Kinetic Energy Calculator

Kinetic energy and the work-energy principle

The work done by a net force equals the change in kinetic energy: F x d = 1/2 m v^2 - 1/2 m u^2. Stopping a 1,200 kg car from 20 m/s removes 240,000 J. If the brakes supply an average force of 8,000 N, the stopping distance is 240,000 / 8,000 = 30 m. At 40 m/s, with the same force, it becomes 960,000 / 8,000 = 120 m, four times as far.

This is a simplified model. Real stopping distance also includes the distance covered during reaction time, and braking force depends on tyres and road surface.

The KE = 1/2 m v^2 formula

Kinetic energy in joules equals half the mass in kilograms times the speed in metres per second, squared. Convert km/h to m/s by dividing by 3.6.

Example: a 1,200 kg car at 72 km/h (20 m/s) has 0.5 x 1,200 x 20^2 = 240,000 J, or 240 kJ. A 0.16 kg cricket ball at 40 m/s carries 0.5 x 0.16 x 1,600 = 128 J.

Why speed matters most

Speed is squared, so doubling speed quadruples energy. The same car at 144 km/h (40 m/s) has 960 kJ. That energy has to be removed by the brakes to stop, which is why stopping distances grow so quickly with speed.