Year: 2011
Paper: 1
Question Number: 11
Course: LFM Pure and Mechanics
Section: Moments
No solution available for this problem.
There were again significantly more candidates attempting this paper than last year (just over 1100), but the scores were significantly lower than last year: fewer than 2% of candidates scored above 100 marks, and the median mark was only 44, compared to 61 last year. It is not clear why this was the case. One possibility is that Questions 2 and 3, which superficially looked straightforward, turned out to be both popular and far harder than candidates anticipated. The only popular and well-answered questions were 1 and 4. The pure questions were the most popular as usual, though there was noticeable variation: questions 1–4 were the most popular, while question 7 (on differential equations) was fairly unpopular. Just over half of all candidates attempted at least one mechanics question, which one-third attempted at least one probability question, an increase on last year. The marks were surprising, though: the two best-answered questions were the pure questions 1 and 4, but the next best were statistics question 12 and mechanics question 9. The remainder of the questions were fairly similar in their marks. A number of candidates ignored the advice on the front cover and attempted more than six questions, with a fifth of candidates trying eight or more questions. A good number of those extra attempts were little more than failed starts, but still suggest that some candidates are not very effective at question-picking. This is an important skill to develop during STEP preparation. Nevertheless, the good marks and the paucity of candidates who attempted the questions in numerical order does suggest that the majority are being wise in their choices. Because of the abortive starts, I have generally restricted my attention to those attempts which counted as one of the six highest-scoring answers in the detailed comments. On occasions, candidates spent far longer on some questions than was wise. Often, this was due to an algebraic slip early on, and they then used time which could have been far better spent tackling another question. It is important to balance the desire to finish a question with an appreciation of when it is better to stop and move on. Many candidates realised that for some questions, it was possible to attempt a later part without a complete (or any) solution to an earlier part. An awareness of this could have helped some of the weaker students to gain vital marks when they were stuck; it is generally better to do more of one question than to start another question, in particular if one has already attempted six questions. It is also fine to write "continued later" at the end of a partial attempt and then to continue the answer later in the answer booklet. As usual, though, some candidates ignored explicit instructions to use the previous work, such as "Hence", or "Deduce". They will get no credit if they do not do what they are asked to! (Of course, a question which has the phrase "or otherwise" gives them the freedom to use any method of their choosing; often the "hence" will be the easiest, though.) It is wise to remember that STEP questions do require a greater facility with mathematics and algebraic manipulation than the A-level examinations, as well as a depth of understanding which goes beyond that expected in a typical sixth-form classroom. There were a number of common errors and issues which appeared across the whole paper. The first was a lack of fluency in algebraic manipulations. STEP questions often use more variables than A-level questions (which tend to be more numerical), and therefore require candidates to be comfortable engaging in extended sequences of algebraic manipulations with determination and, crucially, accuracy. This is a skill which requires plenty of practice to master. Another area of weakness is logic. A lack of confidence in this area showed up several times. In particular, a candidate cannot possibly gain full marks on a question which reads "Show that X if and only if Y" unless they provide an argument which shows that Y follows from X and vice versa. Along with this comes the need for explanations in English: a sequence of formulæ or equations with no explicit connections between them can leave the reader (and writer) confused as to the meaning. Brief connectives or explanations would help, and sometimes longer sentences are necessary. Another related issue continues to be legibility. Many candidates at some point in the paper lost marks through misreading their own writing. One frequent error was dividing by zero. On several occasions, an equation of the form xy = xz appeared, and candidates blithely divided by x to reach the conclusion y = z. Again, I give a strong reminder that it is vital to draw appropriate, clear, accurate diagrams when attempting some questions, mechanics questions in particular.
Difficulty Rating: 1516.0
Difficulty Comparisons: 1
Banger Rating: 1516.0
Banger Comparisons: 1
A thin non-uniform bar $AB$ of length $7d$
has centre of mass
at a point $G$, where $AG=3d$.
A light inextensible string has one end attached to $A$ and
the other end attached to $B$. The string is hung over a smooth
peg $P$ and the bar hangs freely in equilibrium with $B$ lower than~$A$.
Show that
\[
3\sin\alpha = 4\sin\beta\,,
\]
where $\alpha$ and $\beta$ are the angles $PAB$ and $PBA$, respectively.
Given that $\cos\beta=\frac45$ and that $\alpha$ is acute,
find in terms of $d$
the length of the string and
show that the angle of inclination of the bar to the horizontal
is $\arctan \frac17\,$.
This question was attempted by about one-third of candidates, but was fairly poorly answered. About one-third of attempts scored 0 or 1, and the median mark was therefore only 4. Nevertheless, there were many good solutions, reaching about two-thirds of the way through, including a number of perfect scores. An overall comment for this question is again that candidates need to explain their work. Particularly in questions where the answer is given, little credit will be given for simply writing down an equation which leads to the required answer in one step unless a justification can be seen. For the first part of the question, most candidates drew decent diagrams, allowing them to proceed, but a fair number drew something which was either inaccurate (placing A below B or drawing the rod horizontally, for example), severely incomplete (no forces) or too small and scribbly to be useful. These led to subsequent difficulties when attempting to resolve forces or take moments. About 20% of candidates stopped after drawing the diagram, gaining them either 0 or 1 (the modal score). A number of candidates did not appreciate that the tensions in the two parts of the string were equal, and were therefore unable to proceed. A common error seen when taking moments was something like 3d(mg) = 7d(T sin β); this could be made to give the 'correct' answer, but received little credit. Another common error was to forget to include any forces in the moments equation. Of those candidates who proceeded further than the diagram, most made good progress towards finding the length of the string. A common assumption was that ∠APB was a right angle, without giving any justification of this. A similar, though far less common, assertion was that AP/PB = AG/GB. Both of these happen to be true, but candidates are required to prove them to gain any credit for their argument. Many different approaches were seen; the more common ones are described in the solutions above, and there were many variants of these. When it came to finding the angle of inclination, a small number of candidates successfully did so. There were many attempts which fudged their working to reach the stated conclusion. A number stated that the angle APB is bisected by the line PG without giving any justification; such attempts gained few marks.