Calculating kinetic energy: units and orders of magnitude

Maintaining consistency with units

To apply Ec = 1/2 * m * v^2 correctly, you must always use the International System of Units: mass in kilograms (kg) and velocity in metres per second (m/s). The result is then directly in joules (J).

Converting a speed to km/h

Speeds are often given in km/h, so they must be converted before calculating:

v (m/s) = v (km/h) / 3.6

For example, 90 km/h corresponds to 90 / 3.6 = 25 m/s.

Numerical example

A car with a mass of 1000 kg is travelling at 90 km/h, or 25 m/s.

Ec = 1/2 * 1000 * 25² = 1/2 * 1000 * 625 = 312,500 J, or approximately 312.5 kJ.

Table of orders of magnitude

Object	Mass	Speed	Kinetic energy
Tennis ball	0.06 kg	30 m/s	27 J
Cyclist	80 kg	5 m/s	1,000 J
Car	1,000 kg	25 m/s	312,500 J

Common pitfall

A common mistake is to use speed directly in km/h in the formula without converting it. This gives a completely incorrect result, with no physical meaning whatsoever. Always remember to check the units before calculating!