Energy — Science definition
The ability to do work or cause change, measured in joules (J).
Where it is taught
- Kinetic & Potential Energy — Energy & Heat Transfer, Physical Science: Matter, Forces & Energy, Grade 8
In use
Energy is the ability to do work or cause change, and everything that happens — a ball flying, a light glowing, a muscle flexing — involves energy in some form.
Kinetic & Potential Energy
Questions using energy
Problem 1
Find the kinetic energy of a 4 kg cart moving at 5 m/s.
Answer and explanation
50 J
KE = ½mv² = ½ × 4 × 5² = ½ × 4 × 25 = 50 J.
Problem 2
A 10 kg object sits on a shelf 2 m high. Find its gravitational potential energy. (g = 9.8 m/s²)
Answer and explanation
196 J
PE = mgh = 10 × 9.8 × 2 = 196 J.
Problem 3
If you double an object's speed, its kinetic energy…
- doubles
- triples
- quadruples (×4)
- stays the same
Answer and explanation
quadruples (×4)
Since KE = ½mv², speed is squared, so doubling speed multiplies KE by 2² = 4.
Problem 4
Find the kinetic energy of a 1 kg ball moving at 6 m/s.
Answer and explanation
18 J
KE = ½mv² = ½ × 1 × 6² = ½ × 1 × 36 = 18 J.