A student wants to estimate the mass of a domestic cat. Explain how the student could arrive at a reasonable estimate without directly measuring the cat, using everyday objects as references.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Identify a common object of known mass, e.g. a 5‑kg bag of sugar.
Estimate the cat’s mass as 4–5 kg.
Explain that the student can compare the cat’s weight to the bag by lifting both or by feeling the resistance when lifting the cat, noting that the cat feels about as heavy as the bag.
State that the estimate is approximate and will have some uncertainty.
Mention that the student’s prior knowledge that a newborn baby is 3–4 kg helps justify the 4–5 kg estimate for a cat.
Estimate the cat’s mass as 4–5 kg.
Explain that the student can compare the cat’s weight to the bag by lifting both or by feeling the resistance when lifting the cat, noting that the cat feels about as heavy as the bag.
State that the estimate is approximate and will have some uncertainty.
Mention that the student’s prior knowledge that a newborn baby is 3–4 kg helps justify the 4–5 kg estimate for a cat.
Examiner tips
- Use a specific everyday reference mass and state it clearly. Give a numerical estimate within the accepted range. Describe the comparison method. Acknowledge uncertainty. Link to prior knowledge of similar masses.
Common mistakes
- Choosing an object with an unknown or incorrect mass. Giving a value outside the 3–6 kg range. Failing to explain how the comparison is made. Ignoring the uncertainty or range of the estimate. Not relating the estimate to prior knowledge.
Mark scheme (5 marks)
- Identifies a known reference mass (e.g. a bag of sugar or similar everyday object of known mass)
- States a reasonable estimated mass for a domestic cat (accept 3 kg to 6 kg)
- Explains how the reference object is used to judge the cat's mass (e.g. comparing by feel / lifting both)
- Recognises that the estimate may not be exact / acknowledges the uncertainty or range in the estimate
- Links the estimation process to using prior knowledge or experience of similar quantities (e.g. recalling that a newborn baby is about 3–4 kg so a cat is similar)
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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