Kinetic energy

Work, Energy & Power - OCR A-Level Physics

Key Definition
Kinetic energy
The energy an object possesses due to its motion.
$$E_k = \frac{1}{2}mv^2$$
$$E_k = \frac{1}{2}mv^{2}$$
  • Kinetic energyThe capacity to do work. Measured in joules (J).The energyThe capacity to do work. Measured in joules (J). an object possesses due to its motion. is a scalarA quantity with magnitude only and no direction (e.g. mass, time, temperature). quantity (always positive)
  • KE is proportional to $v^{2}$: doubling speed quadruples the kinetic energyThe capacity to do work. Measured in joules (J).The energy an object possesses due to its motion.
  • This is why stopping distance increases dramatically with speed
  • The work-energy theorem: net work doneEnergy transferred when a force moves an object. In electrical circuits, W = QV (chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). times potential difference). on an $object = change$ in its kinetic energyThe energy an object possesses due to its motion.
  • Derivation: from $W = Fs and F = ma with v^{2} = u^{2} + 2as$, substitution gives W = $\frac{1}{2}$$mv^{2}$ - $\frac{1}{2}$$mu^{2}$
Common Mistake MEDIUM
Students often: A typical mistake is to think that doubling speed doubles kinetic energy.
Instead: E_k is proportional to v SQUARED. Doubling speed means 4 times the kinetic energy. Tripling speed means 9 times the kinetic energy.
Examiner Tips and Tricks
  • If asked to derive the kinetic energy equation, start from W = Fs, substitute $F = ma$ and use \(v^{2}\) = \(u^{2}\) + 2as to eliminate a and s.
  • Show every algebraic step clearly.
Work, Energy & Power Overview
Work done 2 of 6 Gravitational potential energy