����ý

As long as no external forces are acting on the objects involved, the total momentum stays the same in explosions and collisions. We say that momentum is conserved.

You can use this idea to work out the mass, velocity or momentum of an object in an explosion or collision.

Momentum calculations

Here is a worked example.

Two railway carriages collide and move off together. Carriage A has a mass of 12,000 kg and moves at 5 m/s before the collision. Carriage B has a mass of 8,000 kg and is stationary before the collision. What is the velocity of the two carriages after the collision?

Step one

Work out the total momentum before the event (before the collision).

Momentum of carriage A before = 12,000 × 5 = 60,000 kg m/s

Momentum of carriage B before = 8,000 × 0 = 0 kg m/s

Total momentum before = 60,000 + 0 = 60,000 kg m/s

Step two

Work out the total momentum after the event (after the collision).

Because momentum is conserved, total momentum afterwards = 60,000 kg m/s.

Step three

Work out the total mass after the event (after the collision).

Total mass = mass of carriage A + mass of carriage B = 12,000 + 8,000 = 20,000 kg

Step four

Work out the new velocity.

p = m × v, but we can rearrange this equation so that v = p ÷ m

Velocity (after the collision) = 60,000 ÷ 20,000 = 3 m/s

Watch this illustrated podcast on momentum for a summary of how momentum and motion are related.