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conservation Energy and momentum?

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A boxcar at a rail yard is set into motion at the top of a hump. The car rolls down quietly and without friction onto a straight, level track where it couples with a flatcar of smaller mass, originally at rest, so that the two cars then roll together without friction. Consider the two cars as a system from the moment of release of the boxcar until both are rolling together.

(a) Is mechanical energy of the system conserved?

(b) Is momentum of this system conserved?

Next, consider only the process of the boxcar gaining speed as it rolls down the hump. Consider the boxcar and the Earth as a system.

(c) Is mechanical energy of this system conserved

(d) Is momentum of this system conserved?

Finally, consider the two cars as a system as the boxcar is slowing down in the coupling process.

(e) Is mechanical energy of this system conserved?

(f) Is momentum of this system conserved?

asked Oct 28, 2013 in Intro College Physics by jakedf13 (170 points)

1 Answer

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(a) the two cars had an inelastic collision (stick together), so the total mechanical energy before and after the collision doesn't conserve.

(b) momentum is always conserved in a collision as long as there is no external forces applied on the system.

(c) without frictional forces and external forces, the total mechanical energy is conserved.

(d) also conserved since the earth is also part of the system so no external forces. The earth will move towards the car with a small velocity in the opposite direction.

(e) there is an external force of friction applied on both cars so the total ME is not conserved for the two cars.

(f) there is an external force of friction so momentum is not conserved.
answered Oct 28, 2013 by PhysicsDude (21,750 points)
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