(DHS) 2024 Prelim Phy P3 Ans_final

1 2024 DHS H2 Physics Prelim Paper 3 Suggested Solutions Section A 1 (a) Resultant force on it must be zero in any direction. B1 Resultant torque on it must be zero about any axis (of rotation). B1 MC: Quite badly done. Missing key words in conditions stated. Marks penalized if there is no mention of about any axis (for torque) or in any direction (for force) in answers. (b) (i) MC: The question specifically mention to draw in Fig 1.1, but some drew a separate diagram instead. BOD awarded if T2 is replaced by weight of the 5.0 kg, Force by wall is not a normal force since it is not 900. Many students drew the force by the wall wrongly, they either assumed its horizontal, or along the beam. (ii) Let x be the distance of c.g of beam from the hinge. Taking moments about hinge, sum of clockwise moments = sum of anticlockwise moments 120 (5 sin 60o) = 5g (5 sin 70o) + 20g (x sin 70o) C1 x = 1.57 = 1.6 m A1 MC: Poorly done, because many could not obtain the correct clockwise or anticlockwise moment about the hinge. Some made mistakes when determining the necessary angle needed for calculations. (iii) Vertical summation of forces: FY + 120 (sin 40o) = 5g + 20g FY = 168.1 N Horizontal summation of forces: weight of beam force by wall on beam 40o tension, T2 tension, T β Any 2 correct pairs of forces B1 Other 2 correct pairs of forces B1 With correct direction and label C1

2 FX = 120 (cos 40o) = 91.93 N F =√FX 2+FY 2 = √91.932+168.12 = 192 N A1 tan  = 91.93 168.1  = 29o A1 Force is at an angle of 41o with the beam A1 MC: Many failed to resolve horizontally and vertically. A few did not calculate the numerical angle of the force by wall, while many did not express the angle correctly with the beam. 2(a) In uniform circular motion, the speed of the metal ball is constant, but its velocity is constantly changing direction. Since acceleration is the rate of change of velocity, the metal ball experiences an acceleration. B1 B1 OR In uniform circular motion, the speed of the metal ball is constant, but there is a net non-zero force acting on the metal ball according to Newton’s 1st law because it is moving in a circular path. From Newton’s 2nd law for constant mass, there must be an acceleration in the same direction as the net force. B1 B1

3 CKW MC • Most candidates demonstrated a lack of understanding regarding the significance of uniform circular motion. Paraphrasing the question, candidates should explain why a metal ball moving at a constant speed in circular motion is still considered to be accelerating. • Some candidates mentioned the presence of a centripetal force without explaining why the centripetal for