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PHYSICSCHAPTER 2 1 CHAPTER 2: Kinematics of Linear Motion (5 hours)

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1 PHYSICSCHAPTER 2 1 CHAPTER 2: Kinematics of Linear Motion (5 hours)

2 PHYSICSCHAPTER Kinematics of Linear motion studies of motion of an objects without considering the effects that produce the motion.  is defined as the studies of motion of an objects without considering the effects that produce the motion.  There are two types of motion: Linear or straight line motion (1-D)  with constant (uniform) velocity  with constant (uniform) acceleration, e.g. free fall motion Projectile motion (2-D)  x-component (horizontal)  y-component (vertical) 2

3 PHYSICSCHAPTER 2 Learning Outcomes : At the end of this chapter, students should be able to:  Define and distinguish between  Distance and displacement  Speed and velocity  Instantaneous velocity, average velocity and uniform velocity  Instantaneous acceleration, average acceleration and uniform acceleration,  Sketch graphs of displacement-time, velocity-time and acceleration-time.  Determine the distance travelled, displacement, velocity and acceleration from appropriate graphs Linear Motion (1 hour)

4 PHYSICSCHAPTER Linear motion (1-D) Distance, d  scalar quantity.  is defined as the length of actual path between two points.  For example : The length of the path from P to Q is 25 cm. 4 P Q

5 PHYSICSCHAPTER 2 5  vector quantity.  is defined as the distance between initial point and final point in a straight line.  The S.I. unit of displacement is metre (m). Example 2.1 : An object P moves 30 m to the east after that 15 m to the south and finally moves 40 m to west. Determine the displacement of P relative to the original position. Solution : Displacement, N E W S O P   30 m 15 m 10 m 30 m

6 PHYSICSCHAPTER 2 The magnitude of the displacement is given by and its direction is 2.1.3Speed, v rate of change of distance  is defined as the rate of change of distance.  scalar quantity.  Equation: 6

7 PHYSICSCHAPTER 2 7  is a vector quantity.  The S.I. unit for velocity is m s -1. Average velocity, v av  is defined as the rate of change of displacement.  Equation:  Its direction is in the same direction of the change in displacement Velocity,

8 PHYSICSCHAPTER 2 8 Instantaneous velocity, v  is defined as the instantaneous rate of change of displacement.  Equation:  An object moves in a uniform velocity when and the instantaneous velocity equals to the average velocity at any time.

9 PHYSICSCHAPTER 2 9  Therefore Q s t 0 s1s1 t1t1 The gradient of the tangent to the curve at point Q = the instantaneous velocity at time, t = t 1 Gradient of s-t graph = velocity

10 PHYSICSCHAPTER 2 10  vector quantity.  The S.I. unit for acceleration is m s -2. Average acceleration, a av  is defined as the rate of change of velocity.  Equation:  Its direction is in the same direction of motion.  The acceleration of an object is uniform when the magnitude of velocity changes at a constant rate and along fixed direction Acceleration,

11 PHYSICSCHAPTER 2 11 Instantaneous acceleration, a  is defined as the instantaneous rate of change of velocity.  Equation:  An object moves in a uniform acceleration when and the instantaneous acceleration equals to the average acceleration at any time.

12 PHYSICSCHAPTER 2 12 Deceleration, a  is a negative acceleration.  The object is slowing down meaning the speed of the object decreases with time.  Therefore v t Q 0 v1v1 t1t1 The gradient of the tangent to the curve at point Q = the instantaneous acceleration at time, t = t 1 Gradient of v-t graph = acceleration

13 PHYSICSCHAPTER 2 13 Displacement against time graph ( s-t ) Graphical methods s t 0 s t 0 (a) Uniform velocity (b) The velocity increases with time Gradient = constant Gradient increases with time (c) s t 0 Q R P The direction of velocity is changing. Gradient at point R is negative. Gradient at point Q is zero. The velocity is zero.

