Question

Give reason and give the law related to these statements-

(a) It is easier to push an empty box than to push the box full of books.(b) It is difficult for a fireman to hold a hose which ejects large amount of water with highvelocity.

Answer 1

(a) The second law of motion is applied here, which states that the net force on an object is equal to the rate of change (that is, the derivative) of its linear momentum p
F=dp/dt=d(mv)/dt=mdv/dt
Or ​F=ma
or a=F/m
The acceleration depends inversely upon the object's mass. Since the box full of books has more mass than an empty box, it will have less acceleration. Thus more force is required to push the heavier box and it is easier to push an empty box.
(b) ​Newton's third law is applied here, which states that to every action there is an equal and opposite reaction.
A large amount of water ejecting from the nozzle of the hose is at very high velocity with a very large force.This large force is due to large momentum developed.
The fireman holding that hose experiences equal force but in opposite direction as reaction. This large force as reaction pushes the fireman. Thus it is difficult for him to hold the hose.