2021 RI Promo Sect BC Soln

Raffles Institution Year 5-6 Physics Department   1   2021 Year 5 H2 Physics Promotional Examination Solution Section B 1 (a) (i)    upthrust 0.040 1410 9.818910 0.062097 0.062 N liquidUV g    (ii) At terminal velocity, the resultant force on the sphere is zero.   1 0.040 9.81 0.062 0.26 0.040 9.81 0.062 0.26 1.2708 1.27 m s T T T mg U v v v       (iii)  0 0.040 1.2708 0 4.5 0.011296 0.0113 N T average mvpF tt    (b) *Curved, increasing gradient (zero gradient at 0 s) from 0 s to 2.0 s, with label *Straight line, constant gradient (smooth transition at 2.0 s) from 2.0 s to 4.5 s, with label (c) When moving at terminal velocity, there is a constant drag force acting upwards on the sphere by the liquid. By Newton’s Third Law , the sphere exerts an additional reaction force downwards on the liquid. This increases the normal contact force bet ween the container of liquid and the mass balance, as compared to X when the sphere is instantaneously at rest and experiencing a downwards acceleration (upthrust less than weight). Hence, Y is larger than X. 4.5 s / m t / s 0 1.0 2.0 3.0 4.0 5.0

Raffles Institution Year 5-6 Physics Department   2   Comments (a) (i) Please note the following for all show questions: 1. All working and numerical substituti on of values have to be written down explicitly. 2. Computation is required and please wr ite down the more precise value prior rounding off to the value given in the question. (ii) A significant handful of students made the following mistakes: 1. Inaccurate free body diagram of the s phere i.e. missing forces such as weight, upthrust or drag force. 2. Careless mistakes in computation. (iii) Partial credit is given for the following: 1. Change in velocity, v is computed as initial finalvv or there is lack of clarity in the substitution forv . 2. Ne gative sign was included when asked for magnitude. (b) Many students did not label the graphs explicitly despite values provided on the horizontal axis. Students tend to forget that the initial gradient for the s–t graph should be zero. A handful of students were confused with the time that the sphere takes to reach terminal velocity. It should be 2.0 s and not 2.5 s. A significant handful of students drew the v–t graph instead of the s– t graph. No credit will be given. (c) This question was very poorly done. Common misconceptions include that the reading of the mass balance is given by the  ‘normal contact force exerted by the sphere on the mass balance’  ‘net force acting on the sphere’  ‘moving sphere exerting a force’  ‘resultant/net force acting on the mass balance’ The mass balance reading is due to the norma l contact force between the container of liquid and the mass balance. There is no cont act force that exists between the sp