Types of energy. Power
A raised body has potential energy: E_p = m · g · h, where h is the height above a chosen zero level; a stretched spring also has potential energy. A moving body has kinetic energy: E_k = m · v² : 2; the greater the mass and speed, the greater the kinetic energy. Energy is not lost but changes from one form to another: a stone falling from a height turns potential energy into kinetic energy. Total mechanical energy E = E_p + E_k. The same work can be done in different times, so power is introduced: power is the ratio of work to the time in which it was done, N = A : t. Its unit is the watt (W), 1 W = 1 J : 1 s; 1 kW = 1000 W. An old unit is the horsepower: 1 hp = 735.5 W. Power is named after the Scottish engineer James Watt. For example, a 35 W fan does 35 · 600 = 21 000 J = 21 kJ of work in 10 minutes.
“Energy changes”: pupils discuss when potential energy turns into kinetic energy in examples such as a ball falling from a height. Safety: use only a soft (tennis or foam) ball, thrown gently; do not use a real bow.