OCR A Level Physics: Development of Practical Skills — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Practical Skills revision notes and the full study guide.

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Questions

1. Distinguish between a control variable and a control experiment. [2]

2. Distinguish between repeatable and reproducible results. [2]

3. A student investigates how the period of a mass–spring system depends on mass, using T = 2π√(m/k).

(a) Identify the independent, dependent and one control variable. [3] (b) State what should be plotted to obtain a straight line through the origin. [2] (c) State what the gradient of that graph represents. [2]

4. The student’s line of best fit is straight but has a small positive y-intercept.

(a) State what this suggests. [1] (b) Suggest a physical cause specific to this experiment. [2]

5. A diameter is measured five times with a micrometer: 0.42, 0.43, 0.42, 0.51, 0.42 mm.

(a) Identify the anomaly and state what should be done with it. [2] (b) Calculate the mean, excluding the anomaly. [2] (c) State the uncertainty, given the micrometer reads to 0.01 mm. [1] (d) Calculate the percentage uncertainty. [2]

6. For each improvement, give the specific reason it helps:

(a) using light gates rather than a stopwatch [2] (b) measuring the diameter at several points along a wire [2] (c) using a fiducial marker at the equilibrium position [2]

7. A pendulum of length 0.800 m ± 0.005 m gives a period of 1.79 s ± 0.02 s, used to find g via T = 2π√(l/g).

(a) Calculate the percentage uncertainty in l. [2] (b) Calculate the percentage uncertainty in T, and hence in T² (the quantity g depends on). [2] (c) State which measurement limits the accuracy of g, and suggest the most effective way to reduce its uncertainty. [2]

8. A student records a results table with the unit written after every individual value instead of once in the column heading, and measures the diameter of a thin wire with a metre rule rather than a micrometer. Identify what is wrong with each choice, and state the correct approach. [4]

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Answers

1. A control variable is a quantity kept constant so it does not affect the result [1]. A control experiment is a parallel run with the independent variable absent, to show the effect observed is genuinely caused by that variable and not by something else [1].

2. Repeatable — the same person gets the same result using the same method and equipment [1]. Reproducible — the same result is obtained by a different person, a different method, or different equipment [1].

3. (a) Independent: mass [1]. Dependent: period [1]. Control: the spring constant / same spring, or the amplitude of oscillation [1]. (b) Square both sides: T² = (4π²/k)m [1], so plot T² against m, giving a straight line through the origin [1]. (c) Gradient = 4π² ÷ k [1], so k can be found from it [1].

4. (a) A systematic error [1]. (b) The mass of the spring itself has been neglected [1], since part of the spring also oscillates and effectively adds to the load’s mass, contributing even at m = 0 [1].

5. (a) 0.51 mm [1]; it should be excluded from the mean and, ideally, the measurement repeated to confirm the remaining readings [1]. (b) (0.42 + 0.43 + 0.42 + 0.42) ÷ 4 [1] = 0.4225 ≈ 0.42 mm [1]. (c) ±0.005 mm — half the smallest division [1]. (d) (0.005 ÷ 0.4225) × 100 [1] = 1.18% [1].

6. (a) They eliminate human reaction time [1], which is primarily a random error but affects manual timing on every reading [1]. (b) The wire may not be uniform along its length [1], so a mean gives a more representative value and reduces random error [1]. (c) The object moves fastest at the equilibrium position [1], so timing there is most consistent and the uncertainty in identifying the moment is smallest [1].

7. (a) (0.005 ÷ 0.800) × 100 [1] = 0.63% [1]. (b) (0.02 ÷ 1.79) × 100 = 1.12% [1]; g depends on T², so the power rule doubles this to 2.24% [1]. (c) Timing dominates the overall uncertainty, contributing far more than the length measurement [1]; timing 20 oscillations and dividing by 20 would reduce it far more effectively than a more precise ruler [1].

8. The unit should be given once, in the column heading, not repeated beside every value [1]. A metre rule cannot resolve a thin wire’s diameter precisely enough; a micrometer, with a much finer resolution appropriate to the size of the quantity, should be used instead [1] [1] — the instrument’s resolution must always match the size of what is being measured [1].

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Where marks are usually lost

• Confusing control variables with a control experiment.
• Not squaring or rearranging to obtain a straight-line plot.
• Including an anomaly in a mean.
• Giving improvements without the specific reason they help.
• Trying to reduce a systematic error by “repeating and taking a mean” — repeats only reduce random error, never systematic error.
• Forgetting that percentage uncertainties multiply by the power when a measured quantity is raised to a power in the equation, as T is when g depends on T².