How it works
Kinetic energy is the energy an object has because it is moving. It grows with the square of speed, so doubling the speed quadruples the energy, while doubling the mass only doubles it.
Switch to grains and feet per second to get bullet muzzle energy in foot-pounds, the figure printed on ammunition boxes. One grain is 1/7000 of a pound and one foot-pound is 1.3558 J.
For a car, kinetic energy shows why stopping distance rises so quickly with speed: the brakes must turn all of that energy into heat.
Formula
KE = ½ × m × v²
Questions people ask
How is bullet energy in ft-lbs calculated?
Using the shortcut KE ≈ weight (gr) × velocity² (fps) ÷ 450,436. A 140 gr bullet at 2,700 fps carries about 2,266 ft·lbf.
Why does speed matter more than mass?
Because speed is squared. Going from 30 to 60 mph quadruples kinetic energy, which is why crash forces and braking distances rise so sharply.
Is kinetic energy the same as momentum?
No. Momentum is m × v and is conserved in collisions. Kinetic energy is ½mv² and is often partly converted to heat, sound and deformation.