# Homework Help: Momentum Question

1. Dec 28, 2007

### niki4d

1. The problem statement, all variables and given/known data
A 82kg girl is coasting along the floor on a large 2.0kg skateboard. If she is moving 1.8 m/s when a 46kg boy jumps on to the same skateboard, what is the speed of the skateboard immediately after the boy jumps on it?

2. Relevant equations

Sum of Initial Momentum = Sum of Final Momentum

3. The attempt at a solution

This question looked pretty easy.. but Im sure Im doing something wrong... here's what I did so far:
(84kg)(1.8m/s) + (130kg)(0m/s) = (84kg + 130kg)Vf

2. Dec 28, 2007

### niki4d

Sorry.. forgot the last part..
I got vf=0.7065m/s

Im not sure if this is right or not.. it SEEMS like an okay speed.. but I havn't done this type of question before.. so just want to be sure. Thanks!

3. Dec 28, 2007

### Gib Z

You should only be looking at the initial and final momentums of the skateboard (and what ever is on it).

So it starts off with a total mass of 84 kg, going at 1.8 m/s. At the end, It has the mass of a 46 kg boy, so the total mass is 130 kg, and v_1 m/s is what we are trying to find. Equating both sides and making v_1 the subject;

$$84 kg \cdot 1.8 ms^{-1} = 130 kg \cdot v_1 ms^{-1}$$

$$v_1 = \frac{84 \cdot 1.8}{130}$$

4. Dec 29, 2007

### niki4d

Hmm.. that makes sense.. but the answer to that would be 1.163m/s. The answer is 0.97m/s. I don't know how to get to that..

Last edited: Dec 29, 2007
5. Dec 29, 2007

### Gib Z

Well, That is somewhat odd...I think it's an error in the textbook, but it has been a while since I've studied Physics so I recommend you wait for someone who has more knowledge in this area to confirm.

6. Dec 29, 2007

### mjsd

unless there are something else in the problem we have not addressed, otherwise I believe Gib Z's correct.

7. Dec 31, 2007

### niki4d

Yeah.. more than likely it is.. thanks for the help!