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Sorry the title should be :

3000Solved Problems in Physics-Shaum's

9.6:

A camper lets fall a heavy mallet of mass M from the height y upon the top of a tent stake of mass m and drives it into the ground a distance d. Find the resistance of the ground, assuming it to be constant and the stake and mallet stay together on impact.

Solution given:

The speed of the mallet on just striking the stake is v= √(2gy). Momentum is conserved at the instant of collision so that Mv=(M+m)v', where v' is the speed of the stake plus mallet just after impact.

My question is why the momentum is conserved since there is external force acting on the system, that is gravity.

**Impulse and Momentum**3000Solved Problems in Physics-Shaum's

9.6:

A camper lets fall a heavy mallet of mass M from the height y upon the top of a tent stake of mass m and drives it into the ground a distance d. Find the resistance of the ground, assuming it to be constant and the stake and mallet stay together on impact.

Solution given:

The speed of the mallet on just striking the stake is v= √(2gy). Momentum is conserved at the instant of collision so that Mv=(M+m)v', where v' is the speed of the stake plus mallet just after impact.

My question is why the momentum is conserved since there is external force acting on the system, that is gravity.

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