Force-Time Graphs

Newton's second law can be written as force = rate of change of momentum, F = Δ(mv)/Δt. Rearranging gives FΔt = Δ(mv). The product of force and the time for which it acts is called the impulse.Impulse equals the change in momentum. Its unit is the newton second (N s), which is equivalent to kg m s^-1.On a graph of force against time, the area under the graph is the impulse, so it gives the change in momentum. This works even when the force changes with time, as in a real collision, where the area can be found by counting squares.Because momentum is a vector, direction matters. If a ball of mass m moving at speed u is hit straight back and leaves at speed v, the change in momentum is m(v + u), not m(v - u).