Forces and Motion

What is the Momentum of an Explosion?

The forces involved in an explosion make objects
move away from each other very quickly.
This is an example of Newton's third law of motion.

The law of conservation of momentum tells us that the
total momentum is the same, before and after the explosion.

Before an explosion the objects are not moving
and so the total momentum is zero.
After the explosion the total momentum is also still zero.
The momentum of objects moving in one direction
must be exactly balanced by the
momentum of objects moving in the opposite direction.

An example of an explosion is a gun firing a bullet.
The same force that pushes the bullet forwards
and gives it a fast velocity
also pushes the gun backwards (called recoil velocity).

Q1. A gun has a mass of 2·5 kg and a recoil velocity of 8 m/s.
What is the velocity of the bullet if it has a mass of 0·02 kg?

A1. The momentum of the gun going backwards
after the explosion, must be the
same as the momentum of the bullet going forwards.

momentum = mass x velocity

For the gun
momentum = 2·5 x 8
                           = 20·0 kgm/s.

For the bullet
20·0 = 0·02 x v 
         v = 20·0 ÷ 0·02
          = 1000 m/s.

The velocity of the bullet = 1000 m/s.

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