Momentum · Grade 9
Momentum is “mass in motion”: the product of mass and velocity. In any collision or explosion where no outside forces act, the total momentum stays the same. This lets us predict what happens when trains couple, snooker balls collide or a rifle recoils.
Key facts
- Momentum p = mv (kg m/s). It is a vector, so direction matters.
- Conservation: total momentum before = total momentum after (if no external forces act).
- Explosions and recoil: total momentum is zero before, so the pieces move off with equal and opposite momenta.
- Take one direction as positive; velocities the other way are negative.
- Elastic collisions conserve kinetic energy too; inelastic collisions lose some kinetic energy (momentum is still conserved).
- Kinetic energy and momentum are linked: KE = (p²)/(2m).