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Kinetic energy

Kinetic energy is the energy of motion. It grows with the mass and with the square of the speed, so double the speed gives four times the energy. The unit is the joule. The energy is the same no matter which way the velocity points, because the speed is squared. When you brake, kinetic energy becomes heat in the brakes and the tyres. Use the speed in meters per second, not in kilometers per hour.

Try it yourself

Energy conservation in free fall

E_k = \frac{1}{2}mv^2

kinetic energy

Symbols

$E_k$	kinetic energy	J
$m$	mass	kg
$v$	speed	m/s

Example

A body of 2 kg has a speed of 6 m/s.

$E_k = \frac{1}{2} \cdot 2 \cdot 6^2 = 36$ J.

If the speed rises from 10 m/s to 20 m/s, the energy becomes four times as large, not twice as large.

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← Friction · Potential energy and falling →

Part of Physics 1: Energy and work.