14 PHYSICSCHAPTER 2 14 Velocity versus time graph ( v-t )  The gradient at point A is positive – a > 0(speeding up)  The gradient at point B is zero – a= 0  The gradient at point C is negative – a < 0(slowing down) t1t1 t2t2 v t 0 (a) t2t2 t1t1 v t 0 (b) t1t1 t2t2 v t 0 (c) Uniform velocity Uniform acceleration Area under the v -t graph = displacement B C A

15 PHYSICSCHAPTER 2 15  From the equation of instantaneous velocity, Therefore Simulation 2.1 Simulation 2.2Simulation 2.3

16 PHYSICSCHAPTER 2 16 A toy train moves slowly along a straight track according to the displacement, s against time, t graph in Figure 2.1. a. Explain qualitatively the motion of the toy train. b. Sketch a velocity (cm s -1 ) against time (s) graph. c. Determine the average velocity for the whole journey. d. Calculate the instantaneous velocity at t = 12 s. e. Determine the distance travelled by the toy train. Example 2.2 : t (s) s (cm) Figure 2.1

17 PHYSICSCHAPTER 2 17 Solution : a. 0 to 6 s : The train moves at a constant velocity of 6 to 10 s : The train stops. 10 to 14 s :The train moves in the same direction at a constant velocity of b t (s) v (cm s  1 )

18 PHYSICSCHAPTER 2 18 Solution : c. d. 10 cm. e. The distance travelled by the toy train is 10 cm.

19 PHYSICSCHAPTER 2 19 A velocity-time ( v - t ) graph in Figure 2.2 shows the motion of a lift. a. Describe qualitatively the motion of the lift. b. Sketch a graph of acceleration (m s  2 ) against time (s). c. Determine the total distance travelled by the lift and its displacement. d. Calculate the average acceleration between 20 s to 40 s. Example 2.3 : t (s) v (m s   1 ) Figure

20 PHYSICSCHAPTER 2 20 Solution : a. 0 to 5 s : Lift moves upward from rest with a constant acceleration of 5 to 15 s : The velocity of the lift increases from 2 m s  1 to 4 m s  1 but the acceleration decreasing to 15 to 20 s :Lift moving with constant velocity of 20 to 25 s :Lift decelerates at a constant rate of 25 to 30 s :Lift at rest or stationary. 30 to 35 s :Lift moves downward with a constant acceleration of 35 to 40 s :Lift moving downward with constant velocity of 40 to 50 s :Lift decelerates at a constant rate of and comes to rest.

21 PHYSICSCHAPTER 2 21 Solution : b. t (s) a (m s  2 )

22 PHYSICSCHAPTER 2 22 Solution : c. i t (s) v (m s   1 ) A1A1 A2A2 A3A3 A4A4 A5A5

23 PHYSICSCHAPTER 2 23 Solution : c. ii. d.

24 PHYSICSCHAPTER 2 24 Figure Figure 2.3 shows a velocity versus time graph for an object constrained to move along a line. The positive direction is to the right. a. Describe the motion of the object in 10 s. b. Sketch a graph of acceleration (m s -2 ) against time (s) for the whole journey. c. Calculate the displacement of the object in 10 s. ANS. : 6 m Exercise 2.1 :

25 PHYSICSCHAPTER A train pulls out of a station and accelerates steadily for 20 s until its velocity reaches 8 m s  1. It then travels at a constant velocity for 100 s, then it decelerates steadily to rest in a further time of 30 s. a. Sketch a velocity-time graph for the journey. b. Calculate the acceleration and the distance travelled in each part of the journey. c. Calculate the average velocity for the journey. Physics For Advanced Level, 4 th edition, Jim Breithaupt, Nelson Thornes, pg.15, no ANS. : 0.4 m s  2,0 m s  2, m s  2, 80 m, 800 m, 120 m; 6.67 m s  m s  1. Exercise 2.1 :

26 PHYSICSCHAPTER 2 Learning Outcome : At the end of this chapter, students should be able to:  Derive and apply equations of motion with uniform acceleration: Uniformly accelerated motion (1 hour)

