The mass of the cylinder was generally indicated correctly and quoted to the correct precision. The majority of candidates repeated their measurement of the diameter of the can and obtained a reasonable value. The better candidates stated that they had determined the diameter at different points along the can or that diameter measurements had been made at right angles to each other; only good candidates could describe the correct technique for the diameter measurement. Examiners expected the set squares and rule to be used like the jaws of vemier calipers with the axis of the can perpendicular to the plane containing the set squares. However, a significant number of candidates had the axis of the can parallel to the plane containing the set squares and gained no marks. Candidates who placed the planes of the set squares parallel and tangential to the sides of the can also gained no marks. The majority of candidates obtained full marks for the velocity determination. When the moment of inertia was determined it was necessary to convert both h and v to SI units as the value of g was given as 9.81 m s-2 on the data sheet. It was not necessary to convert m to kg, provided that the unit of the moment of inertia was consistent with this. Most candidates omitted units or had the wrong units for the value of the moment of inertia. The term in brackets must have no units as the number 1 has no units, hence the units of moment of inertia could have easily been found from md2, i.e. kg m2.
The determination of percentage uncertainties was disappointing. Most candidates could quote Dh as a value between 0.5 mm and 2 mm, although significant mistakes included 5 mm (0.005 m) and 0.05 mm, but Dt was a problem for a large number of candidates. The majority quoted 0.01 s (the precision of the stopwatch) rather than their range of values. Where the range of values was very small, Examiners expected candidates to have some appreciation of reaction time errors in the stopwatch reading and would only accept values in the range 0.05 s to 0.2 s. If the percentage uncertainty in x were negligible then the percentage uncertainty in v would be the same as that in t. Few candidates realised this - perhaps the factor of 2 in the formula for v confused them. Even fewer candidates were able to combine the percentage uncertainties correctly to determine the percentage uncertainty in the expression and only the best candidates determined the absolute uncertainty in the expression by multiplying their percentage uncertainty by the value of the expression. Virtually no candidates could determine the contribution which this made to the uncertainty in the moment of inertia.
The circuit was generally set up correctly and most candidates obtained a reasonable set of values for the a.c. and d.c. voltages. Not all candidates used the full range of the power supply and some candidates did not quote their voltage readings to the precision of the meter that they were using. Most candidates were able to exhibit all the features of the curve from their measurements. Occasionally Vxy values were given negative signs because the voltmeter had been connected the wrong way round and very occasionally very low d.c. voltages were obtained, possibly because the range of the meter had not been changed from a.c. to d.c. Vo was usually determined correctly, but few candidates realised that as the diode and resistor were in series, 0.7 V had to be subtracted from Vo in order to obtain the peak value of Vxy. Most candidates determined the time constant correctly, but units were sometimes omitted or quoted as WF, which was deemed unacceptable for a "time". The period of supply was usually determined correctly, but again units were sometimes omitted. Because it was felt that PH7 (AS) candidates would not necessarily be familiar with the smoothing action of a capacitor, Examiners allowed any sensible comments about the curve for the last two marks.
The position of the line containing the centre of gravity of the stand was determined correctly by the majority of candidates, as was the initial reading on the newton meter. In the precautions a number of candidates stated that they had checked the zero error of the meter before the rod of the stand was attached to it. Better candidates said that they had checked that the rod was horizontal by measuring the height of the rod above the bench at each end. Some credit was given to candidates who checked that the rod was horizontal by checking that the base of the stand was perpendicular to the bench by using the set square. However, this was felt not to be as good as the technique of measuring two heights. In explaining the given expression, many candidates omitted to say that they were taking moments about P; most simply stated that the clockwise moments were equal to the anticlockwise moments. A small number of candidates thought that the expression came from the equilibrium of forces. In describing the experiment, most candidates realised that x and T were the variables but a large number of the lower ability candidates thought that a graph of T against x would yield a straight line. Most candidates took an adequate number of accurate measurements, but only the better ones investigated the full range of values by including at least one x value which was greater than 50 cm and at least one T value which was greater than or equal to 8.0 N. When a graph of T against 1/x was plotted, the plotted data often lay on a good straight line, but the candidates often forced this line through the origin. This made commenting on the graph easier as a straight line through the origin was expected. Often, however, the value of the gradient of the forced line led to an inaccurate value for the weight of the stand. Those candidates who did not mention the origin when commenting on their graph frequently scored 0 out of 3. Those candidates who plotted the appropriate graph could correctly determine the weight of the stand, although this was not always in the range of expected values because of the difficulties stated above. Those candidates who plotted T against x typically lost:
(a) 1 mark in the observations section for not finding 1/x
(b) 3 marks in the comments section
(c) 4 marks when determining the weight of the stand because the gradient which they determined could not be related to the weight.
On balance they would probably have lost fewer marks if they had asked for the card.
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