CJC 2019 A level H2 Physics Answers

1 H2 Physics 9749 – 2019 A Level Exam Paper 1 1 Essential Question(s): What does the unit prefixes represent? Solution: nano - 10-9 micro - 10-6 milli - 10-3 centi - 10-2 deci - 10-1 kilo - 103 giga - 109 Answer: D 2 Essential Question(s):  What does accuracy ±1% mean?  What does “additional” uncertainty of ±10 mV mean?  When recording value together with its uncertainty, how do we represent the absolute uncertainty and how does the value be represented? Solution: The overall absolute uncertainty = 1 % of the value recorded PLUS 10 mV,     3V 4.072 0.01 10 10 0.0502V V 0.05V          Rounding up the uncertainty to reflect the full statement of “additional 10 mV”, and to represent absolute uncertainty to 1 significant figure. Subsequently, the next step is to truncate the value to the same decimal place as the absolute uncertainty,  V V 4.07 0.05 V   Answer: D 3 Essential Question(s):  How to construct the equations of motion using the information provided? Solution: From Start to (t-1)seconds S1 = ½ a (t-1)2 – (1) From Start to t seconds S2 = ½ a (t)2 – (2) (2) / (1) :

2 2 1 22 2 2 1 ( 1)2 1 ( )2 0.25 ( 1) 1 ( ) 1 ( 1) 2 ( ) 2 a tS S a t t t t t t      Answer: D Examiner’s comments: Candidates performed well on this question demonstrating string understanding of kinematics 4 Essential Question(s): o How is the speed and acceleration related to the displacement time graph? o How does acceleration changes in a free fall with air resistance? Solution: Speed increases. Gradient of the graph is increasing. Speed increases at a decreasing rate because the air resistance is increase but the weight is constant. Resultant force decreases and so acceleration decreases. Answer: C 5 Essential Question(s):  How is the conservation of linear momentum applied here? Solution: Initial momentum is zero. So X being less mass move at higher speed, and Y being higher mass moves at lower speed. But the product of the speed and mass is the same. So the centre of mass remains at the same position. Answer: A Examiner’s comments: Options C or D were popular incorrect choices. As the total momentum o f the system is zero before the explosion, the centre of gravity cannot move. If it does, this would mean that an external force must have been applied to the system, which is not the case in this question. 6 Essential Question(s): o Since it is inelastic collision, what are the physics concepts to be used? Solution: Conservation of linear momentum: mu = mv + 2mv u = 3 v

3       2 2 2 2 2 2 2 2 1 1 1( ) ( ) (2 )Initial Total KE Final Total KE 2 2 2 1Initial Total KE ( )2 (3 ) ( ) (2 ) (3 ) 9 1 4 9 4 9         m u m v m v m u v v v v Answer: B 7 Essential Question(s):  What is Hooke’s Law?  How to calculate the work done?  Solution: Hooke’s Law is obeyed when there is a straight line on the F – extension graph. The straight line stop at 20 mm (0.020 m), where the force is 28 N. Work done = ½ (28)(0.020) = 0.28 J Answer: A 8 Essential Question(s): o How does the change in direction of the force affect the contribution of work done to the Ke and GPE? Solution: As the force is applied in the direction of the velocity, as the slope becomes more vertical, more and more of the component of the force is given to the vertical, meaning the GPE increase more and more. Therefore the gain in KE becomes lesser and lesser. Answer: D Examiner’s comments: Candidates performed well in general. Candidates choosing option A thought that the kinetic energy of the object was decreasing, not realizing that the force was being applied to the object continually 9 Essential Question(s):  What form of energy is given to the water?  What happen for the energy given to be minimum? Solution: Power = Energy/ time 91.3 10 9.81 2 24 60 60 295208 300000 GPEPower time       

4 Answer: D 10 Essential Question(s): o What equation relates ω to R? Solution: Rearranging the eqn v = R ω, ω = v R , with v being constant we can see that ω α 1 R  Graph should be a rectangular hyperbola for a graph of ω against R. Answer: B 11 Essential Question(s): o What equation relates acc, ω and R? Solution: Centripetal acc, a = R ω2 ω = a R = 20 9.81 14 2 x      =5.3 rad s-1 Answer: D 12 Essential Question(s): o What does “escape from planet’s gravitational field” mean? [mass goes to infinity where GPE is max and hence a 0 value.] o What is the energy conversion when object goes from surface of planet to infinity? [loss in KE and gain in GPE] Solution: By conservation of energy, Energy of object at surface of planet = Energy of object at infinity KEmax at surface + GPEsurface = KEinfinity + GPEinfinity, max 21 0 02 GMmmv R       2GMv R Answer: A 13 Essential Question(s): o How is Cr.m.s related to temperature, mass and molar mass? Solution: Cr.m.s = 3RT M = -3 3 x 8.31 x (30 + 273.15) 4x10 = 1375 = 1400 m s-1 (2sf) Answer: C Examiner’s comments: The majority of candidates that responded incorrectly chose option A (43 m s-1). This was probably because they did not convert the mass of the helium into kilograms. Since the root-mean-square speed of a typical air molecule is about 500 m s -1, a helium atom is going to be moving more quickly than this. Remember to convert the mass of the helium into kilograms.

5 14 Essential Question(s): o What is internal energy of the air? Solution: Internal energy is the sum of molecular KE due to random motion of molecules and the molecular PE due to intermolecular forces of attraction. N(Ek+Ep) Answer: C 15 Essential Question(s):  Recall the 1st Law of Thermodynamics and the meaning of its symbols.  Be aware that the question is requiring “heat transferred to the surroundings” which is not the definition used for symbol for Q. o Need an equation linking these quantities. Then apply the relevant rules to calculate the percentage uncertainty (product-quotient or addition-subtraction rule). Solution: From the 1st Law of Thermodynamics U = Q + W where: U represents the increase in internal energy of the system Q represents the heat supplied to the system and W represents the work done on the system Therefore from the question, it is defined from the symbols that U is the increase in the internal energy of the gas (the system) and W is the work done on the gas (the system) so rearranging the equation, the heat supplied to the gas (the system) is Q = U – W However the question required the heat supplied to the surroundings, in other words the heat supplied from the gas, i.e. - Q = W – U Answer: B Examiner’s comments: Candidates performed very well in this question. Candidates choosing option C did not consider the direction of heat transfer. 16 Essential Question(s):  Recall the standard information for an object in oscillation. In other words what is the acceleration/velocity/displacement at equilibrium vs amplitude to link it to restoring force/kinetic energy respectively at equilibrium vs amplitude locations. Solution: The solution requires very little calculation but a quick recall that a = -2x where: a represents the acceleration of the object  represents the angular frequency of the object and

6 x represents the displacement of the object from the equilibrium Since Fnet = ma = -m2x Therefore the net force on the object which is the restoring force will be proportional to the acceleration and in turn proportional to the displacement from the equilibrium. So when the object is at displacement = 0 (i.e. equilibrium), x = 0 hence Restoring force (Fnet) = 0 Conversely when the object is at displacement = maximu