Chapter 3
Exercises
8. Without this slack, a locomotive might
simply set still and spin its wheels as it tries to start moving the
entire train all at once. The loose coupling enables the engine to
bring one car at a time into motion. In addition the momentum of the
moving cars helps get the rest of the train moving.
9. The person could through an article of
clothing in the opposite direction they wish to travel, or the could
simply blow air.
15. First let's label the astronauts A, B,
and C, from left to right. A pushes B towards C. Since they are in
outer space both A and B move, in opposite directions and opposite
velocities. As B strikes C the both begin to move away from A, with
half of C's original velocity. B then pushes C towards A, increases
B's velocity while changing the direction of C. C's new velocity is
now in the direction of A but is traveling half as fast. Each
astronaut only gets to push C once.
17. When a rifle with a long barrel is
fired, more work is done as the bullet is pushed through the longer
distance. A greater KE is the result of the greater work, so of
course, the bullet emerges with a greater velocity.
18. The KE of the ball relative to the
passengers on the plane does not depend on the speed of the plane.
The KE of the ball relative to observers on the ground below,
however, is a different matter. Energy, KE or PE is relative to our
frame of reference.
20. KE is maximum at the bottom of the arc,
PE is maximum at its upper most points. When the pendulum swings by
the point that marks half its maximum height it will have half its
maximum KE, and half its maximum PE.
23. Yes. In order to recharge the battery,
more gas must be used.
26. 1000 N. (Force x dist.) in = (Force x
dist.) out. So (100 N x 10 cm) = ( ? x 1 cm).
28. KE before = 1/2 mv2 and KE after =
1/2 (2m)(v/2)2 = 1/4 mv2 . So the cars
have only half the KE possessed by the single car. Most of the "lost"
energy goes into heat.
29. No to all.
30. By altering their decelerations it is possible to use a smaller force to stop the truck than a skateboard. Slow the truck down very gradually, over many miles, may require very little force while bringing a skateboard to a sudden stop could require a greater force.
Problems
1. 4 km/h
4. At 25% efficiency, we'll only get 10 MJ (megajoules), 1/4 of the 40 MJ available.
work = force x dist. or work = 1000N x dist.
= 10MJ ==> dist = 10 km.
5. From Ft = Æmv we get F =
(Æmv)/t. F = 2000N.
8. 1 kilowatt-hour = 1000 joules/second for one hour. There are 3600 seconds in one hour, sot the total number of joules is 1000 J/s x 3600s = 3600000J = 3.6 MJ.