A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Conservation of Energy and Its Applications — mark scheme explained
The short answer
The principle of conservation of energy is a fundamental concept in physics, stating that the total amount of energy in an isolated system remains constant over time. This means that energy can neither be created nor destroyed; it can only change from one form to another.
The question
A 5 kg object is lifted to a height of 10 meters. Calculate the gravitational potential energy (GPE) gained by the object. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Identify the given values: m = 5 kg, g = 9.81 m/s 2 , h = 10 m
- S2
Use the formula for GPE: E GPE = mgh
- S3
Substitute the values into the formula: E GPE = 5 × 9.81 × 10
- S4
Calculate the result: E GPE = 490.5 J
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Identify the given values: m = 5 kg, g = 9.81 m/s 2 , h = 10 m
- S2
Use the formula for GPE: E GPE = mgh
- S3
Substitute the values into the formula: E GPE = 5 × 9.81 × 10
- S4
Calculate the result: E GPE = 490.5 J
Final answer: 490.5 J
Common mistakes
- Forgetting to include the ½ in the kinetic energy formula. — Always remember to use the full formula E KE = ½ mv 2 when calculating kinetic energy.
- Using incorrect units for mass, height, or velocity in calculations. — Always ensure that all units are consistent before performing calculations. Convert units as necessary, especially for velocity.
- Forgetting to consider the direction of forces when calculating work done. — Always check the direction of the force and the displacement. Work done is positive if the force acts in the same direction as the displacement, and negative if it acts in the opposite direction.
- Assuming that all energy is conserved without considering losses due to friction or air resistance. — When applying the principle of conservation of energy, consider any external forces that might cause energy loss, such as friction or air resistance.
- Confusing gravitational potential energy (GPE) with kinetic energy (KE) and applying the wrong formula. — Always double-check the formula for GPE: E GPE = mgh, where m is mass, g is acceleration due to gravity, and h is height.
- Forgetting to convert kilocalories (kcal) to joules when estimating energy from food consumption. — Always remember that 1 kcal = 4184 J. Convert kilocalories to joules before performing any calculations involving energy consumption.
Where the marks go
- Full worked solution (all marking points)3 marks