Momentum

The vector equal to the product of a body’s mass and velocity is its momentum: p = mυ. Momentum points the same way as the velocity; its unit is kg·m/s. The product of a force and the time it acts is the impulse of the force: FΔt, with unit N·s (1 N·s = 1 kg·m/s). From Newton’s second law F = ma = m(υ − υ₀)/Δt, so FΔt = mυ − mυ₀: the impulse of the force equals the change in the body’s momentum. So the same change of momentum can be achieved by a smaller force acting for longer: that is why we bend our knees on landing, and a car airbag lengthens the impact time and reduces the force.