Kinetic Energy

Kinetic energy is the energy a moving object has because it is moving. A faster or heavier object has more kinetic energy. When an object stops, its kinetic energy becomes zero.

Key idea and equation

Kinetic energy is a type of energy store. The equation is:

$$E_k = \tfrac{1}{2} m v^2$$

• Ek is kinetic energy in joules (J)
• m is mass in kilograms (kg)
• v is speed in metres per second (m/s)

How mass and speed affect kinetic energy

• If mass doubles, kinetic energy doubles (when speed is the same).
• If speed doubles, kinetic energy becomes four times bigger, because of the square: $v^2$.

Example idea: a car at twice the speed has four times the kinetic energy, so it needs much more distance to stop.

Energy transfers

Forces can transfer energy to or from the kinetic store. The work done by a force changes kinetic energy: $W = \Delta E$. For example, brakes remove kinetic energy by heating the brake pads.

Common misconceptions

• Kinetic energy depends on speed, not direction. Left or right gives the same $E_k$.
• Stationary objects have $E_k = 0$.
• Kinetic energy cannot be negative.

Real-world connections

• Road safety: higher speed means much more kinetic energy to remove, so longer stopping distances.
• Sports: a faster ball carries more kinetic energy, so it is harder to stop.
• Rollercoasters: gravitational potential energy at the top transfers to kinetic energy on the way down.