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?
750 N and 250 N in the respective cases; Method (b)
Mass of the block, m = 25 kg
Mass of the man, M = 50 kg
Acceleration due to gravity, g=10m/s2
Force applied on the block, F=25×10=250 N
Weight of the man, W=50×10=500 N
Case (a): When the man lifts the block directly
In this case, the man applies a force in the upward direction. This increases his apparent weight.
∴ Action on the floor by the man = 250 + 500 = 750 N
Case (b): When the man lifts the block using a pulley
In this case, the man applies a force in the downward direction. This decreases his apparent weight.
∴ Action on the floor by the man = 500 - 250 = 250 N
If the floor can yield to a normal force of 700 N, then the man should adopt the second method to easily lift the block by applying lesser force.