What is energy?
Energy is the ability to do work or cause change. When something moves, energy is often in a form that helps it keep moving.
Two main kinds of energy in motion
- Kinetic energy — the energy of moving objects.
- Potential energy — stored energy due to position, ready to be released.
Key formulas
- Kinetic energy KE = 1/2 m v^2, where m is mass (kg) and v is speed (m/s). The unit is the joule (J).
- Potential energy PE = m g h, where g ≈ 9.8 m/s^2 on Earth and h is height in meters.
Energy transformations
Energy can change from one form to another. For example, a swinging pendulum converts potential energy to kinetic energy and back. A light bulb turns electrical energy into light and heat.
Conservation of energy
In a closed system, energy is not created or destroyed—it's transformed. The total amount of KE plus PE stays the same if only conservative forces act.
Examples
Example 1: A 2 kg ball moving at 3 m/s has KE = 1/2 × 2 × 3^2 = 9 J.
Example 2: A 5 kg object raised 2 m has PE = m g h ≈ 5 × 9.8 × 2 ≈ 98 J.
Try it yourself
Think of a roller coaster car going up a hill. As it climbs, PE increases and KE decreases. At the top, KE is low; as it speeds down, KE increases again.