A-LevelPhysicsDeformation of solids9702/32 · May/June 2020 · Q220 Marks

In this experiment, you will investigate the bending of a rod under a compressive force.

(a)You are provided with two thin rods. Measure and record the length L and diameter d of the longer rod.[2]
(b)• Assemble the apparatus as shown in Fig. 2.1 using the longer rod. Position the ends of the rod in the shallow holes in the wooden strip and the block. • Adjust the apparatus so that the rod is vertical and the wooden strip is parallel to the bench. • The distance between the nail and the top of the rod is x and the distance between the nail and the string loop is y, as shown in Fig. 2.1. Measure and record x and y.[1]
(c)• Slowly pull the newton meter down until the rod bends in the middle by approximately 1 cm, as shown in Fig. 2.2. • Record the force Fm needed to bend the rod by approximately 1 cm.[2]
(d)Estimate the percentage uncertainty in your value of Fm. Show your working.[1]
(e)Calculate the force F exerted on the rod using F = yFm / x[1]
(f)Justify the number of significant figures you have given for your value of F.[1]
(g)• Measure and record the length L and diameter d of the shorter rod. • Set up the apparatus as shown in Fig. 2.1 using the shorter rod. • Adjust the apparatus so that the rod is vertical and the wooden strip is parallel to the bench. • Repeat (c) and (e).[2]
(h)(i)It is suggested that the relationship between F, L and d is E = 64FL² / (π³d⁴) where E is a constant. Using your data, calculate two values of E.[1]
(h)(ii)Explain whether your results in (i) support the suggested relationship.[1]
(i)(i)Describe four sources of uncertainty or limitations of the procedure for this experiment. 1. 2. 3. 4.[4]
(i)(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 2020). The mark or mark type is shown in brackets.

[1] Value of L to nearest mm, with unit.
[1] Value of d to the nearest 0.01 mm and in the range 2.00–5.00 mm, with unit.
[1] Values of x and y, with x < y.
[1] Value for Fm, with unit, to nearest 0.1 N.
[1] Evidence of repeated readings of Fm.
[1] Percentage uncertainty in Fm based on an absolute uncertainty of 0.2-0.5 N. 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] Correct calculation of F.
[1] Justification based on s.f. in Fm, x and y.
[1] Second values of L, d and Fm.
[1] Quality: Second Fm > first Fm.
[1] Two values of E calculated correctly.
[1] Valid comment relating to the calculated values of E, testing against a criterion specified by the candidate.
[1] Two readings are not enough to draw a (valid) conclusion (not "not enough for accurate results”, “few readings").
[1] Difficult to measure 1 cm bend.
[1] Rod bends suddenly (so difficult to stop at 1 cm bend).
[1] (Whole) weight of newton meter not included in Fm.
[1] Large percentage uncertainty in Fm (for longer rod).
[1] Repeat values of Fm differ because rod stays bent.
[1] Difficult to measure deflection and Fm at the same time.
[1] Take more readings and plot a graph or take more readings and compare k values (not "repeat readings" on its own).
[1] Clamp ruler/use grid behind rod/workable method using stop.
[1] Use motor/other workable method to pull newton meter slowly.
[1] Zero then weigh newton meter and add to reading, or similar.
[1] Increase x/decrease y (for longer rod).
[1] Provide more identical rods.
[1] Video rod and newton meter and replay.