Impulse (Cambridge O Level Physics)

• When a resultant (unbalanced) force acts on a mass, the momentum of that mass will change
• The impulse of a force is equal to that force multiplied by the time for which it acts:

impulse  =  force × change in time

impulse = FΔt

• The change in momentum of a mass is equal to the impulse provided by the force:

impulse = change in momentum

impulse = FΔt  = Δp

• Change in momentum can also be described as:

Δp = Δ(mv)

Δp = mv − mu

• Where:
  • m = mass in kg
  • v = final velocity in m/s
  • u = initial velocity in m/s

• Therefore:

impulse = FΔt  = Δp = mv − mu

• An example in everyday life of impulse is when standing under an umbrella when it is raining, compared to hail (frozen water droplets)
  • When rain hits an umbrella, the water droplets tend to splatter and fall off it and there is only a very small change in momentum
  • However, hailstones have a larger mass and tend to bounce back off the umbrella, creating a greater change in momentum
  • Therefore, the impulse on an umbrella is greater in hail than in rain
  • This means that more force is required to hold an umbrella upright in hail compared to rain

Impulse of Rain & Hail Stones

Since hailstones bounce back off an umbrella, compared to water droplets from rain, there is a greater impulse on an umbrella in hail than in rain

• Force can also be defined as the rate of change of momentum on a body
• The change in momentum is defined as the final momentum minus the initial momentum
• These can be expressed as follows:

• Where:
  • F = force in newtons (N)
  • Δ (Greek letter delta) = change in
  • p = momentum in kilogram metres per second (kg m/s)
  • t = time in seconds (s)