Recall and use the equation to calculate the change in gravitational PE…

The idea

This point requires you to recall the equation for the change in gravitational potential energy when an object is lifted, and to use it in calculations. The change in gravitational potential energy in joules equals mass in kilograms multiplied by gravitational field strength in newtons per kilogram multiplied by the change in vertical height in metres: ΔGPE = m × g × Δh. On Earth g is about 9.8 N/kg, often rounded to 10 N/kg. For example, lifting a 2 kg box by 3 m gives ΔGPE = 2 × 10 × 3 = 60 J. Only the vertical change in height matters, not the path taken. Keep units consistent, convert grams to kilograms and centimetres to metres, and give the answer in joules.

What earns the marks

• Recalls the equation ΔGPE = m × g × Δh with mass in kilograms, gravitational field strength in newtons per kilogram and height in metres.
• Substitutes the correct values into the equation, converting mass to kilograms and height to metres where needed.
• Calculates the change in gravitational potential energy correctly, for example 2 × 10 × 3 = 60 J for a 2 kg object raised 3 m.
• Gives the answer in joules and recognises that only the vertical change in height contributes to the change in gravitational potential energy.

Common mistakes and how to put them right

• Uses the total distance travelled rather than the vertical change in height.

  Put it right: Use only the change in vertical height.

• Leaves mass in grams or height in centimetres.

  Put it right: Convert to kilograms and metres before substituting.

• Omits the unit or gives the answer in newtons.

  Put it right: State the change in gravitational potential energy in joules.

Exam tips

• Write the equation ΔGPE = m × g × Δh before substituting values so the method is clear.
• Check that g is in N/kg and use the value given in the question, often 9.8 N/kg or 10 N/kg.
• Convert all quantities to SI units first, then substitute and state the answer in joules.