# Homework Help: Change in momentum im losttt

1. Aug 20, 2008

### sms14

1. The problem statement, all variables and given/known data

one of my hw problems

you are sitting in a railcart which is moving forward. In your hand is a ball...you throw the ball against the wall of the railcart, and it bounces off of the wall and you catch it...the ball has mass m and was thrown with velocity v...which of the following shows the change in momentum of the rail cart

a. 0 b. mv c. 2mv d. none of above

2. Relevant equations

mv + mv = mv + mv

3. The attempt at a solution

i was thinking the answer is b because if the ball bounces off, it had to have affected the rail cart with a value of mv ???

2. Aug 20, 2008

### LowlyPion

Was there any change in momentum when you threw the ball? What about when you catch the ball? You are part of the system aren't you?

3. Aug 20, 2008

### sms14

well, i assume i'm part of the system...and i guess the final momentum of the ball is 0 since i caught it??..i just am having trouble understanding the problem and the logistics of it

4. Aug 20, 2008

### sms14

actually...now that i think about it..i think the answer is 0 because if the person and ball are located within the railcart, then ultimately...if the ball was thrown and returns at rest...there was no momentum change so the change in momentum of the railcart must be zero.

5. Aug 20, 2008

### sms14

can anyone verify if im thinking along the right track???

6. Aug 20, 2008

### tiny-tim

10/10

7. Aug 20, 2008

### LowlyPion

Good job. You didn't get derailed. You are on the right track now.