To summarize the car s velocity information, let the horizontal axis represent time, and the vertical axis represent velocity.

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1 To summarize the car s velocity information, let the horizontal axis represent time, and the vertical axis represent velocity. The velocity is constant wherever the slope of the distance-vs-time graph is constant. The velocity changes only when the distance graph s slope changes.

2 In the graph shown, is the velocity constant for any time interval? a) Yes, between 0 s and 2 s. b) Yes, between 2 s and 4 s. c) Yes, between 4 s and 8 s. d) Yes, between 0 s and 8 s. e) No, never. The velocity is a constant value between 0 s and 2 s. The velocity is not changing during this interval, so the graph has a zero (flat) slope.

3 A car moves along a straight road as shown. Does it ever go backward? a) Yes, between 0 s and 2 s. b) Yes, between 2 s and 4 s. c) Yes, between 4 s and 6 s. d) No, never. Although the velocity is decreasing between 4 s and 6 s, the velocity is still in the same direction (it is not negative), so the car is not moving backward.

4 How long a distance did this car go? V(m/s) t(sec) The total distance 6.0 m/s X 10 s = 60 m 1/7/2011 4

5 How long a distance did this car go? V(m/s) t(sec) In each slice, velocity is ~ constant. Add all slice up, one have the total distance The total distance is the area covered by the velocity.vs. time graph. 1/7/2011 5

6 During which time interval is the distance traveled by the car the greatest? a) Between 0 s and 2 s. b) Between 2 s and 4 s. c) Between 4 s and 6 s. d) It is the same for all time intervals. The distance traveled is greatest when the area under the velocity curve is greatest. This occurs between 2 s and 4 s, when the velocity is constant and a maximum.

7 Acceleration is the rate at which velocity changes, i.e. Acceleration = Δv/Δt Unit is: length/(time x time), or meter/second 2 (SI) Our bodies don t feel velocity, if the velocity is constant. Our bodies feel acceleration. Feel the difference inside an airplane when it s landing and up in air cruising. Acceleration can be either a change in the object s speed or direction of motion. Acceleration is also a vector quantity, with magnitude and direction. Acceleration is positive when velocity increase. airplane departure It is negative when velocity decrease. airplane landing

8 Acceleration (cont.) The direction of the acceleration vector is that of the change in velocity, v. If velocity is increasing, the acceleration is in the same direction as the velocity. If velocity is decreasing, the acceleration is in the opposite direction as the velocity.

9 Straight line motion with constant acceleration Quantify the relation among distance (d), velocity (v), acceleration (a) and time (t) a = Δv/ Δt = (v-v 0 )/(t-t 0 ). Set t 0 = 0. v = v 0 + at d is the area of the right plot. Sum of large Number of Slices at all instant. d = 0.5 v v a = v v /t t d = v 0 t + ½*Δv t since a = Δv/ Δt d = v 0 t + ½*at 2 9

10 Constant Speed Circular Motion The direction of the acceleration vector is that of the change in velocity, v. If speed is constant but velocity direction is changing, the acceleration is at right angles to the velocity.

11 Average and Instantaneous Acceleration A car starting from rest, accelerates to a velocity of 20 m/s due east in a time of 5 s. a 20 m/s 5 s 4 m/s/s 4 m/s 2

12 Ch 2 CP4 - d + x v 0 = 14 m/s a = 2 m/s 2 v = 24m/s What is the time? What is the distance? Computed at 1 second intervals.? a) v = v 0 + at, 24m/s = 14m/s +2m/s 2 t, t = 5s b) d = v 0 t + ½ at 2 = 14m/s 5s m/s 2 5s 5s = 95m c) 1 sec = 15 2 sec = 32 3 sec = 51 m 4 sec = 72 1/13/2011 Physics 214 Fall

13 For example: a car traveling on a local highway A steep slope indicates a rapid change in velocity (or speed), and thus a large acceleration. A horizontal line has zero slope and represents zero acceleration.

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15 What is the average acceleration a) 1.0 m/s 2 b) 2.0 m/s 2 c) 1.0 m/s d) 2.0m/s e) Impossible to determine between 4 s and 8 s?.

16 For example: the 100-m Dash The runner wants to reach top speed as soon as possible. The greatest acceleration is at the beginning of the race. For the remaining portion of the race, the runner continues at a constant speed (the top speed) so acceleration is zero.

17 In the graph shown, during which time interval is the acceleration greatest? a) Between 0 s and 2 s. b) Between 2 s and 4 s. c) Between 4 s and 8 s. d) The acceleration does not change. The acceleration is greatest between 2 s and 4 s. The velocity is changing fastest, and the graph has the greatest slope, during this interval.

18 At which part is the magnitude of the acceleration the greatest? a) Part A b) Part B c) Part C d) The acceleration does not change. The magnitude of the acceleration is greatest when the velocity is changing the fastest (has the greatest slope). This occurs at point A.

19 At which part the direction of the acceleration is opposite to that if velocity? a) Part A b) Part B c) Part C d) The acceleration does not change. The magnitude of the acceleration is greatest when the velocity is changing the fastest (has the greatest slope). This occurs at point A.

20 The velocity graph of an object is shown. Is the acceleration of the object constant? a) Yes. b) No. c) It is impossible to determine from this graph. The slope of the velocity curve gradually decreases with time, so the acceleration is decreasing. Initially the velocity is changing quite rapidly, but as time goes on the velocity reaches a maximum value and then stays constant.

21 The velocity graph of an object is shown. Is the acceleration of the object constant? a) Yes. b) No. c) It is impossible to determine from this graph.

22 Test Quiz: What is the average acceleration between 0 s and 4 s? a) 1.0 m/s 2 b) 2.0 m/s 2 c) 1.0 m/s d) 2.0m/s e) Impossible to determine.