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If action is always equal to the reaction, explain how a horse can pull a cart.
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.
Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.
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.
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.
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.
Two objects of masses 100 g and 200 g are moving along the same line and direction with velocities of 2 m s−1 and 1 m s−1, respectively. They collide and after the collision, the first object moves at a velocity of 1.67 m s−1. Determine the velocity of the second object.
Mass of one of the objects, m1 = 100 g = 0.1 kg
Mass of the other object, m2 = 200 g = 0.2 kg
Velocity of m1 before collision, 
m/s
Velocity of m2 before collision, 
m/s
Velocity of m1 after collision, 
Velocity of m2 after collision = 
According to the law of conservation of momentum:
Total momentum before collision = Total momentum after collision
Hence, the velocity of the second object becomes 1.165 m/s after the collision.
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