A-LevelPhysicsForces, density and pressure9702/31 · May/June 2019 · Q220 Marks

In this experiment, you will investigate the upthrust on lids placed in water.

(a)(i)You have been provided with two lids and some coins. • Take the larger of the two lids. • The diameter of the lid is d. The height of the lid is t, as shown in Fig. 2.1. Measure and record d and t. d = t =[1]
(a)(ii)Estimate the percentage uncertainty in your value of d. percentage uncertainty =[1]
(b)(i)The volume of the air space within the lid is V. Calculate V where V= πα²t / 4 V=[1]
(b)(ii)Justify the number of significant figures that you have given for your value of V.[1]
(c)• Place the lid on the surface of the water so that it floats with its open face upwards. • Add coins to the inside of the lid. After you have added n coins, the lid will sink. • Record n. n =[2]
(d)Take the smaller lid and repeat (a)(i), (b)(i) and (c). d = t = V = n =[3]
(e)(i)It is suggested that the relationship between n and V is n = kV where k is a constant. Using your data, calculate two values of k. first value of k = second value of k =[1]
(e)(ii)Explain whether your results support the suggested relationship.[1]
(f)Theory suggests that k = ρ/M where the density ρ of water is 1.00gcm⁻³ and M is the mass of a coin. Using your second value of k, calculate M. Give an appropriate unit. M =[1]
(g)(i)Describe four sources of uncertainty or limitations of the procedure for this experiment. 1. 2. 3. 4.[4]
(g)(ii)Describe four improvements that could be made to this experiment. You may suggest the use of other apparatus or different procedures. 1. 2. 3. 4.[4]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

2(a)(i) Value(s) of raw d and raw t to nearest mm with units.
2(a)(ii) Percentage uncertainty in d based on an absolute uncertainty in the range 2–5 mm. If repeated readings have been taken, then the uncertainty can be half the range (but not zero) if the working is clearly shown. Correct method of calculation to obtain percentage uncertainty.
2(b)(i) Correct calculation of V.
2(b)(ii) Justification for s.f. in V linked to s.f. in d and t.
2(c) Value of n.
2(c) Evidence of repeated n.
2(d) Second values of d and t.
2(d) Second value of n.
2(d) Quality: Second value of n less than first value of n.
2(e)(i) Two values of k calculated correctly. The final k values must not be fractions.
2(e)(ii) Valid comment consistent with calculated values of k, testing against a criterion stated by the candidate.
2(f) Correct unit and calculation of M using second k, and in the range 1.0-10.0 g.
2(g)(i) A Two readings are not enough to draw a (valid) conclusion (not "not enough for accurate results”, “few readings”).
2(g)(i) B Lid tilts/sinks more quickly with a reason e.g. cannot stack in one place/tendency to drop coins heavily/force added by hand.
2(g)(i) C Lid has a curved edge leading to inaccurate d/t/V.
2(g)(i) D Parallax error in t.
2(g)(i) E Large percentage uncertainty in t as t is small.
2(g)(i) F Mass of lid needs to be known/masses of two lids are different.
2(g)(i) G Coins have different mass or Coins or lid heavier when readings repeated because they are wet.
2(g)(ii) A Take more readings and plot a graph or take more readings and compare k values (not "repeat readings" on its own).
2(g)(ii) B Improved method of loading coins e.g. tweezers/tongs/forceps/guide for coins mounted centrally.
2(g)(ii) C Improved method of finding V e.g. displacement method/filling with liquid using syringe or pipette.
2(g)(ii) DE Improved method of measuring t e.g. (vernier/digital) calipers/travelling microscope/two blocks/use many stacked lids of the same type.
2(g)(ii) F Method to measure mass e.g. balance/scales.
2(g)(ii) G Method to ensure coins or lid are dry e.g. allow water to evaporate/use hair dryer/use fresh coins or lid each time.