# Momentum/Impulse force vectors

1. Jan 16, 2009

### iluvphys

Hi friends,
I am preparing for my exam and i am stuck with this problem, please help me out.
It wont take too long, I hope.

1. The problem statement, all variables and given/known data
Two bodies A and B have masses mA and mB respectively and collide completely inelastic.
Given that the mass of body A is 1200 kg and body B is 800 kg determine the avergage force vectors acting on each body during the collision. The collision itself lasts for 0.2 s.

2. Relevant equations
Velocity of body A: (5i + 3j) m/s
Velocity of body B: (-i + 4j) m/s

Common velocity after collision I found to be: (2.6i + 3.4j) m/s (this eq was derived in a previous question with mA = 3/2 mB)

3. The attempt at a solution
Well, I have tried using the momentum impulse theorem only to find out that the total momentum was conserved, which is good indeed. However I dont know how to find the answer I have kind of like used up my knowledge on impulse.
The answer is (-14400i + 2400j) N.
I have no clue how this answer was derived.

2. Jan 16, 2009

### Staff: Mentor

To find the force on a given body, use the impulse-momentum theorem:

$$\vec{F}\Delta t = \Delta (m\vec{v})$$

What's the change in momentum of body A?

3. Jan 17, 2009

### iluvphys

I have tried this equation but I just seem to get -4000i+16000j. But this seems to be wrong.
Any further advice? I mean I substituted the final momentum from the initial momentum of A and divided by time, which however is initial momentum of B.

4. Jan 17, 2009

### Staff: Mentor

Do it step by step:
(1) Find the change in velocity of A. (Final velocity minus initial.)
(2) Then find the change in momentum.
(3) Then the force.

5. Jan 19, 2009

### iluvphys

Hi Doc Al,
I am really sorry for the late response but I was so busy studying that i havent been able to check the post.
THank you very much for your help, I finally got it (actually right now). I followed your steps and I am really thankful for your help.

6. Jan 19, 2009

### Staff: Mentor

Excellent. (And you are very welcome.)