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# Conservation of Momentum

Posted on 2006-04-13
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A wagon with two boxes of gold having a total mass of 300kg is cut loose from horses when it is at rest by a robber. The wagon is at rest 50m up a 6.0 degree slope. The outlaw plans to have the wagon roll down the slope and across a level plane and fall into a canyon where his amigos are waiting. But in a tree 40m from the canyon edge two heros wait. One has mass 75.0kg the other has mass 60.0kg. They drop vertically into the wagon as it passes beneath them.

*
*      *        50m                                                   ****
h  *            *                                                              | |  <---tree
*                   *                                                       | |
*           6 degrees *                                                | |
********************************************************************
| ------------- 40m -----------------------|

Hope that helps you understand.

a) if they require five seconds to grab the gold and jump out will they make it before the wagon goes over.?

h = 50 sin 6 = 5.23m
v = sqrt(2gh)
v = 10.1m/s

m_c * v = m_cf * v_f

v_f = m_c * v/ m_cf

v_f = 6.97m/s

6.97m/s * 5s = 34.8m, so it looks like they can make it

b) when they drop into the wagon is the kinetic energy of the system (wagon plus heros) conserved?

m_i * v_i^2 = m_f * v_f^2

they are not equal so no.

c) if kinetic energy is not conserved does it increase, decrease and by how much?

The change in kinetic energy is a decrease of 4800 J i get closer to 10000 J so I am doing something wrong.

Brian
0
Question by:BrianGEFF719

LVL 84

Expert Comment

Did you forget to multiply mv^2 by 1/2?
0

LVL 19

Author Comment

>>Did you forget to multiply mv^2 by 1/2?

Does that matter if we are comparing two kinetic energies

K1 = K2
1/2m_1v_1^2 = 1/2m_2v_2^2

I just canceled the 1/2 from both sides.
brian
0

LVL 84

Accepted Solution

canceling the 1/2 does not affect the result of =
but it can affect the magnitude of the result of -
0

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