a 1.2 x 10^4 kg railroad car is coasting along a level, frictionless track at a constant speed of 25m/s, when a 3000 kg load is dropped vertically onto the car from above. What will its new speed be assuming the load stays on the car??

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Explanation:

Because the track is level and frictionless, the net force on this car-load system will be zero in the horizontal direction. As a result, (by Newton's Second Law of mechanics,) the total momentum of this system in the horizontal direction will stay the same.

Momentum of the car-load system in the horizontal direction, before contact:

Combine the two parts to obtain: .

Because the load stays on the car, the car and the load should have the same horizontal velocity after contact. Let denote that velocity. Momentum of the system after contact:

Combine to obtain:

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Because the total momentum of the system will stay the same:

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Solve for to obtain:

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In other words, the new velocity of the system would be .

there are no contact forces mentioned on the list of choices that you've provided.

the answer is c. the lower legs are levers, and the knees are fulcrums. the ankles hold the loads.