Momentum

What the syllabus demands

• —Define momentum as mass × velocity and calculate it
• —Define impulse as force × time and relate it to the change in momentum
• —Apply the principle of conservation of momentum to collisions in one line
• —Explain safety features (crumple zones, airbags) using impulse

Definitions that earn marks

Clear definitions to practise — check your course mark scheme

Momentum

The product of mass and velocity: p = mv. A vector quantity, measured in kg m/s.

Impulse

The product of force and the time for which it acts: impulse = Ft = change in momentum (Δp).

Principle of conservation of momentum

When two or more bodies interact, the total momentum before the interaction equals the total momentum after, provided no external forces act.

The equations

More equations to practise: the IGCSE formula sheet.

Where the marks die

Common mistakes to check

1. 01

   Ignoring direction. Momentum is a vector: choose a positive direction, and give anything moving the other way a negative velocity before you add.

2. 02

   Forgetting that objects moving together after a collision share one velocity — the combined mass (m₁ + m₂) multiplies the single final velocity.

3. 03

   Explaining airbags with 'they absorb the impact'. The physics: they increase the time over which the momentum changes, so for the same change in momentum the force is smaller (F = Δp ÷ t).

4. 04

   Quoting momentum in newtons. The unit is kg m/s (or the equivalent N s).

One worked example, done properly