A block of mass 25 kg is raised by a 50 kg man in two different ways as shown in Fig. 5.19. What is the action on the floor by the man in the two cases? If the floor yields to a normal force of 700 N, which mode should the man adopt to lift the block without the floor yielding?
Given:
Mass of block, m = 25 Kg
Mass of man, M = 50 Kg
Yielding normal force of floor, Fm = 700 N
Acceleration due to gravity, g = 10 ms-2
In both the cases we will focus on finding the normal force on the floor.
Normal force will change according to the apparent weight of man.
(a) When the block is lifted directly,
The man applies a force in upward direction, which increases his apparent weight as the weight of the block is added to weight of man. This apparent weight is responsible for the normal force on floor.
F = Weight of man + weight of block
F = Mg + mg
F = (50 Kg + 25 Kg) × 10 ms-2
F = 750 N
(b) When the block is lifted with pulley,
The man applies a force in downward direction, which decreases his apparent weight. This apparent weight is responsible for the normal force on floor.
F = Weight of man - weight of block
F = Mg- mg
F = (50 Kg- 25 Kg) × 10 ms-2
F = 250 N
The man should use the pulley to easily lift the block. If block is lifted directly, the force will be greater than the yield force of floor hence the floor can break.