Cambridge A Level Physics 9702 — 2012 May/June Paper 3 · Variant 2

1 In this experiment, you will investigate the equilibrium position of a suspended mass. Use The apparatus has been set up as shown in Fig. 1.1. nail with cardboard disc spring nail string L plumb-line bench Fig. 1.1 (a) Measure and record the distance L between the two nails. L = ............................................. m [1] (b) (i) Attach the crocodile clip to the string so that the string passes through the gap between the jaws of the crocodile clip, as shown in Fig. 1.2. string gap Fig. 1.2 Suspend the mass hanger and masses from the loop attached to the crocodile clip. For Adjust the position of the crocodile clip so that the distance d from the plumb-line to the Examiner’s string loop is about 30 cm, as shown in Fig. 1.3. Use nail crocodile d clip plumb-line h0 bench Fig. 1.3 (ii) Measure and record d. d = .................................................. m (iii) Measure and record the initial height h0 of the bottom of the mass above the bench, as shown in Fig. 1.3. h0 = ............................................. m [1] (c) Reduce d by moving the crocodile clip closer to the plumb-line, ensuring that the string For passes through the gap as shown in Fig. 1.2. Measure and record d and the new height Examiner’s h of the bottom of the mass above the bench. Repeat until you have six sets of values Use for d and h, with d in the range 5 cm to 30 cm. L 2 Include values of – d and (h – h0) in your table. 2 [10] L 2 (d) (i) Plot a graph of – d on the y-axis against (h – h0) on the x-axis. [3] 2 (ii) Draw the straight line of best fit. [1] (iii) Determine the gradient and y-intercept of this line. gradient = ....................................................... y-intercept = ...................................................... [2] For Examiner’s Use where a and b are constants. Using your answers from (d)(iii), determine the values of a and b. Give appropriate units. a = ....................................................... b = ....................................................... [2] You may not need to use all of the materials provided. For Examiner’s

Mark scheme: 1 (a) Value of L in range 0.80 m > L > 0.60 m. Consistent with unit. [1] (b) (iii) Value of h0, less than 50 cm, to the nearest mm. [1] (c) Six sets of readings of d and h scores 5 marks, five sets scores 4 marks etc. Help from Supervisor –1. [5] Range of d: [1] To include 25.0 cm (0.250 m) or more and 10.0 cm (0.100 m) or less Column headings: [1] Each column heading must contain a quantity and a unit The unit must conform to accepted scientific convention e.g. d / m, d(m) or d in m, (h – h0)/m, (L/2 – d)2/m2 Consistency: [1] All values of d and h must be given to the nearest mm. Significant figures: [1] All values of (L/2 – d)2 to 2 or 3 s.f. Calculation: [1] Values of (L/2 – d)2 calculated correctly. (d) (i) Axes: [1] Sensible scales must be used, no awkward scales (e.g. 3:10). Scales must be chosen so that the plotted points occupy at least half the graph grid in both x and y directions. Scales must be labelled with the quantity which is being plotted. Scale markings must be no more than 3 large squares apart. Plotting of points: [1] All observations in the table must be plotted. Diameter of plots must be < half a small square (no blobs). Plots must be accurate to half a small square. Quality: [1] All points in the table must be plotted (at least 5) for this mark to be awarded. Scatter of points must be less than 0.5 cm (0.005 m) of (h – h0) of a straight line. (ii) Line of best fit: [1] Judge by balance of all points on the grid about the candidate’s line (at least 5 points). There must be an even distribution of points either side of the line along the full length. Allow one anomalous point only if clearly indicated by the candidate. Line must not be kinked or thicker than half a small square. GCE AS/A LEVEL – May/June 2012 9702 32 (iii) Gradient: [1] The hypotenuse of the triangle must be at least half the length of the drawn line. Both read-offs must be accurate to half a small square in both x and y directions. Do not allow ∆x / ∆y. y-intercept: [1] Either: Check correct read off from a point on the line and substituted into y = mx + c. Read off must be accurate to half a small square in both x and y directions. Or: Check read-off of the intercept directly from the graph. (e) Value of a = candidate’s gradient. Value of b = candidate’s intercept. [1] Unit for a (e.g. m) and b (e.g. m2) consistent with values. [1] [Total: 20]