27 PHYSICSCHAPTER Uniformly accelerated motion constant  From the definition of average acceleration, uniform (constant) acceleration is given by where v : final velocity u : initial velocity a : uniform (constant) acceleration t : time 27 (1)

28 PHYSICSCHAPTER 2 28  From equation (1), the velocity-time graph is shown in Figure 2.4 : From the graph, The displacement after time, s = shaded area under the graph = the area of trapezium Hence, velocity 0 v u time t Figure 2.4 (2)

29 PHYSICSCHAPTER 2 29  By substituting eq. (1) into eq. (2) thus  From eq. (1),  From eq. (2), (3) multiply (4)

30 PHYSICSCHAPTER 2 30  Notes: equations (1) – (4) can be used if the motion in a straight line with constant acceleration. For a body moving at constant velocity, ( a = 0) the equations (1) and (4) become Therefore the equations (2) and (3) can be written as constant velocity

31 PHYSICSCHAPTER 2 31 A plane on a runway accelerates from rest and must attain takeoff speed of 148 m s  1 before reaching the end of the runway. The plane’s acceleration is uniform along the runway and of value 914 cm s  2. Calculate a. the minimum length of the runway required by the plane to takeoff. b. the time taken for the plane cover the length in (a). Solution : a. Use Example 2.4 :

32 PHYSICSCHAPTER 2 32 Solution : b. By using the equation of linear motion, OR

33 PHYSICSCHAPTER 2 33 A bus travelling steadily at 30 m s  1 along a straight road passes a stationary car which, 5 s later, begins to move with a uniform acceleration of 2 m s  2 in the same direction as the bus. Determine a. the time taken for the car to acquire the same velocity as the bus, b. the distance travelled by the car when it is level with the bus. Solution : a. Given Use Example 2.5 :

34 PHYSICSCHAPTER 2 34 b. From the diagram, c b b b c Therefore

35 PHYSICSCHAPTER 2 35 A particle moves along horizontal line according to the equation Where s is displacement in meters and t is time in seconds. At time, t = 3 s, determine a. the displacement of the particle, b. Its velocity, and c. Its acceleration. Solution : a. t =3 s ; Example 2.6 :

36 PHYSICSCHAPTER 2 36 Solution : b. Instantaneous velocity at t = 3 s, Use Thus

37 PHYSICSCHAPTER 2 37 Solution : c. Instantaneous acceleration at t = 3 s, Use Hence

38 PHYSICSCHAPTER A speedboat moving at 30.0 m s -1 approaches stationary buoy marker 100 m ahead. The pilot slows the boat with a constant acceleration of m s -2 by reducing the throttle. a. How long does it take the boat to reach the buoy? b. What is the velocity of the boat when it reaches the buoy? No. 23,pg. 51,Physics for scientists and engineers with modern physics, Serway & Jewett,6 th edition. ANS. : 4.53 s; 14.1 m s  1 2.An unmarked police car travelling a constant 95 km h -1 is passed by a speeder traveling 140 km h -1. Precisely 1.00 s after the speeder passes, the policemen steps on the accelerator; if the police car’s acceleration is 2.00 m s -2, how much time passes before the police car overtakes the speeder (assumed moving at constant speed)? No. 44, pg. 41,Physics for scientists and engineers with modern physics, Douglas C. Giancoli,3 rd edition. ANS. : 14.4 s Exercise 2.2 :

39 PHYSICSCHAPTER A car traveling 90 km h -1 is 100 m behind a truck traveling 75 km h -1. Assuming both vehicles moving at constant velocity, calculate the time taken for the car to reach the truck. No. 15, pg. 39,Physics for scientists and engineers with modern physics, Douglas C. Giancoli,3 rd edition. ANS. : 24 s 4.A car driver, travelling in his car at a constant velocity of 8 m s -1, sees a dog walking across the road 30 m ahead. The driver’s reaction time is 0.2 s, and the brakes are capable of producing a deceleration of 1.2 m s -2. Calculate the distance from where the car stops to where the dog is crossing, assuming the driver reacts and brakes as quickly as possible. ANS. : 1.73 m Exercise 2.2 :

