What is momentum?
- Any object that has mass and is moving has momentum
- Momentum measures the quantity of motion that an object has
- Momentum is a vector quantity - so it has a magnitude and direction
- The direction of the momentum of a particle is the same as the direction of motion of the particle
- The momentum is negative if the velocity is negative
- If an object is falling vertically then momentum will be downwards
What happens when two objects are in contact?
- If two objects are in contact with each other then by Newton’s Third Law there will be equal and opposite reaction forces
- One object gains momentum from the impact and the other loses momentum
- The change in momentum will be equal for each particle provided there are no external forces
- Remember momentum is a vector and can be negative
- For example, consider the scenario where a tennis ball is struck by a racket, suppose the momentum of the racket and the ball before the impact is 5 kg m s-1 and -3 kg m s-1 and the momentum of the racket afterwards is -2 kg m s-1
- This means the racket has a loss of 7 kg m s-1 in momentum
- Therefore the ball has a gain of 7 kg m s-1 in momentum
- Therefore the momentum of ball after the impact is 4 kg m s-1