CJC 2018 A level H2 Physics Answers

1 H2 Physics 9749 – 2018 A Level Exam Paper 1 1 Essential Question(s):  How to calculate percentage uncertainty of a quantity from other quantities? o Need an equation linking these quantities. Then apply the relevant rules to calculate the percentage uncertainty (product-quotient or addition-subtraction rule). o Note that we are needed to determine the percentage uncertainty of g. We therefore need to make g the subject of the equation. Make g the subject of the equation: T=2πඨL g T2 4π2 = L g g=L4π2 T2 ∆g g × 100% = ൬∆L L + 2 ∆T T ൰ × 100% %g = %L + 2%T %T = %g − %L 2 %T= %g − 0.05 6.25 × 100% 2 %T= 2% − 0.05 6.25 × 100% 2 = 0.6% (1 s.f.) Answer: A Examiner’s comments: Most candidates evaluated the percentage uncertainty in L correctly and also realised that there was a need to halve in order to deduce the percentage uncertainty in T from the percentage uncertainty in T2. The difficulty that candidates had was in realising that the percentage uncertainty in L needed to be subtracted from, rather than added to, the percentage uncertainty in g. 2 Essential Question(s):  What is the difference between distance and displacement, and how can we deduce these quantities from the graphs? Integrating v with respect to t gives change in displacement. Unlike displacement, distance is a scalar, hence the magnitude of the integral is considered to find distance. Change in displacement for P is positive, while for Q it is zero. Hence displacements are different.  either option A or B. For P, the change in displacement is the same as the distance travelled, since the object is travelling on a straight line, in the same direction for the entire journey.

2 For Q, the distance travelled is shorter as the total area bounded by the graph and the time axis is smaller compared to the total area for P. Therefore, both displacement and distance are different for P and Q. Answer: A Examiner’s Comments: Most candidates found this question challenging. The difficulty they experienced was in comparing the two graphs in terms of distance moved. Most candidates correctly deduced that the displacements were different. Only a minority of candidates correctly compared the total magnitudes of the areas of the triangles defined by the graph lines and the time axes to deduce that object P moves through a greater distance than object Q. 3 Essential Question(s):  As momentum is a vector, how do we interpret the directions mathematically for vectors that are in opposite directions?  How to calculate impulse? impulse = ∆p = pf- pi = mvf- mvi = m(vf- vi) = m[vf-(-18 vi)] Taking direction of final velocity as positive. = ൫80×10-3൯[23-(-18 )] = 3.3 Ns Answer: B 4 Essential Question(s):  Is total momentum conserved? What is the condition for total momentum to be conserved?  What kind of collision is this? (Elastic? Inelastic? Perfectly inelastic?)  As