DHS 2021 JC1 Promos Physics (Solutions)

DUNMAN HIGH SCHOOL 2021 PHYSICS H2 (YEAR 5) 1 Answers to Promotional Exam Section A 1 2 3 4 5 6 7 8 9 10 C B C A C B D B A D 11 12 13 14 15 16 17 18 19 20 D A C D D A B A C B 1 C mass of 20c coin: 3.85 g or about 4×10−3 kg (Order of magnitude 10−3 kg) thickness of a sheet of paper: 0.1 mm or 1×10−4 m (Order of magnitude 10−4 m) an average apple has a mass of 100 g or weight of 1 N (Order of magnitude 100 N) temperature of a person’s body: 36.5 oC or 310 K or 3×102 K (Order of magnitude 102 K) 2 B pqf Cm k= ( ) 1 2 1 2 1 2 kg s comparing the exponent: i: of s: n terms of base un o its s 12 f kg: k 0 g qp qq p q p q −− − =−  = = +  =− =− = 3 C X = P – R ⇒ X = P + (−R) ⇒ vector addition of P and –R, i.e. the beginning of vector −R is placed at the end of vector P; the vector sum X is drawn as the vector from the beginning of P to the end point of –R. 4 A ( )( ) ( )( ) o 21 2 2o1 2 After releasing from rest, the mass slides down the inclined surface with constant acceleration sin30 . Using , we have 0 0.80 sin30 0.80 1.6 m (to 2 s.f.) ag s ut at sg = =+ = + = 5 C ( ) ( ) ( ) 22 22 2 Using 2 for uniform acceleration along t he vertical direction, and taking upward direction as positive, we have 0 sin 2 sin 2 v u as v g H vH g   =+ = + − =

DUNMAN HIGH SCHOOL 2021 PHYSICS H2 (YEAR 5) 2 6 B force = rate of change of momentum = gradient of momentum-time graph. Since the resultant force is increasing, the graph has increasing gradient with time. 7 D action-reaction forces act on different objects and are of the same type of force. 8 B at equilibrium W = U 32 = (0.012)(0.3)  (9.81)  = 910 kg m-3 9 A When tension = 400 N, length = 37 mm and extension = 2 mm. strain energy = 1 2Fx = 1 2(400)(2 x 10-3) = 0.40 J 10 D This equation is not valid as there is an acceleration. 11 D efficiency = useful power output/ power input = (260/320) x 100 = 81% 12 A At highest point, T is lowest (=0) and speed is minimum. T + mg = mv2/r so g = v2/r or v = √rg = √(0.25(9.81) = 1.6 m s-1 13 C When released from P, the direction of motion is towards planet A, in the direction of lower potential. By conservation of energy, loss in potential energy = gain in kinetic energy m = 1 2 mv2 v = √2() = √2(62.3 – 13.4) x 106 = 9.89 x 103 m s-1

DUNMAN HIGH SCHOOL 2021 PHYSICS H2 (YEAR 5) 3 14 D total energy = −GMm/2r, potential energy = −GMm/r, kinetic energy = GMm/2r As r increases, total energy