When things start at rest and fly apart, the total momentum stays zero, so the pieces move off with equal and opposite momentum.
What you will learn
- Explain recoil with a total momentum of zero
- Calculate a recoil or explosion velocity
- Handle an object that splits while it is already moving
Zero before, zero after
Before a cannon fires, nothing moves: the total momentum is zero.
Momentum is conserved, so just after firing the total is still zero. The ball goes forwards with some momentum; the cannon must go backwards with the same amount.
0 = m₁v₁ + m₂v₂
so m₁v₁ = m₂v₂ in size, in opposite directions
The heavy cannon recoils slowly; the light ball flies off fast. Their momenta are the same size.
The rest of this lesson and its 9 practice questions are locked.
In this lesson
- Zero before, zero after
- Working it out
- Splitting on the move
- Try it: Your first recoil
9 practice questions, marked as you go; about 20 minutes in all.
In this unit: Momentum
Momentum as mass times velocity, its conservation in collisions and explosions, force as the rate of change of momentum, safety features and kinetic energy in collisions (Higher tier).