1 Chapter 2 Motion in One Dimension

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3 2.1 Kinematics Describes motion while ignoring the agents that caused the motion For now, will consider motion in one dimension Along a straight line Will use the particle model A particle is a point-like object, has mass but infinitesimal size

4 Position 位置 Defined in terms of a frame of reference One dimensional, so generally the x - or y -axis The object’s position is its location with respect to the frame of reference Fig 2.1(a)

5 Position-Time Graph The position-time graph shows the motion of the particle (car) The smooth curve is a guess as to what happened between the data points Fig 2.1(b)

6 Displacement 位移 Defined as the change in position during some time interval Represented as  x SI units are meters (m)  x can be positive or negative Different than distance – the length of a path followed by a particle

7 Vectors and Scalars Vector quantities need both magnitude (size or numerical value) and direction to completely describe them Will use + and – signs to indicate vector directions Scalar quantities are completely described by magnitude only

8 Average Velocity 平均速度 The average velocity is rate at which the displacement occurs The dimensions are length / time [L/T] The SI units are m/s Is also the slope of the line in the position – time graph

9 Average Velocity, cont Gives no details about the motion Gives the result of the motion It can be positive or negative It depends on the sign of the displacement It can be interpreted graphically It will be the slope of the position-time graph

10 Displacement from a Velocity - Time Graph The displacement of a particle during the time interval t i to t f is equal to the area under the curve between the initial and final points on a velocity-time curve Fig 2.1(c)

11 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 2.1

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18 瞬時速度 2.2 Instantaneous Velocity The limit of the average velocity as the time interval becomes infinitesimally short, or as the time interval approaches zero The instantaneous velocity indicates what is happening at every point of time

19 Fig 2.2(a)

20 Instantaneous Velocity, graph The instantaneous velocity is the slope of the line tangent to the x vs. t curve This would be the green line The blue lines show that as  t gets smaller, they approach the green line Fig 2.2(b)

21 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 2.2

22 Instantaneous Velocity, equations The general equation for instantaneous velocity is

23 Instantaneous Velocity, Signs At point A, v x is positive The slope is positive At point B, v x is zero The slope is zero At point C, v x is negative The slope is negative Fig 2.3

24 Instantaneous Speed The instantaneous speed is the magnitude of the instantaneous velocity vector Speed can never be negative Thinking Physics 2.1 (P.43)

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28 Fig 2.4

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Constant Velocity If the velocity of a particle is constant Its instantaneous velocity at any instant is the same as the average velocity over a given time period v x = v x, avg =  x /  t Also, x f = x i + v x t These equations can be applied to particles or objects that can be modeled as a particle moving under constant velocity

34 Fig 2.6

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Average Acceleration Acceleration is the rate of change of the velocity It is a measure of how rapidly the velocity is changing Dimensions are L/T 2 SI units are m/s 2

38 Instantaneous Acceleration The instantaneous acceleration is the limit of the average acceleration as  t approaches 0

39 Instantaneous Acceleration – Graph The slope of the velocity vs. time graph is the acceleration Positive values correspond to where velocity in the positive x direction is increasing The acceleration is negative when the velocity is in the positive x direction and is decreasing Fig 2.7

40 Fig 2.8 QUICK QUIZ 2.3 Match each of the velocity-time graphs on the top with the acceleration-time graphs on the bottom.

41 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 2.8

42 Fig 2.9 The velocity of the car decrease from 30 m/s to 15 m/s in a time interval of 2.0 s a = ?

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44 Fig 2.10

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Negative Acceleration Negative acceleration does not necessarily mean that an object is slowing down If the velocity is negative and the acceleration is negative, the object is speeding up Deceleration is commonly used to indicate an object is slowing down, but will not be used in this text

50 Acceleration and Velocity, 1 When an object’s velocity and acceleration are in the same direction, the object is speeding up in that direction When an object’s velocity and acceleration are in the opposite direction, the object is slowing down

51 Acceleration and Velocity, 2 The car is moving with constant positive velocity (shown by red arrows maintaining the same size) Acceleration equals zero

52 Acceleration and Velocity, 3 Velocity and acceleration are in the same direction Acceleration is uniform (blue arrows maintain the same length) Velocity is increasing (red arrows are getting longer) This shows positive acceleration and positive velocity

