Impulse

Impulsevideo

Impulse

• 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 & Momentum

Force & Momentum

• 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:

$F = \frac{∆p}{∆t}$

$∆p = p_{final} - p_{initial}$

• 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)