# Momentum

AQA A - Notes on momentum

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• Created by: Amy
• Created on: 03-01-13 12:53

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PHY4A
Momentum
If a body of mass m has a velocity v, then its momentum is
=mv
The units of momentum are kgms¯¹ or Newton Seconds (Ns)
Momentum is a vector quantity so it has a direction.
Law of conservation of momentum
Momentum is conserved in all collisions, explosions and interactions. There are no exceptions.
When two objects collide, the changes in their momenta will be equal in size but opposite in
direction. The momentum gained by one body equals the momentum lost by the other.
Tackling problems:
1. Choose which direction is positive
2. Draw before and after sketches of the objects
3. Calculate every momentum you can
4. Apply the law of conservation of momentum for collisions involving 2 bodies.
Recoil and explosions
Guns and cannons recoil when fired because of the law of conservation of momentum. The positive
momentum gained by the bullet or cannon ball is equal to the negative recoil momentum of the gun
or cannon, and so the total momentum before and after the explosion is zero.

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PHY4A
Elastic and inelastic collisions
Kinetic energy is conserved in elastic collisions.
Kinetic energy is not conserved in inelastic collisions.
Momentum is conserved in all collisions
Total energy is conserved in all collisions
In inelastic collisions some kinetic energy is converted to other forms of energy ( mainly heat). Large
scale collisions are inelastic: collisions between hard steel spheres are nearly elastic: some particle