A Short Guide to Writing Long Answers

Posted on 17-04-18 in Physics

Writing clear (and correct!) answers to long physics questions can be challenging. In this post, we will outline techniques for effectively answering them and some of the common pitfalls students face.

Identifying what a question is asking for And then read it again. Slowly this time. Do you understand what the question is asking for?

In many long questions it can be surprisingly easy to answer the wrong thing. Have a look at the following question from A2 EdExcel physics (paper 4R, June 2014). Try to think how you would answer. I will put my answer at the end of this [post][1].

Particle accelerators accelerate particles to very high speeds before collisions occur. New particles are created during the collisions. Two particles of the same type can undergo two kinds of collision.
Fixed target: a high speed particle hits a stationary particle.
Colliding beams: two particles travelling at high speeds, in opposite directions, collide head-on.
By considering the conservation of energy and momentum, explain which type of collision is able to create a new particle with the largest mass.
[6 Marks]

So first off - let's not rush into answering the central question. There is a lot of information here and more worryingly there's a lot of information in your head! So many students will often leap before looking at this point and dash off hoping to hit the points as they rattle out of their brain.

Do not do this!

In reality this question has very little to do with particle physics and more in common with mechanics. If you had started writing the second you saw particle accelerator and started listing interesting details of linacs you would score zero!

So what is step one? I would recommend highlighting or underlining key physics terms in the question. This can be a good way of forcing yourself to observe what is actually there. The key terms for me in this question would be fixed target, colliding beams, energy, momentum, collision and largest mass. From these key terms we can begin to think about the points that we would like to make. In this case, the question is about momentum conservation.

2. Explicit is better than implicit

This leads us into our second area. It is generally insufficient in an exam to simply state a fact or property without relating it to the system you are trying to describe. Put simply,

Consider the following question.

Why is a ruler an appropriate tool for measuring the length of a piece of A4 paper?

A common answer I would see from students would be to simply state the precision of the rule. To these students their intent is to state the key fact needed, namely that the rule's precision is 1 mm. This however does not relate the 'obvious' point to the examiner - Namely that the precision of the ruler is much smaller than the length of a piece of paper. To gain the marks you must state this explicitly in the question[2]. Annoying as this maybe, this kind of logical reasoning is essential.

For the particle detector question above, a statement that energy and momentum are conserved is insufficient. This point must relate to the problem of the particle accelerators. For instance, the net momentum of colliding beams can be zero but the net momentum of the fixed target cannot.

In all these long form questions, a good technique to remember is pairing. Pair a related physics principle to content in the question.

3. Plan ahead - Don't waste space

Every question you do will have a number of marks associated with it. This can be a useful way of trying to see how many points you should aim to make. For instance for a question with five marks, you should aim to make at least five points. Whilst perhaps an obvious point it can be easy to simply write and hope. This write-and-forget strategy is not optimal for a couple of reasons. Firstly the space given can be easily wasted. Please don't copy out or reword the question in your answer! This often happens when running on automatic. The examiner knows the question - they're looking for answers! Secondly we want to arrange the points we wish to make in a logical order. In doing so we can ensure we make a watertight argument. And it also ensures we don't forget a point we need to make. Towards these aims I would recommend,

Write short sentences!

Each sentence should have a purpose and point you wish to make. Each sentence should link to the previous. And you should be able to make your point succinctly. This makes it easier for your examiner to understand your reasoning. Importantly nobody expects you to be perfect. If you can't make enough points simply move on. This also means if you wish to add more points later in the examination (as your memory returns!) you have space left.

Finally one point about the particle detector question above. In this exam two thirds of an A4 page were given to write an answer. Personally I consider this to be almost an intimidation tactic! Write more it says. Look at all this space you're wasting by not writing enough!

Focus on what you want to say, think of the points you need to make and write short sentences to get there.

4. Vital Information

Physics examination questions rarely have single parts. Information given in a part of the question may be important in another. Diagrams especially can be vital.

Be sure to check for vital information through the whole question!

5. Don't Panic!

Practice, practice, practice. I would also strongly recommend reviewing examiner's reports on the exam questions you are looking at if possible. These list the general mistakes and performance of all students on an exam.

Finally, don't panic. You're not stupid, you're not slow, you're just stressed! If you do come across a question you don't understand, leave yourself any notes you can recall and move on. Don't get trapped on something you find difficult. You can always come back at the end if required.

Summary

1. Make sure you're answering the question
The net momentum before collision can be zero for the colliding beams. Therefore the net kinetic energy after the collision ($$E=\frac{p^2}{2m}$$) can also be zero. This is not the case for the fixed target collision. Since energy and mass are related by $$E=mc^2$$, the more energy we have, the larger the mass we can produce. Therefore the colliding beams can produce a higher mass than the fixed target, as no energy is lost to kinetic energy.