2022 A Level H2 Physics P3 Soln

2022 A Levels H2 Physics 9749/02 Paper 3 suggested solutions 1a Inertia is the tendency for bodies to resist changes in its motion. 1b      11 22 1 22 6 6.67 10 7.35 10 1.62 N kg 1.74 10 GMg r        1ci Since a force of 10 kN is exerted on the rocket, by Newton’s 3rd law, a force of 10 kN is also exerted on the gases in the opposite direction.   on gas 1 10000 70.0 143 m s dmF v dt v v     1cii After 15.0 s, mass of rocket   4000 15.0 70.0 2950 kg        2 10000 2950 1.62 2950 1.77 m s netF ma T mg ma a a        1ciii Actual acceleration is greater. At a greater height, the gravitational field strength is lesser since gravitational field strength is inversely proportional to the square of distance from centre of planet. Thus the weight of the rocket will be lesser, resulting in a larger net force on the rocket. Hence its acceleration is greater. 2a 2 3 6 1.021.73 10 2 1.41 10 m R A R A                  l l 2bi time after being switched on / s ΔU q w 0-59 positive negative positive 60-100 zero Negative positive 2bii At constant temperature, ΔU = 0 work done by the power supply = Power x time Hence for 40 s, w = Pt = IVt = 12 x 230 x 40 =110400 = 1.10 x 105 J By 1st law of thermodynamics, q = -1.10 x 105 J 3ai -1 2 12002 ( ) 60 126 rad s f      3aii 2 2 21.20(0.230)(126) 1.2(9.81) 4370 N F W mr F mr W         

3aiii Washing machines vibrate vigorously when spinning. The large masses serve as a mass damper by reducing the amplitude of vibration. The masses also reduce the natural frequency so that it will be below the frequency of spinning, to prevent resonance from happening. The masses are also placed at the bottom to lower the centre of gravity of the washing machine, increasing the stability to prevent toppling when it vibrates. By Newton’s third law, the force on the towel by the machine (a)(ii) is equal in magnitude to the force on the machine by the towel. Hence, large masses placed at the bottom can also prevent such a large force from cracking the base of the machine. 3b At the holes, there is no normal contact force exerted on the water droplet, so the only force it experiences is its weight which acts vertically downwards. There is no normal contact force present at the holes to provide for centripetal force, hence the water droplet is not able to remain in circular motion. The velocity of the water droplets at the holes will be tangential to the drum. 4a The 2 spheres have the same sign. Based on Fig 4.2, the potential gradient of the graph changes from negative to zero before turning positive. This implies that electric field also changes direction since E=-dV/dx. Electric field between 2 charges will only be zero if the charges are of the same sign. Since potential between the 2 spheres is always positive and potential is a scalar, the 2 spheres are positively charged. 4b Using F