Energy transfer & conservation
The idea
Energy never appears from nowhere and never vanishes — it only moves between objects and changes form. That rule, conservation of energy, is one of the most reliable accounting tricks in all of science. You already know energy shows up as motion, heat, light, and sound; conservation says that if you add up every form before and after an event, the totals match. A falling ball trades stored gravitational energy for kinetic energy, joule for joule, all the way down.
The misconception to drop is that energy gets 'used up.' When your phone battery dies or a ball stops bouncing, the energy is not gone — it has leaked into less useful forms, mostly heat spread thinly into the surroundings, plus a little sound. Physicists say it has dissipated. The skill to build is bookkeeping: name the energy store the system starts with, name where each joule goes, and refuse to let any of it disappear off the books.
Worked example
A 2 kg ball is nudged off a shelf 5 m above the floor. Using 10 N/kg for gravity and ignoring air resistance, how fast is the ball moving just before it hits the floor?
- Find the energy stored at the start: potential energy = mass × gravitational strength × height = 2 × 10 × 5 = 100 J.
- Apply conservation: with no air resistance, every joule of potential energy becomes kinetic energy during the fall, so just before impact the kinetic energy is 100 J.
- Set up the kinetic energy formula and solve for speed: 1/2 × 2 × speed² = 100, which simplifies to speed² = 100, so speed = √100 = 10 m/s.
- Sanity-check the endpoints: at the shelf the ball has 100 J potential and 0 J kinetic; at the floor it has 0 J potential and 100 J kinetic. The total is 100 J at both moments, exactly as conservation demands.
- Interpret the aftermath: when the ball smacks the floor, that 100 J does not vanish — it spreads out as sound and a tiny amount of heat in the ball and floor.
Answer. The ball hits the floor moving at 10 m/s, carrying the full 100 J as kinetic energy.
Check your understanding
- When a bouncing ball finally comes to rest, where exactly has all of its original energy gone?
- How would air resistance change the energy bookkeeping of a falling object, and which numbers in the chain would shrink?
- Why is 'energy efficient' a better phrase than 'energy saving' once you know energy is conserved no matter what?
- How would you trace the energy in a flashlight beam all the way back to its earlier stores, step by step?
Build the foundations first
Energy transfer & conservation builds on these concepts. If any feel shaky, start there.