# Homework Help: Momentum question

1. Oct 26, 2006

### rkslperez04

I need a nudge in the right direction.

A 50-kg boy runs at a speed of 10.0 m/s and jumps onto a cart originally at rest. The cart, with the boy on it, then takes off in them same direction in which the boy was running. If the cart with the boy has a veolocity of 2.50 m/s, what is the mass of the cart?

Would this me a Final Momentum = Intital Momentum problem?
How do I go about setting this up?

:)

2. Oct 26, 2006

Hint: while calculating 'final momentum', the boy and the cart form a system.

3. Oct 26, 2006

### rkslperez04

ok... this is the formula I have

m1v1i + m2v2i = (m1 + m2)vf -----

I take it they are asking for the m2 and m2v2i willl cancel out being v2i = rest which is 0.

So do I pull everything to oneside and solve for m2

m2 = (m1 + m2)vf - m1v1i / v2i

I think I am doing something wrong

Last edited: Oct 26, 2006
4. Oct 26, 2006

You set the equation up correct:

m1v1i + m2v2i = (m1 + m2)vf

The indices 1 are referring to the boy, and 2 to the wagon. As you said v2i equals zero, so you only have to solve for m2. So, $$m_{2} = \frac{m_{1}v_{1i}-m_{1}v_{f}}{v_{f}}$$.

5. Oct 26, 2006

### rkslperez04

ok.. I think i got it...Its soo funny.. I would never think of this during the winter... usually you just sled and go.. not caculate and go..LOL

I guess in a sled race.. this is all very important

(50)(10) - (50)(2.5) / 2.5 =

500 - 125 / 2.5 =

150 <~~ wieght of cart

6. Oct 26, 2006