Impulse & Momentum

This lesson covers impulse and momentum at A-Level. Momentum is a fundamental concept in mechanics that is especially useful for analysing collisions and impacts. The principle of conservation of momentum is one of the most important laws in physics.

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Momentum

The momentum of an object is the product of its mass and velocity:

$p = mv$p=mv

Property	Detail
Unit	kg m/s (or Ns)
Type	Vector (has direction)
Direction	Same as the velocity

Exam Tip: Momentum is a vector quantity. Always define a positive direction and use signs consistently. A common error is to forget that velocity (and hence momentum) can be negative.

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Impulse

Impulse is the change in momentum of an object:

$I = \Delta p = mv - mu$I=Δp=mv−mu

Impulse is also equal to force multiplied by time:

$I = Ft \quad \text{(for a constant force)}$I=Ft(for a constant force)

More generally (for a variable force):

$I = \int F \, dt$I=∫Fdt

The unit of impulse is Ns (newton-seconds) or equivalently kg m/s.

Impulse-Momentum Theorem

$Ft = mv - mu$Ft=mv−mu