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# Why does an ordinary rifle recoil (kick backward) when

ISBN: 9781938168000 42

## Solution for problem 18CQ Chapter 4

College Physics | 1st Edition

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Problem 18CQ

Why does an ordinary rifle recoil (kick backward) when fired? The barrel of a recoilless rifle is open at both ends. Describe how Newton’s third law applies when one is fired. Can you safely stand close behind one when it is fired?

Step-by-Step Solution:

Solution 18CQ

This question can be explained on the basis of Newton’s third law of motion.

Step 1 of 3</p>

When a bullet is fired from a rifle, the rifle recoils. The firing of the bullet is an action. According to Newton’s third law of motion, every action is accompanied by an equal and opposite reaction. The bullet is much lighter than the gun. So, its acceleration is much larger than the latter. The latter being much heavier, experiences a smaller acceleration. So, there is a slight displacement of the same backward.

Step 2 of 3</p>

This step can also be explained with momentum conservation principle.

Since there is no external force acting on the system initially, the linear momentum is zero.

Consider  be the mass of the rifle and  is its recoil speed.

Consider  be the mass of the bullet and  is its velocity.

Therefore, initial momentum

Since, the initial momentum is zero

Since , hence in magnitude the recoil speed . This explains why the rifle recoils with a lower velocity than the outgoing velocity of the bullet.

Step 3 of 3

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