Study guides & original practice
Gravitational potential energy practice
Near Earth’s surface, a change in gravitational potential energy is ΔU = mgΔh when g can be treated as constant. The chosen zero height is a reference, not a physical boundary.
By ExamCat · Original study lesson · Published September 24, 2026
Understand the method
Only changes in potential energy matter for many energy calculations. If gravity is the only force doing work, a decrease in gravitational potential energy equals an increase in kinetic energy. Friction or applied forces require additional energy accounting.
Worked example, step by step
A 2 kg object is lifted vertically by 3 m. Use g = 9.8 m/s².
- Calculate the energy change: 2 × 9.8 × 3 = 58.8 J.
- The sign is positive because height increases.
- If lifted at constant speed, the lifting force does approximately +58.8 J while gravity does −58.8 J.
- Its kinetic energy does not change at constant speed, so the net work is zero despite the potential-energy increase.
A mistake to watch for
Do not set the work of the lifting force equal to net work without considering gravity. Potential energy belongs to the gravitational interaction; it is not an extra force added to the force diagram.
Try it yourself: three questions
Write an answer before opening the explanation. For calculations, keep your units and intermediate steps; for concepts, state the reason for your choice.
1. A 1 kg object drops 5 m. Find ΔU using g = 9.8 m/s².
Show answer and explanation
−49 J. Height change is negative.
2. If that object starts from rest and air resistance is negligible, what is its final kinetic energy?
Show answer and explanation
49 J, because the lost gravitational potential energy becomes kinetic energy.
3. Does changing the reference height change ΔU between the same endpoints?
Show answer and explanation
No. Both endpoint values shift by the same constant, leaving their difference unchanged.
Build your next practice session
Mark the first step where your reasoning differed from the explanation. Turn that step into a question, answer it without looking, then change one condition in the example and explain what would change. A correct guess is a reason to revisit the method.
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Further reading
OpenStax Physics provides textbook background. The worked scenario and practice questions on this page were written independently for ExamCat.
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