Energy Examples in Physics

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Energy.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Physics.

Concept Recap

The capacity to do work or cause change in a physical system, measured in joules (J).

The 'currency' that makes things happen. It's what you need to move, heat, or change anything.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Energy can change forms but never disappears—it's conserved.

Common stuck point: Energy is not a substance you can touch—it's a calculated quantity that's conserved.

Sense of Study hint: When solving an energy problem, first identify all forms of energy present at the start and end of the process. Then apply conservation of energy: total energy before equals total energy after, accounting for any energy transferred in or out (work, heat). Finally, check units — energy is always in joules.

Worked Examples

Example 1

easy
A 2 \text{ kg} ball is held 10 \text{ m} above the ground and then dropped. What is its total mechanical energy at the top? Use g = 9.8 \text{ m/s}^2.

Solution

  1. 1
    At the top, the ball is at rest, so kinetic energy is zero: KE = 0.
  2. 2
    Potential energy: PE = mgh = 2 \times 9.8 \times 10 = 196 \text{ J}
  3. 3
    Total mechanical energy: E = KE + PE = 0 + 196 = 196 \text{ J}

Answer

E = 196 \text{ J}
Energy is the capacity to do work. Mechanical energy is the sum of kinetic and potential energy. At the top, all energy is stored as gravitational potential energy.

Example 2

medium
A 1000 \text{ kg} car traveling at 20 \text{ m/s} brakes to a stop. How much energy is dissipated as heat?

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy
Name the energy transformations when a ball is thrown upward and comes back down.

Example 2

medium
A 3 \text{ kg} rock at 15 \text{ m} above the ground is moving at 4 \text{ m/s}. What is its total mechanical energy? Use g = 10 \text{ m/s}^2.
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