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1.7.3Motion, Forces and Energy

Gravitational Potential Energy — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 8 parts Lesson 70 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Curriculum Equation Key Circular Motion & Gravitation LawsFast Exam Solution
a_c = v² / r | F_c = (m × v²) / r | F_g = G × (m₁ × m₂) / r² | v_orbit = √(G×M / r)
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Lesson summary

1.7.3 Gravitational Potential Energy — IGCSE PHYSICS notes: Calculate the change in gravitational potential energy using ΔEp = mgΔh and apply it to lifting and falling.Gravitational potential energy = the energy stored in an object because of its position in a gravitational field…

  • What the symbols mean
  • Hydroelectric dams
  • Pumped storage
  • Different planets
  • Defining the system
  • Gravitational field strength (g)

Part 1 of 6Gravitational potential energy

Gravitational potential energy
Gravitational potential energy
Gravitational potential energy the energy stored in an object because of its position in a gravitational field.
Lifting an object requires work against its weight, and the work is stored as gravitational potential energy.
The energy returns as kinetic energy if the object falls.

Part 2 of 6The equation

Equation
  • ΔEp = mgΔh.
    What the symbols mean: Ep = gravitational potential energy gained (J) · m = mass (kg) · g = 9.81 N kg−1 · h = change in height (m)
  • ΔEp is the change in gravitational potential energy in joules (J).
  • m is mass in kilograms (kg).
  • g is gravitational field strength, 9.8 N/kg on Earth.
  • Δh is the change in vertical height in metres (m).
mg is the weight, so ΔEp is weight × height lifted.

Part 3 of 6Only vertical height counts

1
The change depends on the vertical height difference, not on the path.
2
A box lifted vertically or carried up a ramp or stairs gains the same energy.
3
A ramp reduces the force needed over a longer distance, not the energy needed.
4
A walk of 200 m along a slope that rises 15 m uses 15 m in the calculation.
5
Height is measured from a chosen reference level, as only changes in energy matter.

Part 4 of 6Gravitational potential energy and kinetic energy

When an object falls, the decrease in gravitational potential energy equals the increase in kinetic energy if air resistance is negligible.
Equation
  • Equation mgΔh = ½mv2, so v = √(2gΔh).
The mass cancels, so objects of different mass reach the same speed.
Pendulum
  • Pendulum maximum gravitational potential energy at the ends of the swing and maximum kinetic energy at the lowest point.
Roller coasters, balls on slopes and pendulums swap the two stores, with small losses to the thermal store from friction.

Part 5 of 6Applications

Hydroelectric dams
  • Hydroelectric dams water falls through pipes, its gravitational potential energy becomes kinetic, the turbine turns and the generator produces electricity.
Pumped storage
  • Pumped storage spare electricity pumps water uphill, storing energy until demand is high.
Different planets
  • Different planets on the Moon g is about 1.6 N/kg, so the same lift gains much less gravitational potential energy.
Always use the value of g given in the question.

Part 6 of 6More from the specification

1Gravitational work and rate of power
  • Gravitational work and rate of power lifting loads through vertical heights delivers mechanical power, providing a means to calculate potential energy gain.
2Efficiency
  • Useful output divided by total input.
3Surroundings
  • Everything outside the system.
4Wasted energy
  • Energy transferred to stores that are not useful, usually thermal in the surroundings.
5Transfer pathway
  • The way energy moves between stores, such as heating or work done.
6Defining the system
  • Defining the system choosing the system first gives a start and end point for the energy transfers you must consider.

Key terms

TermMeaning
Gravitational potential energyEnergy stored because of the height of an object in a gravitational field.
Gravitational field strength (g)Force per unit mass; 9.8 N/kg on Earth.
Vertical heightHeight difference measured straight up or down.
Reference levelChosen height where the energy is taken as zero.
Free fallMotion under gravity alone.
WeightForce of gravity on an object, W = mg.

Avoid these mistakes

1
Using the length of a ramp instead of the vertical height.
2
Forgetting g or using the mass in grams — use kg.
3
Thinking a heavier object reaches a higher speed when dropped — mass cancels.
4
Believing gravitational potential energy has a true zero — it is measured from a chosen level.
5
Mixing up mgh with ½mv².

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