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touches the ground as ( ) v f = − v i2 + 2a( ∆y ) = − 0 + 2 − 9.80 m s 2 ( −1.5 m ) = −5.4 m s .
Then, using the second assumption, the acceleration while coming to rest is found to be a= v 2 − v i2
2 ( ∆y ) = 0 − ( −5.4 m s ) ( 2 5.0 × 10 −3 m 2 ) = 2.9 × 103 m s 2 , or ~ 103 m s 2 . 48 CHAPTER 2 Answers to Even Numbered Conceptual Questions
2. Yes. Zero velocity means that the object is at rest. If the object also has zero acceleration,
the velocity is not changing and the object will continue to be at rest. 4. You can ignore the time for the lightning to reach you because light travels at the speed of
3 x 108 m/s, a speed so fast that in our day-to-day activities it is essentially infinite. 6. The average velocity of an object is defined as the displacement of the object divided by
the time interval during which the displacement occurred. If the average velocity is zero,
the displacement must also be zero. 8. In Figure (b), the images are equally spaced showing that the object moved the same
distance in each time interval. Hence, the velocity is constant in (b).
In Figure (c,) the images are farther apart for each successive time interval. The object is
moving toward the right and speeding up. This means that the acceleration is positive in
In Figure (a), The first four images show an increasing distance traveled each time interval
and therefore a positive acceleration. However, after the fourth image, the spacing is
decreasing showing that the object is now slowing down (or has negative acceleration). 10. Velocities are equal only if both magnitude and direction are the same. These objects are
moving in different directions, so the velocities are not the same. 12. The rule of thumb assumes constant velocity. If the car(s) move with constant acceleration,
the velocity would continually be changing. This would mean the distance between the
cars would continually have to change for the rule of thumb to be valid, which could
require a slowing down, which would imply a change in the value of the acceleration. 14. (a) The car is moving to the east and increasing in speed.
(b) The car is moving to the east but slowing in speed.
(c) The car is moving to the east at constant speed.
(d) The car is moving to the west but slowing in speed.
(e) The car is moving to the west and speeding up.
(f) The car is moving to the west at constant speed. (g) The car starts from rest and begins to speed up toward the east.
(h) The car starts from rest and begins to speed up toward the west.
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