According to Newton’s third law, forces must come in equal but opposite pairs.

By extension, impulses and changes in momentum must also occur in equal but opposite pairs.

This means that total momentum of a system must be conserved in the absence of a net external force.

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Principle of conservation of momentum is the reason behind recoils. During the firing of the bullet, we have the rifle and the bullet pushing each other. As far as the bullet-rifle system is concerned, this is a pair of internal forces. Without any external force acting on the bullet-rifle system, its total momentum remain unchanged. Since we started with the rifle and bullet both at rest, the total momentum must remain at zero.

The bullet, though small in mass, does carry a large forward momentum thanks to its speed. This forward momentum must be matched by a backward momentum of the same magnitude.

If you are sitting in a sailing boat, and there is not the slightest wind, can you get the boat moving by blowing air (with your breath) at the sail? Watch the video below to check your prediction.

Surprisingly, the outcome depends on whether the collision is elastic or not (besides the shape of the sail). Read xmdemo.wordpress.com/028 for the explanation.