What is Energy? Energy is the ability to do work. Think of it as a bank account where you can deposit or withdraw energy but the total amount stays the same unless you add or remove it from the system.
The total energy in an isolated system remains constant. Energy can change form (e.g., kinetic ↔ potential) but the sum is unchanged.
Mathematically: \$\sum E{\text{initial}} = \sum E{\text{final}}\$
Rolling Ball – A ball released from height \$h\$ has potential energy \$Ep = mgh\$ at the top. At the bottom, all that energy is kinetic: \$Ek = \tfrac{1}{2}mv^2\$. If no friction, \$mgh = \tfrac{1}{2}mv^2\$.
Pendulum – At the highest point, the pendulum has maximum potential energy. At the lowest point, it has maximum kinetic energy. The sum \$Ep + Ek\$ stays constant.
Battery & Lightbulb – Chemical energy stored in a battery is converted into electrical energy, then light and heat. The total energy before and after remains the same (ignoring losses).
| Input | Output | Loss (if any) |
|---|---|---|
| Chemical energy (battery) | Electrical energy → Light + Heat | Minor heat loss to surroundings |
| Potential energy (ball at height) | Kinetic energy (ball in motion) | None if frictionless |
Imagine a bank account where you can deposit energy (e.g., by lifting a weight) and withdraw it (e.g., by letting the weight fall). The account balance never changes unless you add or remove money (energy) from outside the system.