Sec 3 Combined Science (Physics) Energy, Work & Power notes

Energy is not a substance you can hold — it is a capacity to do things, measured in joules (J). We picture it sitting in stores: a moving car in its kinetic store, a book on a shelf in its gravitational potential store, food and fuel in chemical stores, a stretched catapult band in its elastic store, hot soup in a thermal (internal) store, and nuclei in nuclear stores.

Energy gets between stores by only four pathways: (1) mechanical working — a force moves through a distance (pushing a trolley, a falling weight); (2) electrical working — an electric current carries energy from a power station or battery to a lamp or motor; (3) heating — energy flows from hot to cold by conduction, convection or radiation; (4) waves — energy carried by light or sound leaving the source. Every energy story you will describe is just stores × pathways: name the emptying store, the filling store and the pathway.

Anything moving carries energy in its kinetic store: Ek = ½mv², mass in kg, speed in m/s, answer in J. The squared speed is the headline feature: double the speed and the kinetic energy quadruples — the physics behind why road-crash damage grows so fast with speed. Doubling the mass only doubles the KE.

Lift anything against gravity and you pay energy into its gravitational store: Ep = mgh — mass in kg, g = 10 m/s² (N/kg) in this course, height in m. Height is always measured from a chosen reference level (usually the ground); pick one and stay consistent through the question.

The principle of conservation of energy: energy cannot be created or destroyed — it can only be transferred from one store to another. Friction does not "use up" energy; it transfers energy into thermal stores. That is why a bouncing ball never returns to its drop height: on each impact some energy leaves the kinetic and gravitational stores and warms the ball and floor (plus a click of sound).

The most useful application is a falling object: gravitational store empties, kinetic store fills. Ignoring air resistance, the two amounts are equal — so ½mv² = mgh, which rearranges to v = √(2gh): the landing speed depends on the height fallen, not on the mass.

Work done is the energy transferred when a force moves its point of application: W = force × distance moved in the direction of the force, in joules — the same unit as energy, because work is an energy transfer (the mechanical working pathway). Push a crate with 80 N for 3.0 m and you have transferred 240 J. But hold the crate still and you do zero work on it: the distance is zero, however tired your arms feel.

The direction clause matters: only the distance along the force counts. Carrying a box at constant speed along a corridor, your upward holding force is perpendicular to the horizontal motion, so that force does no work on the box. Lifting it up the stairs does work — against gravity, at the rate mgh per flight.

Power is the rate of energy transfer: P = energy transferred ÷ time taken, measured in watts — 1 W = 1 J/s. A 40 W lamp transfers 40 joules every second. Power compares machines fairly: two motors may transfer the same total energy, but the more powerful one does it in less time.