Question

A 13.0-lb block rests on the floor.

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

magnitude	lb
direction	---Select--- upward downward The floor does not exert force 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 10.5-lb weight, what is the force exerted by the floor on the 13.0-lb block?

magnitude	lb
direction	---Select--- upward downward The floor does not exert force on the block.

(c) If we replace the 10.5-lb weight in part (b) with a 31.5-lb weight, what is the force exerted by the floor on the 13.0-lb block?

magnitude	lb
direction	---Select--- upward downward The floor does not exert force on the block.

Answer 1

(a) The floor exerts a reaction force equal to the force exerted by the weight of the block on the floor.

This equals - F = m*g

where,

• F : Force exerted
• m : mass of the block = 13 lb = 5.897 kg
• g : acceleration due to gravity = 9.8 m/s²

F = 5.897 * 9.8 N = 57.79 N

The floor will exert a reaction force of 57.79 N opposite in direction to the force exerted by the block on the floor.

(b) Let the mass of the second floor be (m')

Given, m' = 10.5 lb = 4.763 kg

The net force exerted by the net weight of the first block acting on the floor is given by -

Net force (F_net) = (m*g) - ((m')*g)

F_net = (5.897*9.8) - (4.763*9.8)

F_net = 11.11 N

So, the floor will exert a reaction force of 11.11 N on the block in the upward direction.

(c) The weight of the new block (31.5 lb) is much higher than that of the first block with weight 13.0 lb.

This will result in the upward movement of the first block (13.0 lb). Hene, the first block (13.0 lb) will be exerting no force on the floor as a result of which, the floor will also not exert any reaction force on the 13.0 lb block.

Or, in simple words, we can say that the force exerted by the floor is equal to zero.