# Impulse Momentum Theorem

1. Oct 21, 2006

### shaka23h

Hi, I'm kinda lost on these 2 problems.

A space probe is traveling in outer space with a momentum that has a magnitude of 7.15 x 107 kg·m/s. A retrorocket is fired to slow down the probe. It applies a force to the probe that has a magnitude of 1.81 x 106 N and a direction opposite to the probe's motion. It fires for a period of 9.56 s. Determine the momentum of the probe after the retrorocket ceases to fire.

On this first question I have no idea if I'm going to have to find the mass and than use the impulse momentum theorem? If so how would I go about finding the mass for it?

A student (m = 65 kg) falls freely from rest and strikes the ground. During the collision with the ground, he comes to rest in a time of 0.017 s. The average force exerted on him by the ground is +15000 N. From what height did the student fall? Assume that the only force acting on him during the collision is that due to the ground.

On this problem I think I'm suppose to apply the constant acceleartion equation to it? But I have no idea how to find the t in this problem. I don't think the 0.017s is the T that I'm suppose use?

Any help would be greatly appreciate it

2. Oct 21, 2006

### Hootenanny

Staff Emeritus
Do you really need to know that mass? What is the impulse equal to?

Think about conservation of energy. What must the velocity be just before the impact? What's the kinetic energy at this point?

3. Oct 21, 2006

### shaka23h

Thanks to your help I figured out what i was doing wrong on my first problem. I didn't take into consideration that the opposite direction accounts for a negative value. I know that Impulse = Change in momentum which is also equal to net force x change in time. After finding the impulse I added the initial momentum value to it and found my final momentum. :) thanks a lot.

I am still having trouble on number 2 because I don't know how to apply the conservation of energy principle to this problem because thi swas in the impulse-momentum chapter. I could not find a equation in this chapter that would allow me to find the distance of anything. Maybe I'm just stupid or just thinking it wrong. I'm thinking this problem as a free fall problem yet when I looked at that equation it didn't seem very logical either. :(

Last edited: Oct 21, 2006
4. Oct 22, 2006

### Hootenanny

Staff Emeritus
Sounds good to me. There's no need for the Mr. Advisor (I'm not a teacher ), Hoot will do. And welcome to the Forums.
Okay, firstly you can use impulse/momentum to find the velocity of the student just before the collision, yes? Now, with this information you can find the kinetic energy of the student just before the collision. As we are ignoring drag what can you say about the (gravitational) potential energy of the student before he/she fell?