Beginner · 11 min
Work and kinetic energy
Calculate energy transferred by a force and energy stored in motion.
Work transfers energy
In physics, work is energy transferred when a force moves an object. When a constant force acts in the same direction as the displacement, work equals force multiplied by distance moved: W = F × d. Work and energy are measured in joules (J). One joule equals one newton metre. If the object does not move, that force does no mechanical work on it, even if holding it feels tiring.
Worked example
A constant 5 N force pulls a box 4 m in the direction of the force. Work done by the force = 5 × 4 = 20 J
Energy of motion
A moving object has kinetic energy. For everyday speeds, kinetic energy equals half its mass multiplied by the square of its speed: E = ½ × m × v². Use mass in kilograms and speed in metres per second to obtain joules. Doubling the mass doubles the kinetic energy at the same speed. Doubling the speed makes the kinetic energy four times larger.
Worked example
A 4 kg cart travels at 3 m/s. Kinetic energy = ½ × 4 × 3² = 18 J
Compare moving carts
- Calculate the kinetic energy of a 2 kg cart at 2 m/s and then at 4 m/s.
- Compare the two answers. Explain why doubling speed does not simply double kinetic energy.