Physics, visually · Work, Energy & Power · Concept 05
Energy Conservation
Why does a swing slow down at the top and speed up at the bottom, and why does it eventually stop?
Energy is never made or lost, only moved between forms. A pendulum shows the swap in slow motion.
- Release from a high angle with Air drag off. Watch the bars: when PE falls, KE rises by exactly the same amount. The total never changes.
- Switch Air drag on. The total bar shrinks every swing. Where did the energy go? (Heat in the air.)
- Change the mass. The bars get taller but the swing time stays the same. Mass does not change how fast a pendulum swings.
Height is stored, speed is spent. Total stays the same.
PE = m g h is energy stored by height. KE = ½ m v² is energy of motion. At the top: all PE. At the bottom: all KE. In between: a mix, always adding to the same total (if there is no friction).
Roller coasters
The first hill is the tallest because that is where all the energy is put in. Every later hill must be lower.
Regenerative drives
A lift coming down or a motor braking turns KE back into electricity instead of wasting it as heat in a resistor.
Flywheels and presses
A flywheel stores KE slowly and gives it back in a burst: that is how a press punches through steel with a small motor.