Gravitational Potential Energy (Cambridge (CIE) IGCSE Physics): Revision Note

Note that $∆h$ is always the change in vertical height, not the distance travelled along a slope or ramp.

Remember that the change in gravitational potential energy has units of joules (J). Any mass given in grams must be converted into kilograms before substituting.

A man climbs a flight of stairs that is, in total, 3.0 m higher than the floor. The man has a mass of 72 kg, and the gravitational field strength on Earth is approximately 9.8 N/kg.

Calculate the energy transferred to the man's gravitational potential energy store.

Answer:

Step 1: List the known quantities

• Mass of the man, $m=72\mathrm{kg}$

• Gravitational field strength, $g=9.8\mathrm{N}/\mathrm{kg}$

• Change in height, $∆h=3.0\mathrm{m}$ 

Step 2: Write down the equation for gravitational potential energy

$∆E_p=mg∆h$

Step 3: Calculate the gravitational potential energy

$∆E_p=72\times 9.8\times 3.0$

$∆E_p=2116.8\mathrm{J}$

Step 4: Round to an appropriate number of significant figures

• The input values are to 2 s.f. so the answer is also given to 2 s.f.

$∆E_p=2100\mathrm{J}\left(2\mathrm{s}.\mathrm{f}.\right)$

In your IGCSE exam, you will generally be expected to round your answers to an appropriate number of significant figures. In theory papers, any answer given to two or more significant figures is accepted unless the question says otherwise.

Your answer can only be as precise as your least precise input value. Therefore, your final answer should round to the lowest number of significant figures of all your input values.