a) A 15 lb. block rests on the floor. What force does the floor exert on tbe blo

Question

a) A 15 lb. block rests on the floor. What force does the floor exert
on tbe block?
In this case the normal force (Nf) is equal to the weight (W) of the block.
I will assume that the 15 lb is the weight of the block.
Nf = W = 15 lb
b) If a rope is tied to the block and runs vertically over a pully,
and the other end is attached to a free hanging 10 lb weight, what is
the force of the floor on the 15 lb block?

c) If we replace the 10lb weight with a 20lb weight, - now what is the
force of the floor on the block?

Explanation / Answer

a) When the block is at rest on the floor , weight of the block is the normal reaction force . Hence Nf = 15 lb b) In the second case 10 lb + N = 15 lb Hence N = 15-10 = 5 lb c) In the third case 20 lb+ N = 15 lb Hence N = 15-20 = -5 lb It indicates that normal force acts in the down ward direction opposite to the applied force.

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