What is the mass of the cart?

  • Thread starter helen3743
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  • #1
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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!!
 

Answers and Replies

  • #2
Doc Al
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It's much simpler than you think. Hint: What's conserved when the boy "collides" with the cart?
 
  • #3
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Sum of external forces = 0! thank you!!!
 
  • #4
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
Hootenanny
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Science Advisor
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DaMastaofFisix said:
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.

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

~H
 
Last edited:

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