A-LevelPhysicsPhysical quantities and units9702/31 · Oct/Nov 2018 · Q220 Marks

In this experiment, you will investigate the movement of a chain of paper clips.

(a)(i)You have been provided with a chain of paper clips. Measure and record the mass M of the chain.[1]
(a)(ii)Using your answer in (a)(i), calculate the mass m of one paper clip.[1]
(a)(iii)Justify the number of significant figures that you have given for your value of m.[1]
(b)(i)You have been provided with two identical spheres of modelling clay. Measure and record the mass S of one of the spheres.[1]
(b)(ii)Calculate c where c = S/m.[1]
(c)(i)Place the wooden strip on the bench so that one end of the strip overhangs the bench, as shown in Fig. 2.1. Place the chain on the wooden strip and place one of the spheres on the paper clip at the end of the chain, as shown in Fig. 2.2. The sphere must not touch the wooden strip. Gradually move the paper clips off the end of the wooden strip so that they hang down, as shown in Fig. 2.3. When the chain starts to slip, count and record the number p of paper clips hanging off the end of the wooden strip and the number q of paper clips remaining on the strip.[2]
(c)(ii)Estimate the percentage uncertainty in your value of p.[1]
(d)Replace the chain on the wooden strip and place both spheres on the paper clips at the end of the strip, as shown in Fig. 2.4. Determine p and q.[2]
(e)(i)It is suggested that the relationship between p, q and c is p = k(q + nc) where n is the number of spheres on the chain and k is a constant. Using your data, calculate two values of k.[1]
(e)(ii)Explain whether your results support the suggested relationship.[1]
(f)(i)Describe four sources of uncertainty or limitations of the procedure for this experiment.[4]
(f)(ii)Describe four improvements that could be made to this experiment. You may suggest the use of other apparatus or different procedures.[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 (Oct/Nov 2018). The mark or mark type is shown in brackets.

[1] Value of M with unit and to the nearest 0.1 g or better.
[1] Correct calculation of m dividing the value in (a)(i) by 25.
[1] Justification for significant figures in m linked to the s.f. in M.
[1] Value of S in range 1.0g < S ≤ 2.0g with consistent unit.
[1] Correct calculation of c.
[1] Values p and q such that p + q = 25.
[1] Evidence of repeated values of p.
[1] Percentage uncertainty in p based on absolute uncertainty of 1 or 2. 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.
[1] Second values of p and q.
[1] Quality: second value of q less than first value of q.
[1] Two values of k calculated correctly.
[1] Valid comment consistent with calculated values of k, testing against a criterion stated by the candidate.
[1] Two readings are not enough to draw a (valid) conclusion (not "not enough for accurate results”, “few readings”).
[1] Difficulty linked to board during movement of clips e.g. paper clips flip over when moving affecting friction/clips move to the side/clips catch on end of board/board irregular and clips catch on edge/board moves on table as well as clips.
[1] Difficulty linked to placing or keeping the spheres on the clips e.g. spheres fall off/clips too small to hold the sphere/ difficult to get two neighbouring paper clips level enough to take the two spheres/spheres touch table.
[1] Difficulty linked to size of clips with a reason e.g. large increments of mass gives large uncertainty in the value of p or q/the exact force needed to cause movement may not be equal to a whole number of clips.
[1] Masses of clips or spheres may be different.
[1] Take many readings (for different values of n) and plot a graph or take more values of k and compare.
[1] Workable method e.g. provide a guide/over a pulley wheel/tape or clamp board to bench/sand the end of the board/sand out irregularities.
[1] Workable method of adding/holding spheres e.g. use cuboid/disc shaped plasticine/tie paper clips together with thin cotton to keep flat.
[1] Improved method to deal with large clips e.g. use more and smaller/lighter clips.
[1] Check masses using a balance.