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Lesson · Grade 10 · Science

Gravitational Potential Energy: Ep = mgh

  • Grade 10
  • Science
  • 1 min 59 s
  • British Columbia
  • BC10-SC-37

A 10 kg box is lifted onto a shelf 2 m up and Ep = mgh is filled in one term at a time to get 196 J, then lifting it to 4 m gives 392 J, showing that doubling the height doubles the energy. Measured from the lower shelf instead of the floor, the same unmoved box has only 196 J, showing that Ep depends on the chosen reference level and not on the box alone.

Concept: Gravitational potential energy: mass, gravity and height

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In one sentence

The question this lesson answers

When you lift a box onto a shelf, what energy does it gain, and how much?

Gravity pulls this 10 kilogram box down, toward Earth.

Transcript

What the lesson says

When you lift a box onto a shelf, what energy does it gain, and how much?

Gravity pulls this 10 kilogram box down, toward Earth.

To lift it 2 metres onto the shelf, you push up against that pull.

The box gains gravitational potential energy, E p: energy stored because of its height.

To calculate it, use E p equals m times g times h.

The m stands for mass, in kilograms. Our box is 10 kilograms.

The g is the acceleration due to gravity: about 9.8 metres per second squared on Earth.

The h is the height in metres above a reference level, where height is zero. Here that is the floor, so h is 2 metres.

Energy is measured in joules, written J.

So E p equals 10 times 9.8 times 2.

That is 196 joules, measured from the floor.

Now lift the same box twice as high, to 4 metres.

This time, E p equals 10 times 9.8 times 4, which is 392 joules.

Double the height, double the energy: 392 is 2 times 196.

But choose the lower shelf as the reference level instead, and the box is only 2 metres up.

From there, E p is 10 times 9.8 times 2, which is just 196 joules.

The box has not moved, yet E p changed: its value depends on the reference level you choose.

E p equals m g h, with h measured from a chosen reference level.

Curriculum

Where this fits

British Columbia, Grade 10

Science 10, Physics: potential and kinetic energyBC10-SC-37

Previous: Conservation of Energy: Tracking the TotalNext: Kinetic energy: why doubling speed gives four times the energy

The outcomes and strands follow the British Columbia curriculum.

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