# Car velocity question

Im studying on my own and don't have the benefit of input from an instructor, so any help with these few questions would be very appreciated.

1. If a car traveling at a constant velocity has a net force of 0, why can it be said that work is being done on the car when Work equals force multiplied by distance?

2. A golf ball is thrown at and bounces off a very massive bowling ball that is initially at rest. Which ball, after impact has greater momentum, and which has greater kinetic energy?

On question two, I think the answer is the bowling ball has greater momentum, since the golf ball changed velocity double what it would have relative to the bowling ball if it had not bounced, and since momentum is conserved, twice the momentum would be imparted into the bowling ball in order for net momentum = 0. And since KE=.5mv^2, I would think the golf ball would have more kinetic energy, having a much higher velocity. Am I correct here, or am I missing something. Thanks.

## Answers and Replies

Staff Emeritus
Gold Member
Originally posted by Cpt. Bob
1. If a car traveling at a constant velocity has a net force of 0, why can it be said that work is being done on the car when Work equals force multiplied by distance?

In the real world, there is air resistance to the motion of a car. That is a force against the motion, and so an additional force has to be supplied by the engine to make up for it. Seperately, the two forces do work on the car. The work done by the air is negative, and the work done by the engine is positive. For zero net force, the work done by each force has precisely the same magnitude, so they add up to zero.

2. A golf ball is thrown at and bounces off a very massive bowling ball that is initially at rest. Which ball, after impact has greater momentum, and which has greater kinetic energy?

On question two, I think the answer is the bowling ball has greater momentum, since the golf ball changed velocity double what it would have relative to the bowling ball if it had not bounced, and since momentum is conserved, twice the momentum would be imparted into the bowling ball in order for net momentum = 0. And since KE=.5mv^2, I would think the golf ball would have more kinetic energy, having a much higher velocity. Am I correct here, or am I missing something. Thanks.

I would do this explicitly.

Let:
m=mass of golf ball
M= mass of bowling ball
vi=initial velocity of golf ball
vf=final velocity of golf ball
V=final velocity of bowling ball

Conservation of Momentum:
mvi=mvf+MV

If the collision is elastic, then we also have...

Conservation of Kinetic Energy:
(1/2)mvi2=(1/2)mvf2+(1/2)MV2

You have two equations and two unknowns (vf and V).