40 PHYSICSCHAPTER 2 Learning Outcome : At the end of this chapter, students should be able to:  Describe and use equations for freely falling bodies. For upward and downward motion, use a =  g =  9.81 m s  Freely falling bodies (1 hour)

41 PHYSICSCHAPTER Freely falling bodies  is defined as the vertical motion of a body at constant acceleration, g under gravitational field without air resistance.  In the earth’s gravitational field, the constant acceleration known as acceleration due to gravity or free-fall acceleration or gravitational acceleration. the value is g = 9.81 m s  2 the direction is towards the centre of the earth (downward).  Note: In solving any problem involves freely falling bodies or free fall motion, the assumption made is ignore the air resistance. 41

42 PHYSICSCHAPTER 2 42  Sign convention:  Table 2.1 shows the equations of linear motion and freely falling bodies. Table 2.1 Linear motionFreely falling bodies From the sign convention thus,

43 PHYSICSCHAPTER 2 43  An example of freely falling body is the motion of a ball thrown vertically upwards with initial velocity, u as shown in Figure 2.5.  Assuming air resistance is negligible, the acceleration of the ball, a =  g when the ball moves upward and its velocity decreases to zero when the ball reaches the maximum height, H. H u v velocity = 0 Figure 2.5

44 PHYSICSCHAPTER 2 44  The graphs in Figure 2.6 show the motion of the ball moves up and down. Derivation of equations  At the maximum height or displacement, H where t = t 1, its velocity, hence therefore the time taken for the ball reaches H, Figure 2.6 t 0 v u uu t1t1 2t 1 t 0 a gg t1t1 t s 0 H t1t1 v =0 Simulation 2.4

45 PHYSICSCHAPTER 2 45  To calculate the maximum height or displacement, H : use either maximum height,  Another form of freely falling bodies expressions are Where s = H OR

46 PHYSICSCHAPTER 2 46 A ball is thrown from the top of a building is given an initial velocity of 10.0 m s  1 straight upward. The building is 30.0 m high and the ball just misses the edge of the roof on its way down, as shown in figure 2.7. Calculate a. the maximum height of the stone from point A. b. the time taken from point A to C. c. the time taken from point A to D. d. the velocity of the ball when it reaches point D. (Given g = 9.81 m s  2 ) Example 2.7 : A B C D u =10.0 m s  m Figure 2.7

47 PHYSICSCHAPTER 2 47 Solution : a. At the maximum height, H, v y = 0 and u = u y = 10.0 m s  1 thus b. From point A to C, the vertical displacement, s y = 0 m thus A B C D u 30.0 m

48 PHYSICSCHAPTER 2 48 Solution : c. From point A to D, the vertical displacement, s y =  30.0 m thus By using A B C D u 30.0 m OR Time don’t have negative value. a b c

49 PHYSICSCHAPTER 2 49 Solution : d. Time taken from A to D is t = 3.69 s thus From A to D, s y =  30.0 m Therefore the ball’s velocity at D is A B C D u 30.0 m OR

50 PHYSICSCHAPTER 2 50 A book is dropped 150 m from the ground. Determine a. the time taken for the book reaches the ground. b. the velocity of the book when it reaches the ground. (Given g = 9.81 m s -2 ) Solution : a. The vertical displacement is s y =  150 m Hence Example 2.8 : u y = 0 m s  m

51 PHYSICSCHAPTER 2 51 Solution : b. The book’s velocity is given by Therefore the book’s velocity is OR

52 PHYSICSCHAPTER A ball is thrown directly downward, with an initial speed of 8.00 m s  1, from a height of 30.0 m. Calculate a. the time taken for the ball to strike the ground, b. the ball’s speed when it reaches the ground. ANS. : 1.79 s; 25.6 m s  1 2.A falling stone takes 0.30 s to travel past a window 2.2 m tall as shown in Figure 2.8. From what height above the top of the windows did the stone fall? ANS. : 1.75 m Exercise 2.3 : Figure 2.8 to travel this distance took 0.30 s

