Forces and Motion

Momentum - Explosion - Calculations.

The force of an explosion makes objects
move away from each other very quickly.
This is an example of Newton's Third Law of motion.

The 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 must also still be 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).

For example, 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?

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.

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