Ask a friend to drive a small nail into a piece of wood placed on top of a pile of books on your head. Why doesn't this hurt you?

Because the nail has a sharp point and thus, exert less pressure on wood

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The force is exerted on the wood, thus it has no effect on out head

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The relatively large mass of the books and block on your head resists being moved

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Action-reaction forces cancel out each other

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Solution

The correct option is D The relatively large mass of the books and block on your head resists being moved
The hammer delivers a big force for a very short time. That force on a very small nail may change its velocity quickly (big acceleration with big force because of small mass.)
However the big force does not last long enough to change the speed of the pile of books much (big force, big mass, small acceleration.) Therefore the pile of books does not transmit much force on to your heard because they have large inertia .

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