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
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For calculations, show all steps clearly and include units. For explanations, be concise but thorough, addressing all parts of the question. Always check your work for consistency in units and logical flow.
What the command words demand
- Calculate
- Perform a numerical calculation using the given data and appropriate formula.
- Explain
- Provide a clear and concise explanation, often including reasons or causes.
- Describe
- Give a detailed account of something without necessarily explaining why it happens.
- Estimate
- Make an approximate calculation based on given information.
- Determine
- Find out or establish the value or nature of something.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 2-3 minutes per mark. For a 4-mark question, spend approximately 8-12 minutes.
- Identify the given values: m = 5 kg, g = 9.81 m/s 2 , h = 10 m0 marks
- Use the formula for GPE: E GPE = mgh1 mark
- Substitute the values into the formula: E GPE = 5 × 9.81 × 101 mark
- Calculate the result: E GPE = 490.5 J1 mark
Final answer: 490.5 J
Work through every step correctly and you earn all 3 marks.
Another worked example
A car of mass 1200 kg is moving at a speed of 30 m/s. Calculate the kinetic energy (KE) of the car.
- Identify the given values: m = 1200 kg, v = 30 m/s0 marks
- Use the formula for KE: E KE = ½ mv 21 mark
- Substitute the values into the formula: E KE = ½ × 1200 × 30 21 mark
- Calculate the result: E KE = 540,000 J1 mark
Final answer: 540,000 J
Work through every step correctly and you earn all 3 marks.
Common mistakes
Forgetting to include the ½ in the kinetic energy formula.
Why it happens: Students often forget that the kinetic energy formula includes a factor of ½. This can lead to incorrect calculations and loss of marks.
Fix: 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.
Why it happens: Students sometimes use inconsistent units (e.g., using kilograms and meters but forgetting to convert velocities from km/h to m/s). This can lead to incorrect answers.
Fix: 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.
Why it happens: Students may overlook the fact that work done is a scalar quantity and can be positive or negative depending on the direction of the force relative to the displacement.
Fix: 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.
Why it happens: In real-world scenarios, some energy is always lost due to resistive forces. Students may forget to account for these losses in their calculations.
Fix: 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.
Why it happens: Students sometimes confuse GPE with kinetic energy and apply the wrong formula. For example, they might apply the kinetic energy formula E KE = ½ mv 2 to a height-change problem instead of the correct GPE formula E GPE = mgh.
Fix: 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.
Why it happens: Students may forget the conversion factor between kcal and joules, leading to incorrect calculations of energy derived from food.
Fix: Always remember that 1 kcal = 4184 J. Convert kilocalories to joules before performing any calculations involving energy consumption.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Calculate Energy | Calculate gravitational potential energy using mass, gravity and height with correct units. | 3 |
| Calculate Kinetic Energy | Use the kinetic energy formula to find the car's KE with correct units. | 3 |
| Calculate Work Done | Multiply the applied force by the distance moved to find work done, with units. | 2 |
| Calculate Kinetic Energy | Find the ball's kinetic energy just before impact using energy conservation from its dropped height. | 4 |
| Conservation Of Energy | Use energy conservation to find an object's speed after falling from a given height. | 5 |
| Total across these question types | 17 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.