(A)
(B)
(C)
(D)
(E)
Answers
1 . A —The area (integral) of the force-time graph equals the impulse, and the impulse equals the
spheres’ change in momentum:
Note that the impulse is directed to the left on the 10 kg sphere, which is the opposite direction of its
velocity. This makes the impulse negative:2 . E —This a perfectly inelastic collision; therefore, kinetic energy is not conserved. Thus, we can
ignore answer choices that contain kinetic energy. Momentum is only conserved in the horizontal
direction because the ground does not allow the cat-skateboard system to move vertically. Using
conservation of horizontal momentum, we can see that the skateboard speed must decrease because
the cat adds mass to the system:
Note: The equal and opposite forces between the cat and the skateboard do not cancel! The force
from the cat on the skateboard slows the skateboard down, while the force from the skateboard on the
cat speeds the cat up. Both end up with the same final velocity and move off together.