2.3.2Forces, Space and Radioactivity
KE, GPE and Elastic Energy — WJEC GCSE Science (Double Award) - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 14 of 37
Part 1 of 4Three energy stores
Kinetic energy (KE)
- Kinetic energy (KE) The energy an object has because it is moving.
- Speeding up increases it and slowing down decreases it.
Gravitational potential energy (GPE)
- Gravitational potential energy (GPE) The energy an object has because of its height above the ground.
- Lifting an object increases it, and a falling object loses it.
Elastic energy
- Elastic energy The energy stored in a spring or elastic band when it is stretched or compressed.
Part 2 of 4A falling ball
1At the top
- At the top The ball is held still, so KE is zero and GPE is at its maximum.
2Falling
- Falling The ball loses height, so GPE is transferred to KE.
3Speeding up
- Speeding up As GPE decreases, KE increases and the ball moves faster.
4Just before landing
- Just before landing GPE is almost zero and KE is at its maximum.
- With no air resistance, the KE equals the GPE lost.
Part 3 of 4Mass on a spring
| Position | Kinetic energy | Gravitational potential energy | Elastic energy |
|---|---|---|---|
| Highest point (spring slightly compressed) | Zero, mass momentarily at rest | Maximum | Some |
| Middle (resting position) | Maximum, moving fastest | Medium | Some |
| Lowest point (spring stretched most) | Zero, mass momentarily at rest | Minimum | Maximum |
Part 4 of 4Conservation in the system
Total energy stays constant
Ideal case
- Ideal case With no friction or air resistance, the total of KE, GPE and elastic energy stays the same throughout.
Real case
- Real case Some energy is transferred to the thermal store by friction and air resistance, so the oscillations gradually die away.
Energy never lost
- Energy never lost The energy dissipated spreads out into the surroundings and is hard to use, but it is not destroyed.
Key terms
| Term | Meaning |
|---|---|
| Kinetic energy | The energy of an object due to its motion. |
| Gravitational potential energy | The energy of an object due to its height above the Earth's surface. |
| Elastic energy | The energy stored in an object that has been stretched, compressed or twisted. |
| Energy transfer | The movement of energy from one store to another. |
| Oscillation | A repeated back and forth movement about a central position. |
Avoid these mistakes
1
Saying a ball has no energy when it is held still at the top: it has maximum GPE.
2
Believing energy is lost as an object slows: it is transferred to other stores, such as thermal.
3
Saying KE is largest at the highest point of a bouncing mass: it is zero there because the mass has stopped.
4
Forgetting that a mass on a spring has elastic energy at nearly every position.
5
Writing that speed depends on energy 'running out' rather than describing the transfer between stores.
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