Energy is the capacity to do work or cause change. It exists in two broad categories: kinetic energy (the energy of motion) and potential energy (stored energy due to position or configuration). Kinetic energy depends on mass and velocity: KE = ½mv². A moving car has kinetic energy proportional to its mass and the square of its speed — doubling the speed quadruples the kinetic energy.
Potential energy comes in several forms. Gravitational potential energy depends on height: PE = mgh, where g is gravitational acceleration. Elastic potential energy is stored in stretched or compressed springs. Chemical potential energy is stored in the bonds of molecules — this is the energy released when we burn fuel or eat food. Nuclear potential energy is released during fission (splitting atoms) or fusion (combining atoms), powering the Sun and nuclear reactors.
Thermal energy is the kinetic energy of particles in motion at the microscopic level. Temperature measures the average kinetic energy of particles. When you heat water, you increase the speed of water molecules. Phase changes (melting, boiling) require energy input without changing temperature — the energy breaks intermolecular bonds rather than speeding up molecules. This is called latent heat.
The law of conservation of energy states that energy cannot be created or destroyed, only transformed from one form to another. A swinging pendulum continuously converts between kinetic energy (at the bottom) and gravitational potential energy (at the peaks). A flashlight converts chemical potential energy into light and thermal energy. In every transformation, some energy is inevitably converted to thermal energy due to friction — this is why perpetual motion machines are impossible.
Work and power connect force to energy. Work (W) is done when a force moves an object over a distance: W = Fd cos θ, where θ is the angle between the force and the displacement. Power (P) is the rate at which work is done or energy is transferred: P = W/t. A more powerful machine does the same work in less time. The unit of power is the watt (W), where 1 watt = 1 joule per second.