A student is investigating the density of different materials. She measures the mass of a rectangular block of aluminium using a balance. She then measures the length, width and height of the block using a ruler to find its volume. She repeats each measurement three times and records her results. Describe how the student should use her repeated measurements to obtain reliable values for mass and volume, and explain why repeating measurements improves the quality of her results.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Density is defined as the mass per unit volume of an object. A student wants to determine the density of an aluminium block as accurately as possible.
Model answer (5 marks)
1. Calculate the mean of the three mass readings to give a single mass value.
2. Calculate the mean of the three length readings, the mean of the three width readings and the mean of the three height readings.
3. Identify any outlier readings that are far from the others and exclude them before calculating the means.
4. Use the three mean dimensions to calculate the volume: V = mean length × mean width × mean height.
5. Repeating the measurements reduces random errors; the mean of the repeated values is closer to the true value and therefore gives a more reliable density.
2. Calculate the mean of the three length readings, the mean of the three width readings and the mean of the three height readings.
3. Identify any outlier readings that are far from the others and exclude them before calculating the means.
4. Use the three mean dimensions to calculate the volume: V = mean length × mean width × mean height.
5. Repeating the measurements reduces random errors; the mean of the repeated values is closer to the true value and therefore gives a more reliable density.
Examiner tips
- Use the word ‘mean’ and show the calculation for each dimension and mass.
- Mention outliers and that they are excluded before averaging.
- Explain that repeated measurements reduce random error and improve accuracy.
Mark scheme (5 marks)
- The student should calculate the mean (average) of the three repeated measurements for each dimension (or mass).
- Any anomalous (outlier) results should be identified and excluded before calculating the mean.
- Use the mean values for length, width, and height to calculate the volume of the block (length × width × height).
- Repeating measurements reduces the effect of random errors.
- Taking the mean of repeated measurements gives a more accurate / reliable value (closer to the true value).
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
More Practical skills assessed questions
- A student is investigating how the resistance of a wire changes with its length.…
- A student is investigating the specific heat capacity of cooking oil. She pours …
- A student investigates the specific latent heat of vaporisation of water. She he…
- A student is investigating how the temperature of water changes as it is heated …