# Impulse needed to stop

1. Apr 4, 2009

### phys99

1. The problem statement, all variables and given/known data
What is the impulse needed to stop a 10kg bowling ball moving at 5m/s?

2. Relevant equations
Impulse = -initial momentum and
momentum = mass * v = 50

3. The attempt at a solution

2. Apr 4, 2009

### Dr.D

Did you try putting some units on that?

3. Apr 5, 2009

### phys99

ummm... what kind of units?? because so far, i thought that unit of impulse is Kg.m/s?! am i wrong?

4. Apr 5, 2009

### Dr.D

That will do, as will N-s, but what you put down above for your answer showed no units at all. Is that perhaps why your answer was rejected?

5. Apr 5, 2009

### phys99

No, I dont have to answer units at all... but are you saying that the unit should be N*s at the end?? my ans was in units kg.m/s

6. Apr 5, 2009

### Dr.D

If you work through it, you will discover that N-s are exactly the same thing as kg-m/s.

I suspect that the problem lies in the fact that the impulsive force applied to stop the ball is aided by the friction force that is maintianing the rolling constraint since both of the act against the forward motion. The friction force is hard to evaluate since it is a constraint, rather than a known value.