A 13.5-lb block rests on the floor. (a) What force does the floor exert on the b

Question

A 13.5-lb block rests on the floor.

(a) What force does the floor exert on the block?

(b) If a rope is tied to the block and run vertically over a pulley, and the other end is attached to a free-hanging 11.0-lb weight, what is the force exerted by the floor on the 13.5-lb block?

(c) If we replace the 11.0-lb weight in part (b) with a 36.0-lb weight, what is the force exerted by the floor on the 13.5-lb block?

Explanation / Answer

(a)

In this case, the block is at rest , that means the net force is zero. The weight of the block is 13.5 lb downward. The floor must therefore exert a force 13.5 lb upward.

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(b)

The block is still at rest. The weight is 13.5 lb downward, the rope pulls with 11 lb upward due to the tension in the rope. The floor must therefore exert a force 13.5 lb - 11 lb = 2.5 lb upward.

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(c)

The weight is 13.5 lb downward, the rope pulls with 36.0 lb upward due to the tension in the rope. Therefore, The block will accelerate upward,as the net force is upward.

In this case, block is not pushing on the ground, and the ground is not pushing back. The ground exerts zero force on the block.

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