Lecture Notes: Calculating Gravitational Potential Energy

Introduction

• Objective: Calculate the amount of gravitational potential energy (GPE) gained by an object raised above ground level.

Gravitational Potential Energy (GPE)

• Definition: Energy stored in an object due to its position above the Earth's surface.
• Cause: Due to the force of gravity acting on the object.

Example Explanation

1. Initial State: A ball sitting on the ground.
2. Action: Lifting the ball onto a shelf.
   • Transformation: Chemical energy (from muscles) → Gravitational potential energy (in the ball).
3. After Falling: The ball falls back to the ground.
   • Energy Conversion: GPE → Thermal and kinetic energy.
   • Result: The ball has no GPE.

Calculating Gravitational Potential Energy

• Formula: [ \text{GPE (Joules)} = \text{mass (kg)} \times \text{gravitational field strength (N/kg)} \times \text{height (m)} ]
• Note: The gravitational field strength is provided in the question.
• Exam Tip: Memorize the GPE equation, as it is not provided in exams.

Sample Problem

• Scenario: A crane lifts a 75 kg mass to a height of 8 meters.
• Given: Gravitational field strength = 9.8 N/kg
• Calculation:
  • Mass = 75 kg
  • Height = 8 meters
  • GPE = 75 kg × 9.8 N/kg × 8 m = 5880 Joules

Practice Question

• Question: A ball with a mass of 500 grams is lifted onto a shelf 1.5 meters above the ground.
• Steps:
  1. Convert mass from grams to kilograms (500 g = 0.5 kg).
  2. Use gravitational field strength = 9.8 N/kg.
  3. Height = 1.5 meters.
  4. Calculate GPE: 0.5 kg × 9.8 N/kg × 1.5 m = 7.35 Joules.

Additional Resources

• More practice questions available in the revision workbook (link provided in the lecture).

Conclusion

• Goal: Ability to calculate GPE for objects raised above ground level.