes Please do not ask questions about this problem at discussion sessions before 10/19 Provided in lecture notes on: 10/6


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