# Physics momentum/ speed question. Need help

1. Oct 2, 2013

### astru025

Physics momentum/ speed question. Need help!!!

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

A woman with a mass of 40 kg runs at a speed of 9 m/s and jumps onto a giant 30 kg skateboard initially at rest. What is the combined speed of the woman and the skateboard?

2. Relevant equations
Momentum= mass x velocity. I can use this to find the momentum of the woman before she jumps onto the skateboard. But what do I do after this??

3. The attempt at a solution
Momentum of woman= 40 kg x 9m/s. 360 kg * m/s. what I do from here?! Any help would be nice :)

2. Oct 2, 2013

### Staff: Mentor

Do you know the combined mass after they join?

Is there some law involving momentum?

3. Oct 3, 2013

### astru025

The combined mass after they join would be 40 kg and 30 kg =70 kg correct?
There is the law of conservation of momentum...,

4. Oct 3, 2013

### henryc09

Momentum is conserved, i.e. for any process, the total momentum before = the total momentum afterwards. Basically the woman and skateboard can be considered as one object moving with speed v after she has jumped on, whose total momentum must be the same as the momentum before (of the two objects separately).

5. Oct 3, 2013

### astru025

The woman's momentum before she jumps on the skateboard is, 360. The total momentum of the skateboard is 0. The skateboard weight plus the woman's weight equals 70 kg. total momentum before equals total momentum after so how do I find total momentum before?.

6. Oct 4, 2013

### Staff: Mentor

Before:
total momentum
= momentum of woman + momentum of board
= 40 * 9 + 30 * 0

After:
total momentum
= momentum of woman + momentum of board
=

7. Oct 4, 2013

### astru025

Total momentum= 360
Momentum= mass x velocity
360=70 kg x Velocity
Divide each side by 70 and the velocity equals 5.1 thanks for the help!!