Knowledge Organiser: Gravitational Potential Energy
Key Terms
- GPE: energy stored due to position in a gravitational field
- g: gravitational field strength (10 N/kg on Earth)
- h: vertical height above reference point (m)
- Reference point: level from which height is measured
Key Equation
- E_p = mgh
- Rearranged: h = E_p / (mg)
- Rearranged: m = E_p / (gh)
- Conservation: GPE lost = KE gained
Key Facts
- GPE ∝ mass and ∝ height (both direct)
- g = 10 N/kg on Earth
- Height must be VERTICAL, not slope distance
- In free fall (no friction): all GPE → KE
Exam Tips
- Use vertical height only — not slope length
- mgh = ½mv² for falling objects — mass cancels!
- Always square root at the end when finding v
- Check g value given — may differ from 10 N/kg
Common Mistakes
- Using slope instead of vertical height: h in Eₚ = mgh must be the vertical height — not the length of a ramp or slope
- Wrong units for g: g = 10 N/kg (or 9.8 N/kg) — check which value the exam provides; using 9.8 when the question says 10 loses marks
- Forgetting to convert mass: Mass must be in kg, height in metres — always convert grams and centimetres first
- Misapplying KE = GPE: When mass cancels, students sometimes forget to square root when finding v — always show full working
- Confusing GPE with work done: GPE gained equals work done against gravity — they are equal only when no energy is wasted
Memory Aid
- Remember E_p = mgh as: "Masses on Great Hills" — mass, gravitational field strength, height.
- Or think of it as: the bigger the object (m), the stronger gravity (g), and the higher it is (h), the more "potential danger" it has — all three multiply together.
- Hoover Dam memory hook: "The higher the water, the more power the dam can generate." Height = stored energy potential.