Overview
This lecture explains gravitational potential energy, the conditions under which it changes, and how to calculate it using a standard formula.
Gravitational Potential Energy Basics
- Gravitational potential energy (GPE) is the energy stored in an object due to its position above Earth's surface.
- GPE exists because of the force of gravity acting on the object.
- When you lift an object, chemical energy from muscles is converted into GPE.
Changes in Gravitational Potential Energy
- Placing an object higher increases its GPE; dropping it converts GPE into kinetic and thermal energy.
- An object on the ground has no GPE (relative to that ground).
Calculating Gravitational Potential Energy
- The formula: GPE (Joules) = mass (kg) × gravitational field strength (N/kg) × height (m).
- Gravitational field strength on Earth is typically 9.8 N/kg.
- You must memorize the GPE formula for exams, as it is not provided.
Example Calculations
- Example 1: A 75 kg mass lifted 8 meters with g = 9.8 N/kg:
GPE = 75 × 9.8 × 8 = 5,880 J.
- Example 2: A 500 g ball (0.5 kg) lifted 1.5 meters with g = 9.8 N/kg:
GPE = 0.5 × 9.8 × 1.5 = 7.35 J.
- Always convert grams to kilograms by dividing by 1,000 before using the formula.
Key Terms & Definitions
- Gravitational Potential Energy (GPE) — Energy stored in an object due to its height above Earth.
- Gravitational Field Strength (g) — Force per kilogram exerted by gravity (typically 9.8 N/kg on Earth).
- Joule (J) — Standard unit of energy.
- Mass (kg) — The amount of matter in an object, measured in kilograms.
- Height (m) — Distance above the ground in meters.
Action Items / Next Steps
- Memorize the GPE formula for exams.
- Practice with additional GPE calculation problems in the revision workbook.