Two kinds of energy

A fast cycle can hurt if it hits you. So can a brick that falls from a wall.

Rule

Each has energy. Energy lets a thing do work on what it hits. In the rule, m is mass in kg. The letter v is speed in m/s. The letter h is height in m. The letter g is the speed a falling thing gains each second.

With no friction, work on an object equals its energy gain. Moving objects have kinetic energy, ½ m v². Raised objects have potential energy, m g h. So do stretched springs and bands.

• Energy — the capacity to do work. It is measured in joules. Capacity means ability.
• Kinetic energy (KE) — the energy of motion. KE = ½ × m × v².
• Potential energy (PE) — stored energy. Near the ground, PE = m × g × h.

Take g = 10 m/s². Ignoring air, a falling thing gains 10 m/s each second.

A 2 kg trolley moves at 3 m/s. Here m = 2 and v = 3. KE = ½ × 2 × 3 × 3. Now ½ × 2 = 1, and 3 × 3 = 9. So KE = 9 J.

A 0.5 kg ball is 4 m up. Here m = 0.5, g = 10 and h = 4. PE = 0.5 × 10 × 4. Now 0.5 × 10 = 5, and 5 × 4 = 20. So PE = 20 J.

Double the speed and the KE becomes 4 times as much. KE depends on v × v, and 2 × 2 = 4.