Energy
What you must know
Energy is the capacity to cause change and appears in forms: kinetic (motion), potential (position or stored), thermal, chemical, electrical, light, and sound. Energy is conserved: it transfers between objects and transforms between forms, but the total stays constant. Friction and air resistance transform useful motion into thermal energy, which is why a rolling ball eventually stops.
Distinguish kinetic energy (depends on mass and speed, growing with the square of speed) from gravitational potential energy (depends on mass, height, and gravity). In a falling object, potential energy converts to kinetic energy. On a ramp or roller coaster, energy shifts back and forth, with some lost to thermal energy each time.
Understand conduction, convection, and radiation as heat transfer methods: direct contact, fluid circulation, and electromagnetic waves. Know that heat flows from warmer to cooler objects until temperatures equalize, and that temperature measures average particle kinetic energy, not total energy.
Traps to watch
- Energy is not "used up"; it is transformed, often into thermal energy.
- Doubling speed quadruples kinetic energy; doubling mass only doubles it.
- Temperature is not the same as heat or total thermal energy; a small hot object can have less total energy than a large cool one.
- Radiation needs no medium; it can travel through space.
Rule of thumb
Track energy with a chain: where it started, where it went, and what form it became. If something speeds up while dropping, potential became kinetic.
Worked example
A ball at 5 m height has potential energy. As it falls, potential energy decreases while kinetic energy increases; just before impact nearly all of it is kinetic, with a small share converted to thermal energy and sound.