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2 In this experiment, you will investigate the adhesive strength of Blu-Tack. Use (a) (i) You have been provided with two plastic rulers, one with a loop of string attached. Take the ruler without string and clamp it along the edge of the bench with a flat face uppermost. (ii) Use some of the Blu-Tack to make a ball of diameter about 8 mm. (iii) Using the calipers provided, measure and record the diameter of the ball. ball diameter = ...................................................... (iv) Place the ball of Blu-Tack on the clamped ruler with a thin rod each side of it. Press the flat face of the second ruler on to the Blu-Tack until the ruler touches the rods. The Blu-Tack will form a disc, as shown in Fig. 2.1. G-clamp disc of rod Blu-Tack ruler rod ruler string loop bench Fig. 2.1 (b) (i) Measure and record the diameter d of the disc of Blu-Tack. d = ................................................. [2] (ii) Estimate the percentage uncertainty in d. For Examiner’s Use percentage uncertainty = ................................................. [1] πd 2 (iii) Calculate the contact area A using A = . 4 A = ................................................. [1] (c) (i) Attach the newton-meter to the string loop and lay it horizontally on the bench. (ii) Pull the newton-meter horizontally (parallel to the rulers). Measure and record the force F when the top ruler detaches. F = ................................................. [2] (d) (i) Remove the Blu-Tack from the rulers. (ii) Make a ball with a slightly different diameter and repeat (a)(iii), (a)(iv), (b)(i), (b)(iii) and (c). You should be aware that a large change in diameter could result in a reading outside the range of the newton-meter. ball diameter = ...................................................... d = ................................................. [1] A = ................................................. [1] F = ................................................. [2] (e) It is suggested that the relationship between F and A is For Examiner’s F = kA Use where k is a constant. (i) Using your data, calculate two values of k. first value of k = ...................................................... second value of k = ...................................................... [1] (ii) Explain whether your results support the suggested relationship. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. ..............................................................................................................................[1]

Mark scheme: 2 (b) (i) Value of ball diameter or d to the nearest 0.1 mm (or 0.01 mm). [1] Values of ball diameter and d in range 5 mm < d < 25 mm. [1] (ii) Absolute uncertainty is between 2 mm and 5 mm. [1] If repeated readings have been taken, then the absolute uncertainty can be half the range. Correct method shown to find the percentage uncertainty. (iii) Correct calculation of A with consistent unit. [1] (c) (ii) Value of F, with unit. [1] Evidence of repeat measurements of F here or in (d)(ii). [1] (d) (ii) Second value of d. [1] Second value of A is given to the same number of s.f. (or one more s.f.) than d2. [1] Second value of F. [1] Quality: When d increases (second d value is larger than first d value) F also increases (second F value is larger than first F value) and vice versa. [1] (e) (i) Two values of k calculated correctly. [1] (ii) Sensible comment relating to the calculated values of k, testing against a criterion specified by the candidate. [1] GCE AS/A LEVEL – May/June 2012 9702 32 (f) (i) Limitations 4 max. (ii) Improvements 4 max. No credit/not enough A two results not enough take more readings and plot a ‘repeat readings’ on its own/ graph/ few readings/ calculate more k values and take more readings and compare (calculate) average k/ only one reading B difficult to form a perfect method to make uniform pre-sized spheres/ sphere or disc/diameter of spheres/discs e.g. moulds repeat diameter sphere or disc varied measurement and average C reason for difficulty in method to improve eyes in line measuring d e.g. viewed measurement of d e.g. through ruler/parallax error in travelling microscope d D difficult to pull newton-meter method to ensure force is parallel to ruler/bench parallel to ruler e.g. use a long string/pulley and weights* E difficult to judge reading on method to read force at video to take reading/ newton-meter when detaches detachment e.g. newton digital (electronic) newton with reason e.g. ruler moves meter with a ‘max hold’ meter/ suddenly/without warning (so facility/video and playback or parallax related to newton difficult to read newton-meter freeze frame/ use system of meter/ at the instant the ruler starts pulley and weights or sand to difficult to measure force/ to move)/force drops to zero measure F*/ use force sensor issue of viewing ruler and immediately after detachment and datalogger or computer* meter simultaneously F contact area less than calculated disc area/bulging disc G difficult to zero newton-meter improved method to measure zero error in newton-meter/ when used horizontally F: e.g. use system of pulley just a pulley and weights or sand*/use force sensor with datalogger or computer* Do not allow: reaction time/human error/using vernier caliper/helpers/use of micrometer screw gauge/effect of temperature/change in stickiness of Blu-Tack. *This answer can be credited as D, E or G (but not more than once). [Total: 20]

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