53 PHYSICSCHAPTER A ball is thrown directly downward, with an initial speed of 8.00 m s  1, from a height of 30.0 m. Calculate a. the time taken for the ball to strike the ground, b. the ball’s speed when it reaches the ground. ANS. : 1.79 s; 25.6 m s  1 2.A falling stone takes 0.30 s to travel past a window 2.2 m tall as shown in Figure 2.8. From what height above the top of the windows did the stone fall? ANS. : 1.75 m Exercise 2.3 : Figure 2.8 to travel this distance took 0.30 s

54 PHYSICSCHAPTER 2 Learning Outcomes : At the end of this chapter, students should be able to:  Describe and use equations for projectile, Calculate: time of flight, maximum height, range and maximum range, instantaneous position and velocity Projectile motion (2 hours)

55 PHYSICSCHAPTER Projectile motion  A projectile motion consists of two components: vertical component (y-comp.)  motion under constant acceleration, a y =  g horizontal component (x-comp.)  motion with constant velocity thus a x = 0  The path followed by a projectile is called trajectory is shown in Figure v u  s x = R s y =H uxux v 2y uyuy v 1x v 1y v 2x v1v1 11 v2v2 22 t1t1 t2t2 B A P Q C y x Figure 2.9 Simulation 2.5

56 PHYSICSCHAPTER 2 56  From Figure 2.9, The x-component of velocity along AC (horizontal) at any point is constant, The y-component (vertical) of velocity varies from one point to another point along AC. but the y-component of the initial velocity is given by

57 PHYSICSCHAPTER 2 VelocityPoint PPoint Q x-comp. y-comp. magnitude direction 57  Table 2.2 shows the x and y-components, magnitude and direction of velocities at points P and Q. Table 2.2

58 PHYSICSCHAPTER 2 58  The ball reaches the highest point at point B at velocity, v where x-component of the velocity, y-component of the velocity, y-component of the displacement,  Use Maximum height, H

59 PHYSICSCHAPTER 2 59  At maximum height, H Time, t =  t’ and v y = 0  Use Time taken to reach maximum height,  t’ Flight time,  t (from point A to point C)

60 PHYSICSCHAPTER 2 60  Since the x-component for velocity along AC is constant hence  From the displacement formula with uniform velocity, thus the x-component of displacement along AC is Horizontal range, R and value of R maximum and

61 PHYSICSCHAPTER 2 61  From the trigonometry identity, thus  = 45  sin 2  = 1  The value of R maximum when  = 45  and sin 2  = 1 therefore Simulation 2.6

62 PHYSICSCHAPTER 2 62  Figure 2.10 shows a ball bearing rolling off the end of a table with an initial velocity, u in the horizontal direction.  Horizontal component along path AB.  Vertical component along path AB Horizontal projectile h x A B Figure 2.10 Simulation 2.7

63 PHYSICSCHAPTER 2 63 Time taken for the ball to reach the floor (point B), t  By using the equation of freely falling bodies, Horizontal displacement, x  Use condition below : The time taken for the ball free fall to point A The time taken for the ball to reach point B = (Refer to Figure 2.11) Figure 2.11

64 PHYSICSCHAPTER 2 64  Since the x-component of velocity along AB is constant, thus the horizontal displacement, x  Note : air resistance is negligible In solving any calculation problem about projectile motion, the air resistance is negligible. and

65 PHYSICSCHAPTER 2 65 Figure 2.12 shows a ball thrown by superman with an initial speed, u = 200 m s -1 and makes an angle,  = 60.0  to the horizontal. Determine a. the position of the ball, and the magnitude and direction of its velocity, when t = 2.0 s. Example 2.9 : Figure 2.12 x O u  = 60.0  y R H v 2y v 1x v 1y v 2x Q v1v1 P v2v2