53 Acceleration and Velocity, 4 Acceleration and velocity are in opposite directions Acceleration is uniform (blue arrows maintain the same length) Velocity is decreasing (red arrows are getting shorter) Positive velocity and negative acceleration

54 Fig 2.11

Kinematic Equations Fig 2.12

56 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 2.11

57 Kinematic Equations The kinematic equations may be used to solve any problem involving one- dimensional motion with a constant acceleration You may need to use two of the equations to solve one problem The equations are useful when you can model a situation as a particle under constant acceleration

58 Kinematic Equations, specific For constant a, Can determine an object’s velocity at any time t when we know its initial velocity and its acceleration Does not give any information about displacement

59 Kinematic Equations, specific For constant acceleration, The average velocity can be expressed as the arithmetic mean of the initial and final velocities Only valid when acceleration is constant

60 Kinematic Equations, specific For constant acceleration, This gives you the position of the particle in terms of time and velocities Doesn’t give you the acceleration

61 Kinematic Equations, specific For constant acceleration, Gives final position in terms of velocity and acceleration Doesn’t tell you about final velocity

62 Kinematic Equations, specific For constant a, Gives final velocity in terms of acceleration and displacement Does not give any information about the time

63 Graphical Look at Motion – displacement – time curve The slope of the curve is the velocity The curved line indicates the velocity is changing Therefore, there is an acceleration Fig 2.12(a)

64 Graphical Look at Motion – velocity – time curve The slope gives the acceleration The straight line indicates a constant acceleration Fig 2.12(b)

65 The zero slope indicates a constant acceleration Graphical Look at Motion – acceleration – time curve Fig 2.12(c)

66 If you can't see the image above, please install Shockwave Flash Player.Shockwave Flash Player. If this active figure can’t auto-play, please click right button, then click play. NEXT Active Figure 2.12

67 Problem-Solving Strategy – Constant Acceleration Conceptualize Establish a mental representation Categorize Confirm there is a particle or an object that can be modeled as a particle Confirm it is moving with a constant acceleration

68 Problem-Solving Strategy – Constant Acceleration, cont Analyze Set up the mathematical representation Choose the instant to call the initial time and another to be the final time These are defined by the parts of the problem of interest, not necessarily the actual beginning and ending of the motion Choose the appropriate kinematic equation(s)

69 Problem-Solving Strategy – Constant Acceleration, Final Finalize Check to see if the answers are consistent with the mental and pictorial representations Check to see if your results are realistic

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77 Consider the car For the trooper

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Kinematic Equations – Summary

Freely Falling Objects A freely falling object is any object moving freely under the influence of gravity alone It does not depend upon the initial motion of the object Dropped – released from rest Thrown downward Thrown upward

81 Galileo Galilei( )

82 Acceleration of Freely Falling Object The acceleration of an object in free fall is directed downward, regardless of the initial motion The magnitude of free fall acceleration is g = 9.80 m/s 2 g decreases with increasing altitude g varies with latitude 9.80 m/s 2 is the average at the earth’s surface

83 Acceleration of Free Fall, cont. We will neglect air resistance Free fall motion is constantly accelerated motion in one dimension Let upward be positive Use the kinematic equations with a y = g = m/s 2 The negative sign indicates the direction of the acceleration is downward

84 Fig 2.14

85 Emily challenges David to catch a dollar bill as follows. She holds the bill vertically, as in Figure 2.15, with the center of the bill between David’s index finger and thumb. David must catch the bill after Emily releases it without moving his hand downward. The reaction time of most people is at best about 0.2 s. Who would you bet on?

86 We will assume that it can be modeled as a particle falling through a vacuum. Because the bill is in free-fall and undergoes a downward acceleration of magnitude 9.80 m/s 2, in 0.2 s it falls a distance of y = (1/2)gt 2  0.2 m = 20 cm This distance is about twice the distance between the center of the bill and its top edge ( 8 cm). Therefore, David will be unsuccessful.

87 Free Fall Example Initial velocity at A is upward (+) and acceleration is g (-9.8 m/s 2 ) At B, the velocity is 0 and the acceleration is g (-9.8 m/s 2 ) At C, the velocity has the same magnitude as at A, but is in the opposite direction The displacement is –50.0 m (it ends up 50.0 m below its starting point)

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