Page No 126: - Chapter 9 - Force & Laws Of Motion - Ncert Solutions class 9 - Science
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Question 1:
If action is always equal to the reaction, explain how a horse can pull a cart.
Answer:
A horse pushes the ground in the backward direction. According to Newton’s third law of motion, a reaction force is exerted by the Earth on the horse in the forward direction. As a result, the cart moves forward.
Question 2:
Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.
Answer:
Due to the backward reaction of the water being ejected
When a fireman holds a hose, which is ejecting large amounts of water at a high velocity, then a reaction force is exerted on him by the ejecting water in the backward direction. This is because of Newton’s third law of motion. As a result of the backward force, the stability of the fireman decreases. Hence, it is difficult for him to remain stable while holding the hose.
Question 3:
From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 m s−1. Calculate the initial recoil velocity of the rifle.
Answer:
Mass of the rifle, 
Mass of the bullet, 
Recoil velocity of the rifle = 
Bullet is fired with an initial velocity,
Initially, the rifle is at rest.
Thus, its initial velocity, v = 0
Total initial momentum of the rifle and bullet system 
Total momentum of the rifle and bullet system after firing:
According to the law of conservation of momentum:
Total momentum after the firing = Total momentum before the firing
The negative sign indicates that the rifle recoils backwards with a velocity of 0.4375 m/s.
