Introduction
Energy is one of the most powerful tools in A-Level Mechanics because it allows you to relate speed and height without needing to track every force at every instant. In the 9709 exam, questions on kinetic energy (KE) and gravitational potential energy (GPE) appear regularly — often combined with the work–energy principle you have already met. Mastering these two formulae, and understanding exactly what they measure, is essential for tackling problems involving particles moving along slopes, projectiles, and connected systems.
Core Concept
Kinetic Energy
Kinetic energy is the energy a body possesses by virtue of its motion. Any object with mass (kg) moving at speed (m s) carries kinetic energy. It is a scalar quantity measured in joules (J).
The link to work done is direct: the work done by the net force acting on a body equals the change in its kinetic energy (the work–energy principle). Here we focus on the energy itself.
Gravitational Potential Energy
Gravitational potential energy is the energy a body possesses by virtue of its position in a gravitational field — specifically, its height above a chosen reference level. When a body of mass is raised through a vertical height , the work done against gravity equals ; this work is stored as GPE.
Key points:
- Only the vertical height matters — not the path taken (slope, curve, or straight up).
- The reference level (where GPE ) is chosen for convenience; only changes in GPE are physically meaningful.
- unless the question states otherwise.
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