CJC 2016 A level H2 Physics Answers

1    H2 Physics 9646 – 2016 A Level Exam Highlight in YELLOW the recommended qns to go through in Class Paper 1 1 No specific keywords in questions, but prefixes is the key in evaluating the answer. Answer: B 2 Keywords  Estimate (need not be exact, but choose the nearest answer)  Photon, Green light  Energy Essential Questions:  How do we calculate the energy of a photon?  What is an estimate of the wavelength of green light? Answer: C 3 Keywords  700 km/h airspeed  250 km/h from west  Resultant Directly North Essential Questions:  How should the aircraft fly towards so that the resultant velocity is directly north?  What is the vector diagram and what are the vectors to be drawn? (name the 3 vectors to be drawn: initial velocity of aircraft, change in velocity of caused by the wind, final/resultant velocity of the aircraft) Answer: B 4 Keywords  velocity-time graph  displacement-time graph  acceleration-time graph  velocity-distance graph Essential Questions:  How do we translate from velocity-time graph to other graphs?  What are the information from a velocity-time graph can we get from the features of such a graph? A possible method to solve this question is to evaluate each graph with the features from the given graph. For example, for a acceleration-time graph, there must be a horizontal line at 0. Answer: D 5 Keywords  Flat golf course  falls back to the original height  Horizontal distance  Maximum height  Angle  (thematic words indicating kinematics and projectile motion) Essential Questions  What are implicit info at the ‘max height’ and ‘when it returns to the original height’?  What kinematics equations are applicable?

2    Answer: D 6 Keywords:  Total momentum  After collision Essential Questions:  Is total momentum conserved? Is there any net external force on the system? Answer: C 7 Keywords:  Free to move without friction Essential Questions:  What is the relationship between force, speed and time?  Does the gradient and/or the area under a F-t graph represent anything useful?  What is the gradient of speed–time graph? Answer: D 8 Keywords:  Range of minimum forces Essential Questions:  What are the forces acted on the block when it is completely submerge?  What are the forces acted on the block when it is completely lifted out of water?  When does minimum force occur? (  lifted at constant velocity) Answer: D 9 Keywords:  Force from the slope on the skier Essential questions: 1. Draw the free body diagram of the skier 2. Draw the vector diagram of the forces in equilibrium Answer: B 10 Keywords:  Couple of torque M  Torque of couple Essential  How to determine the torque of a couple? Answer: C 11 Key info: - Constant speed - Level road - Efficiency is 16% - Travel 1 km - 1 kg of fuel provides 48 MJ of energy Essential questions:  How much energy is required from the fuel?  Which approach to use? Dynamics or Ener gy approach? (since information given in terms of energy efficiency, try energy approach)  What are the energy changes? What are the key words in the question that suggest what the energy supplied by the fuel is used for? Constant speed  no change in KE Level road  no change in GPE

3    So energy from fuel goes to doing work against resistive forces. Answer: A 12 Key info: - Angular velocity, ω - earth Essential questions:  How is angular velocity related to period? ( ω = T 2 )  How long does earth take to rotate about its axis? (24 hrs to be converted to sec) Answer: D 13 Key info: - Corner is an arc of a circle - Horizontal road - When road is wet, maximum frictional force is halved Essential Questions:  What provides the centripetal force? (frictional force)  How is the centripetal force related to linear speed? (F = r mv2 ) where m and r are constant Answer: D 14 Key info: - Point P is midway - From diagram, mass of earth is larger than mass of moon. Essential Question:  At point P, whose gravitational field will be stronger?  What is the direction of the resultant gravitational field?  How is gravitational potential at a point determined?  When is gravitational potential zero? Answer: C 15 Essential Questions:  How is orbital radius related to period? (T 2  R3) Answer: C 16 Essential questions: 1. How does the PE change with position of the oscillating object? 2. How does the KE change with position of the oscillating object? 3. Can this change be represented as a mathemat ical formula to verify the answers to the above 2 questions? PE: The PE is maximum at the amplitude positions of the oscillations. KE: The KE is maximum at the equilibrium position of the oscillations. Formula for PE:

4    EP = ½ m2x2 (upright parabola) EK = ½ m2(x02 – x2) (inverted parabola) Thus, equations verified the descriptions above. Answer: A 17 Answer: D Essential questions: 1. How is angular speed related to linear speed of the pendulum? 2. How can we find the speed of the pendulum at a specific angular displacement? Using conservation of energy, TE at vertical height y0 = TE at vertical height y1 GPEy0 = KEy1 + GPEy1 KEy1 = GPEy0 - GPEy1 ½ mv2 = mgh v = √(2gh) = √[2x 9.81 x (1cos(0.030) - 1cos(0.050))] = 0.1257 m s-1 since v = r   = v/r = 0.1257/1.0 = 0.1257 rad s -1  18 Answer: A Essential question: 1. What equation describes an ideal gas? 2. What are the constants in both situations? As the gas is ideal, P 1V1 = n1RT1 - - (eqn 1) P2V2 = n2RT2 - - (eqn 2) Dividing (1) by (2): (P1V1) / (P2V2) = (n1RT1) / (n2RT2) as the amount of gas is kept the same, (P1V1) / (P2V2) = (T1) / (T2) V 2 = (T2P1V1) / (P2T1) = (600 x 1.0 x 105 x 0.025) / (5.0 x 105 x 300) = 0.010 m3  19 Answer: B Essential questions: 1. What is common for both processes?  same change in U (Why?) Using 1st law of Thermodynamics, Process PRQ: U1 = W1 + Q1 Process PSQ: U2 = W2 + Q2

5    Since both processes have the same resultant change, W 1 + Q1 = W2 + Q2 Q 2 = W1 + Q1 – W2 = (-3) + 8 – (-1) = +6 J 20 Answer: D Essential questions: 1. What is the simplest way to calculate the mean KE of a molecule at sea level? 2. What are the properties at sea level? At sea level, the temperature is about 20 oC, which is about 300 K. KE = 3/2 kT = 3/2 (1.38 x 10-23) (300) = 6.2 x 10-21 J 21 Keywords:  Intensity I corresponding to amplitude A  Polarization  Relative angles between polarizing axes of the filters  beam emerges from second filter with amplitude A √2  To find: intensity (not amplitude) of beam after passing the third filter Essential Question:  How are the amplitudes and intensities of the incident beam and emergent beam related? The ‘simple’ answer if we ignore the first information that I and A are the intensity and amplitude of the unpolarised light would be: Vector resolution and recalling that Intensity α Amplitude2 can be applied. Since a beam of amplitude A corresponds to an intensity I, and since Intensity α Amplitude2, therefore for the emergent beam with amplitude A ଶ , corresponding intensity would be I ସ Answer: C B [Note: CPDD highlighted that Examiners’ Report is wrong. Correct answer is B not C. This is an unfair question for A-level as students are not expected to understand that after the first polarisor, while intensity is halved, amplitude stays the same.] What it should be (Actual correct answer): 2nd polarising axis  3rd polarising axis 45⁰  A √2 A √2 cos 45⁰ = A 2

6    Intensity Amplitude Unpolarised I A After 1st filter I ଶ since only half the energy is transmitted A Amplitude stays the same. Conceptual understanding of this is not really required at A-level. After 2nd filter To apply the relationship Intensi