How Do You Calculate the Mass of a Cart in a Momentum Problem?

[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!

 

[Doc Al]

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

 

[helen3743]

Sum of external forces = 0! thank you!

 

[DaMastaofFisix]

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

 

[Hootenanny]

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

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

~H