# What is the mass of the cart?

1. May 4, 2006

### helen3743

Thank you--

Problem:
A 50 kg boy runs at a speed of 10 m/s and jumps onto a cart. The cart is initially at rest. If the speed of the cart with the boy on it is 2.50 m/s, what is the mass of the cart?

I don't know what to do. This probelem seems very simple but I don't know where to start.

These I know are the knowns:
vo(cart) = 0 m/s
vf(cart) = 2.50 m/s
vo(boy) = 10 m/s
vf(boy) = 2.5 m/s

So I know velocities and I know the mass of the boy.
Should I start off by finding the acceleration from F=ma, since I know a mass? But then I don't know how to find F. I know Fn = mg in the y component, but I don't know what I would use Fn next for. Am I supposed to use kinematics equations? But I don't know a, x, or time. Direction is needed, thanks!!

2. May 4, 2006

### Staff: Mentor

It's much simpler than you think. Hint: What's conserved when the boy "collides" with the cart?

3. May 4, 2006

### helen3743

Sum of external forces = 0! thank you!!!

4. May 5, 2006

### DaMastaofFisix

SO yeah, the net force is zero. This clue leads us to what principle pf physics, ya know, with its own set of silly equations and all.

5. May 5, 2006

### Hootenanny

Staff Emeritus
Care to expand / explain your comment DaMasta? Edit. perhaps this is best discussed in the pf lounge actually.

~H

Last edited: May 5, 2006