AQA GCSE Physics Physics Paper 2 (Higher), June 2022: Question 5

✅ Correct Answer

Either of the following standard AQA definitions:

• The point from which weight may be considered to act.
• The point where the mass appears to be concentrated.

💡 Key Knowledge

• Every object behaves as if all its mass is focused at one single point.
• Gravity pulls down on all parts of an object, but we represent the total weight force as an arrow drawn downwards directly from the centre of mass.

🧠 Exam Technique

Be careful not to mix up "mass" and "weight". Notice how the words pair up:

• "Point where mass is concentrated"
• "Point from which weight acts"

❌ Common Errors

• Vague answers like "the exact middle of the tomato" (only true for symmetrical objects of uniform density).
• Saying "where gravity starts" (unscientific phrasing that scores 0).

📐 Step-by-Step Calculation

1 Read scale and convert mass to kg:
Pointer indicates 425 g (each major mark is 100 g; the needle points halfway between 400 g and 450 g).
Total mass = 425 ÷ 1000 = 0.425 kg [1 mark]

2 Find mass of one tomato:
There are 5 identical tomatoes:
Mean mass = 0.425 kg ÷ 5 = 0.085 kg [1 mark]

3 Calculate weight using W = mg:
W = 0.085 kg × 9.8 N/kg = 0.833 N [1 mark]

❌ Calculation Traps

• Forgetting to divide by 5: Calculating the weight of all 5 tomatoes (4.165 N) instead of the mean weight of a single tomato.
• Scale misread: Mistaking the pointer reading as 450 g or 420 g. Look closely at the interval divisions.
• Unit error: Multiplying grams directly by 9.8 (e.g., 85 × 9.8 = 833 N). In physics equations, mass must always be in kilograms (kg)!

📐 Step-by-Step Calculation

1 Calculate compression (extension e):
Compression = Original length - Compressed length
e = 5.0 cm - 3.5 cm = 1.5 cm
Convert to metres: 1.5 ÷ 100 = 0.015 m

2 Substitute into Hooke's Law:
Equation: F = k × e
6.0 = k × 0.015 [1 mark]

3 Rearrange and solve for k:
k = 6.0 ÷ 0.015 [1 mark]
k = 400 N/m [1 mark]

🧠 Exam Technique & Units

• Check the printed unit on the answer line: it specifies N/m.
• If you keep extension as 1.5 cm, your answer would be 4 N/cm, but the given unit line says N/m, which will lose you the final mark! Always convert cm to m first.
• Error Carried Forward (ECF): If you calculated compression wrong (e.g., 2.0 cm), you can still get 2 marks for correctly rearranging and dividing 6.0 by your value.

✅ Acceptable Answers (Choose ONE pair)

Option 1 (Elasticity):

• Property: It deforms elastically [1 mark]
• Explanation: (so) it will return to its original length / shape once the tomatoes are removed [1 mark]

Option 2 (Linearity / Hooke's Law):

• Property: Compression is directly proportional to the force applied [1 mark]
• Explanation: (so) it gives a linear scale / evenly spaced markings / is easy to calibrate [1 mark]

❌ Common Misconceptions

• Writing everyday descriptions like "it is flexible", "it bounces back", or "it is stretchy". Use the technical physics term: deforms elastically.
• Only giving the property without the explanation: stating "it obeys Hooke's Law" only earns 1 mark unless you explain that this provides an evenly spaced / linear dial scale.