Question

Question 5.32 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?

Chapter Laws Of Motion Page 112

Answer 1

Given,
mass of block (m ) = 25 Kg
mass of man ( M) = 50 Kg

Force required to lift the block (F) = weight of block = mg = 25 × 10 = 250 N

also weight of man = Mg
= 50 × 10 = 500 N

Case 1.
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see figure (a) , if block raised by man as shown in figure (a) then, force is applied by man in upward direction by which weight of mass increases .
hence, Normal reaction on the floor by man = Force applied by man + weight of man
= F + weight of man
= 250 N + 500 N = 750 N

case 2
__________________
if block raised in such as shown in figure (b). then, force is applied by man in downward direction . hence, weight of man decreases in this situation .
Normal reaction applied on the floor applied by man = weight of man - force required to lift the block
= 500 - 250 = 250 N

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if the floor yield a normal reaction force of 700 N , it means action on the floor exceed 700 N in case (a) and less then 700 N in case (b) .
hence, 2nd case has adopted to lift the block without the floor yielding .

_________________________________________________

Answer 2

Answer:

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 =  

Force applied on the block, F = 25 x 10 = 250 N

Weight of the man, W = 50 x 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.

Explanation: