A force is a push or a pull. Energy is something different: it is what lets things change, by moving, heating up, lighting up or making a sound. Energy comes in many forms and can change from one form to another, and it is measured in joules (J).
Key idea
Energy shows up as motion (kinetic), height (gravitational potential), stretch or squeeze (elastic), the jiggling of particles (thermal), and stored in fuel and food (chemical), plus electrical, light, sound and nuclear energy. In every change, energy moves from one form or place to another; it is never used up.
Why it works
Follow the energy in a flashlight. The battery stores chemical energy. Switched on, it pushes charge through the wires: electrical energy. The bulb turns that into light, and some into thermal energy, which is why bulbs get warm. Add up all the forms at the end and you get the same amount you started with. "Using" energy really means changing it into forms that are less useful to us.
Worked example: A bouncing ball
A ball is dropped, bounces and rises again, but not quite as high. Trace its energy.
- Held up high: gravitational potential energy.
- Falling: potential energy turns into kinetic energy.
- Squashed against the floor: elastic energy, which pushes it back up.
- Each bounce turns a little energy into thermal energy and sound, so it rises a bit less each time.
Answer: Potential → kinetic → elastic → kinetic → potential, with some thermal and sound
Common mistake
Thinking energy is a substance that gets used up, or mixing up energy with force. A force is a push between two objects at one moment; energy is an amount that can be stored and passed along. When a battery "runs out," its energy has not vanished: it has become light, sound and heat in the room.
Food labels count energy in food Calories. One food Calorie is 4,184 J, so a 100-Calorie banana holds about 418,000 J of chemical energy.