2025 DHS H2 Prelim P3 Ans_w MC

2025 DHS Prelim Paper 3 Suggested Solutions 1 (a) Gravitational field strength at a point is defined as the gravitational force exerted per unit mass placed at that point. B1 (b) Since gravitational field strength g is the gravitational force F exerted per unit mass, Fg m= and the gravitational force exerted by the point mass on another body with mass m is 2 GMmF r= 2 2 GMm F GM rg mm r  = = = Note: Do not accept answers with just working but no explanations. M1 M1 A0 Marker’s Comment: Credit requires explanation of the physics steps. Candidates must state Newton's law, reference the field strength definition from part (a), and explain the substitution process. Simply writing equations and algebra without explaining the reasoning shows m athematical manipulation but not physics understanding. (c) (at surface,) lines (of force) are radial Earth has large radius / height above surface is small, so lines are (approximately) parallel and equally spaced B1 B1 B1 Marker’s Comment: Candidates must answer the question as asked. The requirement is to explain using lines of gravitational force (field lines), not mathematical expressions. Credit requires describing how field lines near the surface appear parallel and uniformly spaced, sh owing constant field strength. Using g = GM/r² arguments ignores the specified approach. 2 (a) The rate of change of momentum of a body is directly proportional to the resultant force acting on the body and occurs in the direction of the resultant force. B1 Marker’s Comment:

Many did not write the correct Newton’s 2nd law of motion. A few stated the force is ma. (b) (i) p = ∆(mv) = −65 x 10-3 (5.2 + 3.7) = −0.58 N s C1 A1 (ii) F = (0.58) / (7.5 x 10-3) = 77 N A1 (c) (i) 1. Force on the wall from the ball is equal to the force on ball from the wall B1 but in the opposite direction B1 (general statement of Newton’s third law only score 1 mark) 2. momentum change of ball is equal B1 and opposite to momentum change of the wall OR since there is no external resultant force acting on the system (ball and wall), the total momentum of the system is conserved OR total change in momentum of ball and wall is zero Marker’s Comment: Common mistakes such as the word ‘resultant’ was missed out in stating Principle of Conservation of Linear Momentum. Change in momentum was wrongly calculated without considering momentum as a vector. (ii) kinetic energy (of ball and wall) is reduced / not conserved, inelastic. (allow relative speed of approach does not equal to relative speed of separation) B1 3 (a) The moment of a force about a point is the product of the force and the perpendicular d