VJC 2024 H2 Physics CA2 Practical sample answers and common mistakes

2024 CA2 Physics P4 Mark Scheme 1 In this experiment, you will investigate the effect of friction on a simple pulley system. (a) You are provided with a spring of unstretched length 0L , as shown in Fig. 1.1. Fig. 1.1 Measure and record 0L . 0L = 4.1 cm [1] • Some students did not measure from the extreme ends of the rings. Read the question carefully! • When using ruler to measure length, instrument uncertainty = 0.1 cm. So L0 should be recorded to 1 dp. (b) Set up the apparatus as shown in Fig. 1.2. Fig. 1.2 L0 boss bench clamp boss mass hanger pvc pipe string retort standretort stand rod of clamp spring

Ensure that the pipe remains horizontal at all times, and that the strings on each side of the pipe are vertical and parallel with each other. The spring should only stretch minimally at this instant. (i) Place a 50 g slotted mass onto the mass hanger and slowly lower the mass hanger, allowing the spring to extend to a new equilibrium length 1L , Measure and record 1L . 1L = 6.1 cm [1] (ii) Calculate the extension 1x of the spring from its unstretched length, where =−1 1 0x L L = − = − =1 1 0 6.1 4.1 2.0 cmx L L 1x = 2.0 cm [1] • You should show the working for your calculation. (iii) Pull the mass hanger downwards by about 10 cm, causing the spring to extend further. Hold the hanger while allowing it to slowly rise upwards until the spring retracts to another equilibrium length 2L , where 2L is larger than 1L . Measure and record 2L . 2L = 11.3 cm (iv) Calculate the new extension 2x of the spring from its unstretched length, where =−2 2 0x L L = − = − =2 2 0 11.3 4.1 7.2 cmx L L 2x = 7.2 cm [1]

(c) Theory suggests that 1x and 2x are related by the expression =2 1 ln 2x x where  is a constant and  is the angle of contact (expressed in radians) between the string and the pipe, as shown in Fig. 1.3. Fig. 1.3 (i) State the value of θ for the set-up in Fig. 1.2.  =  rad [1] • Some students recorded the answer in degrees, when the question asked for radians. (ii) Determine a value for . ( ) ( )  −= = = 21 1ln ln 7.2 2.0 0.204 rad22 xx  = −10.204 rad [1] 

(iii) The experiment is repeated with different numbers of slotted masses to obtain more values of 1x and 2x . State how a straight line graph can be plotted and used to determine the value of , assuming the theory is correct. =2 1 ln 2x x  = 22 1 x ex  ( ) = 2 21x e x Plot a graph of 2x against 1x where the gradient is 2e and the vertical intercept is zero. Hence, ( ) = 1 ln gradient2 . or =2 1 ln 2x x  −=21ln ln 2xx  =+21ln ln 2xx Plot a graph of 2ln x against 1ln x where the gradient is 1 and the vertical intercept is 2 . Hence, ( ) = 1 vertical intercept2 . [2] Many students