What is Potential Energy?
Gravitational potential energy is the stored energy an object has because of its height above a reference point. A book on a shelf, water behind a dam, or a person at the top of a hill all have potential energy ready to convert into motion.
Potential energy is PE = mgh, measured in joules (J), where m is mass in kg, g is gravitational acceleration (9.8 m/s² on Earth), and h is height above a chosen reference point.
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Step-by-step worked examples
A 2 kg book sits 3 m above the floor. Find its potential energy (g = 9.8 m/s²).
PE = m×g×h = 2×9.8×3 PE = 58.8 J
A 50 kg elevator is raised 10 m. Find its potential energy.
PE = m×g×h = 50×9.8×10 PE = 4900 J
A 5 kg object is lifted 2 m. Find its potential energy.
PE = m×g×h = 5×9.8×2 PE = 98 J
Flashcards
Quick quiz
Q1.A 5 kg object is 4 m above the ground (g = 9.8 m/s²). Find its potential energy.
Q2.Potential energy depends on which factors?
Q3.How is the reference point (h = 0) for potential energy chosen?
Q4.If height doubles and other factors stay constant, potential energy…
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Common mistakes
Forgetting to measure height from a defined reference point. — Correct: Height must always be measured relative to a chosen zero level.
Using g in the wrong units (e.g. m/s instead of m/s²). — Correct: g is an acceleration, measured in m/s².
Mixing up the potential energy and kinetic energy formulas. — Correct: PE = mgh (height-based); KE = ½mv² (motion-based) — they are different.
Ignoring sign when an object is below the reference point. — Correct: Below the reference, h — and PE — is negative.
FAQ
What is potential energy?
Potential energy is stored energy an object has due to its position, most commonly its height above a reference point: PE = mgh.
What is the potential energy formula?
PE = mgh, where m is mass in kg, g is gravitational acceleration in m/s², and h is height in metres, giving energy in joules.
How do you calculate potential energy?
Multiply mass by gravitational acceleration by height. Example: a 2 kg object at 3 m has PE = 2×9.8×3 = 58.8 J.
What are examples of potential energy?
A book on a shelf, water held behind a dam, a stretched bow, and a ball held above the ground all have potential energy.