Impact forces acts on falling objects hitting ground, crashing cars and similar. The dynamic kinetic energy of a moving object, like a falling ball or a driving car, can be expressed as. E = 1/2 m v2 (1) where. E = kinetic (dynamic) energy (J, ft lb) m = mass of the object (kg, slugs) v = velocity of the object (m/s, ft/s) In an impact - like a