66 PHYSICSCHAPTER 2 66 b. the time taken for the ball reaches the maximum height, H and calculate the value of H. c.the horizontal range, R d. the magnitude and direction of its velocity when the ball reaches the ground (point P). e. the position of the ball, and the magnitude and direction of its velocity at point Q if the ball was hit from a flat-topped hill with the time at point Q is 45.0 s. (Given g = 9.81 m s -2 ) Solution : The component of Initial velocity :

67 PHYSICSCHAPTER 2 67 Solution : a. i.position of the ball when t = 2.0 s, Horizontal component : Vertical component : (200 m, 326 m) therefore the position of the ball is (200 m, 326 m)

68 PHYSICSCHAPTER 2 68 Solution : a. ii.magnitude and direction of ball’s velocity at t = 2.0 s, Horizontal component : Vertical component : Magnitude, Direction, from positive x-axis anticlockwise

69 PHYSICSCHAPTER 2 69 Solution : b. i.At the maximum height, H : Thus the time taken to reach maximum height is given by ii.Apply

70 PHYSICSCHAPTER 2 70 Solution : c. Flight time = 2  (the time taken to reach the maximum height) Hence the horizontal range, R is d.When the ball reaches point P thus The velocity of the ball at point P, Horizontal component: Vertical component:

71 PHYSICSCHAPTER 2 71 Solution : Magnitude, Direction, therefore the direction of ball’s velocity is e.The time taken from point O to Q is 45.0 s. i.position of the ball when t = 45.0 s, Horizontal component : from positive x-axis anticlockwise

72 PHYSICSCHAPTER 2 72 Solution : Vertical component : (4500 m,  2148 m) therefore the position of the ball is (4500 m,  2148 m) e. ii.magnitude and direction of ball’s velocity at t = 45.0 s, Horizontal component : Vertical component :

73 PHYSICSCHAPTER 2 73 Solution : Magnitude, Direction, therefore the direction of ball’s velocity is from positive x-axis anticlockwise

74 PHYSICSCHAPTER 2 74 A transport plane travelling at a constant velocity of 50 m s  1 at an altitude of 300 m releases a parcel when directly above a point X on level ground. Calculate a. the flight time of the parcel, b. the velocity of impact of the parcel, c. the distance from X to the point of impact. (Given g = 9.81 m s -2 ) Solution : Example 2.10 : 300 m d X

75 PHYSICSCHAPTER 2 75 Solution : The parcel’s velocity = plane’s velocity thus a.The vertical displacement is given by Thus the flight time of the parcel is and

76 PHYSICSCHAPTER 2 76 Solution : b.The components of velocity of impact of the parcel : Horizontal component : Vertical component : Magnitude, Direction, therefore the direction of parcel’s velocity is from positive x-axis anticlockwise

77 PHYSICSCHAPTER 2 77 Solution : c.Let the distance from X to the point of impact is d. Thus the distance, d is given by

78 PHYSICSCHAPTER 2 78 Figure 2.13 Use gravitational acceleration, g = 9.81 m s  2 1.A basketball player who is 2.00 m tall is standing on the floor 10.0 m from the basket, as in Figure If he shoots the ball at a 40.0  angle above the horizontal, at what initial speed must he throw so that it goes through the hoop without striking the backboard? The basket height is 3.05 m. ANS. : 10.7 m s  1 Exercise 2.4 :

79 PHYSICSCHAPTER An apple is thrown at an angle of 30  above the horizontal from the top of a building 20 m high. Its initial speed is 40 m s  1. Calculate a. the time taken for the apple to strikes the ground, b. the distance from the foot of the building will it strikes the ground, c.the maximum height reached by the apple from the ground. ANS. : 4.90 s; 170 m; 40.4 m 3.A stone is thrown from the top of one building toward a tall building 50 m away. The initial velocity of the ball is 20 m s  1 at 40  above the horizontal. How far above or below its original level will the stone strike the opposite wall? ANS. : 10.3 m below the original level. Exercise 2.4 :

80 PHYSICSCHAPTER 2 80 THE END… Next Chapter… CHAPTER 3 : Momentum and Impulse


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