Impulse (Cambridge (CIE) IGCSE Physics): Revision Note

Impulse

Extended tier only

• When an external resultant force acts on an object for a very short time and changes the object's motion, we call this impulse

  • For example:

    • Kicking a ball

    • Catching a ball

    • A collision between two objects

The impulse equation

• Impulse is the product of the force applied and the time for which it acts

$\mathrm{impulse}=F∆t$

• Where:

  • $F$ = resultant force, measured in newtons (N)

  • $∆t$ = change in time, measured in seconds (s)

• Therefore, the units of impulse are newton seconds (Ns)

• The impulse of the resultant force is equal to the change in momentum of the object

$\mathrm{impulse}=F∆t=∆p$

• Change in momentum (opens in a new tab) can also be described as:

$∆p=∆\left(mv\right)$

$∆p=mv-mu$

• Where:

  • $∆p$ = change in momentum, measured in kilogram metres per second (kg m/s)

  • $m$ = mass, measured in kilograms (kg)

  • $v$ = final velocity, measured in metres per second (m/s)

  • $u$ = initial velocity, measured in metres per second (m/s)

• Therefore:

$\mathrm{impulse}=F∆t=∆p=mv-mu$

Examples of impulse

• When rain and hail (frozen water droplets) hit an umbrella, they feel very different due to the impulse of the force

• Water droplets tend to splatter and roll off the umbrella because there is only a very small change in momentum

• Hailstones have a larger mass and tend to bounce back off the umbrella, because there is a greater change in momentum

• Therefore, the impulse that the umbrella applies on the hail stones is greater than the impulse the umbrella applies on the raindrops

• This means that more force is required to hold an umbrella upright in hail compared to rain

The impulse exerted by the umbrella on the hail stones is greater than the impulse exerted on the raindrops.

Force & momentum

Extended tier only

• Newton's second law is defined by the equation:

$F=ma$

• Momentum is given by the equation:

$p=mv$

• Combining these equations gives Newton's second law in terms of momentum:

$F=\frac{∆p}{∆t}$

• Therefore, force can also be defined as the rate of change of momentum on a body

  • Or the change in momentum per unit time