Solving Momentum Questions: Intital vs Final Momentum

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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?

:)
 
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rkslperez04 said:
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?

:)

Hint: while calculating 'final momentum', the boy and the cart form a system. :smile:
 
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:
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}}.
 
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
 
rkslperez04 said:
150 <~~ wieght of cart

Yup, that's it.
 
To solve this, I first used the units to work out that a= m* a/m, i.e. t=z/λ. This would allow you to determine the time duration within an interval section by section and then add this to the previous ones to obtain the age of the respective layer. However, this would require a constant thickness per year for each interval. However, since this is most likely not the case, my next consideration was that the age must be the integral of a 1/λ(z) function, which I cannot model.
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