Impulse and Force-time Graphs

Newton's second law states that the resultant force on an object equals its rate of change of momentum, F = Δp/Δt. Rearranging gives FΔt = Δp. The product of the force and the time for which it acts is called the impulse.Impulse is equal to the change in momentum. Its unit is the newton second (N s), which is equivalent to kg m s^-1, the unit of momentum.On a force-time graph, the area under the graph is the impulse. This is true even when the force varies, as it does during a real collision, where the force rises to a peak and then falls. The area can be found by counting squares or by splitting it into simple shapes.The same change in momentum can be produced by a large force for a short time or a small force for a long time. Crumple zones, airbags and crash mats increase the time of an impact, so the average force on a person is smaller for the same impulse.