Minds Set In Motion

The Age

Monday May 5, 2008

Melanie Lane

Clarity and conciseness are vital in physics exams, writes Melanie Lane.

Tips

The mid-year exam is approaching and you are probably wondering "What makes a good answer? How can I best show the examination assessors what I know? How can I get full marks for my answer to a question?"

? Make your answers clear and concise.

? Don't ramble. Rambling answers, that try to touch on many, often irrelevant, points end up being contradictory and do not score well.

? Answers written in point form are acceptable and often preferred by the assessors.

? Don't simply copy material from your A4 sheets.

? It's a good idea to re-read your final answer, to check you have answered the question asked.

? Show all working.

In the area of study "Motion in one and two dimensions", question 2 seemed to present some difficulty for students sitting the 2007 paper.

Questions, involving frames of reference are often found quite challenging by students. This question required students to sketch the tail-light as seen by Mary, and to indicate the final position of the tail-light.

Students who sketched a parabolic path from the tail-light's initial position in A and landing directly below the seat in B, received the full two marks. Some students had the parabola going backwards. Others drew a straight line to the landing point. Many missed the essential point that if the bike was travelling at a constant speed, the light would land directly under the position from which it fell, having travelled the same horizontal distance as the bike in the 0.45s.

The other challenge in motion included the set of questions about projectile motion, questions 14-17. Students often confuse the vertical and horizontal components of the projectile's motion and sometimes have difficulty using the constant acceleration formulas correctly. Calculations involving simple trigonometry often seem to be a problem.

It is strongly recommended that students analyse the horizontal and vertical motion of a projectile independently. Those students who did had a better chance of obtaining full marks for question 17.

Most students realised that the acceleration was the same, i.e. acceleration is due to gravity if air resistance is ignored. However, very few students realised the time of flight would also be the same. Of those students who treated the components of motion separately, and analysed the vertical component, a few students realised that if the same maximum height above ground was reached in both scenarios, and if v = 0 ms-1 at this height, then the time to reach this maximum height would also have to be the same (given a = g = - 10 m s-2). Hence the total time of flight would be the same for both scenarios.

In the "electronics and photonics" area students once again found the n-p-n transistor voltage amplifier circuit the most difficult. Remember you need to be able to analyse the circuit for DC voltages and currents, and then for the small voltage and current changes that allow the AC voltage amplification to be determined.

Remember that capacitors are open circuits (breaks) for DC current and closed circuits (shorts) for AC. This may help you explain how capacitors act as decouplers to separate AC from DC.

In question 1 most students realised the output signal had an amplitude of 1.6 V and the same period as the original but quite a few students forgot to invert the signal or had it centred around 3 V, overlooking the effect of the capacitor.

Most students gave an inverted output signal with the same frequency and centred on 0 V.

Some students showed some sort of clipping, while very few students showed VOUT clipped at 3 V.

Answers, which require explanations and/or descriptions, need to be specific to the situation presented in the preamble. In question 3 many students were able to give generic answers of the meaning of the term "saturation" - such as "If the input voltage either is too large or too small (too negative) then VOUT is clipped and the amplifier is said to be in saturation. The output signal will be distorted." - leaving assessors to wonder if students were merely copying definitions straight from their A4 sheets. A question worth three marks requires more depth.

A clear and comprehensive answer for question 3 would discuss that in this situation the collector current is maximum, therefore the voltage drop across the collector resistor is maximum, so that VOUT reaches the minimum possible voltage and cannot go any lower. VOUT is clipped at the bottom. The transistor is operating like a closed switch.

Some students continue to experience difficulty with numerical calculations. This was evident in question 8 in this section. Most students identified that the "voltage divider" equation was the correct equation to apply. However, many did not substitute the correct values for R1 and R2. Other students were unable to calculate the final answer, i.e. to transpose the variables or simply unable to use the calculator.

In the detailed study of "Einstein's special relativity", students often have difficulty with the substitution and transposition of the time dilation and length contraction formulas. Students are usually able to identify the appropriate formula to use but have difficulty in determining which is the proper frame of reference.

In question 8, students were asked to estimate how long the pion existed as measured in its own frame of reference. The proper frame of reference, in this question, is the pion's frame of reference. The experimenters measure a longer time, 4.16 x 10-17 s. Hence to obtain the proper time, students had to divide by the Lorentz factor to get 2.6 x 10-8 s, A.

Another interesting question in this section is question 9. It requires you to show the average speed of the pion was 0.998 c, rather than to calculate the average speed (i.e. you are given the solution). Don't try to fool the assessors in similar questions (that now may only be found in the core!).

They watch very carefully for "fudging", and will examine each step closely. You must show sufficient working. Substituting 16 for g in the Lorentz factor equation, and transposing to make v the subject, will eventually give the required answer. Many students struggled with the mathematics. This question was not generally tackled well.

Every year, questions involving translational and rotational effects of forces (torque) in the "Investigating materials and their use in structures" detailed study prove difficult. Make sure you set your answers out correctly and clearly identify the points on which you are taking torques. Don't forget to include the gravitational field strength "g" when converting from mass into weight force, nor to multiply W by r when finding the torque.

Students often incorrectly substitute and transpose the turns ratio formula NP/NS = VP/VS in the "further electronics" section of the paper. The reciprocal of the correct answer is often given. Many students incorrectly chose D, instead of C, for this reason. Recognising the differences between linear components, where Ohm's law can be applied, and non-linear components, where the relationship is acquired from a graph (I-V characteristics), still proves to be an area of concern in both this detailed study and area of study 2 in the core.

Melanie Lane is a teacher of physics and mathematics. She is an assessor in VCE physics.