In this part of our Physics Practical Skills Series, we explain how to nail practical exams.
Students are often told, “you can’t study for prac exams because they test skills, not knowledge.”
This is simply not true!
Students must prepare themselves for a practical assessment, just as for any other assessment. In this post, we will outline how to prepare for a practical assessment.
Physics Practical exams can be:
In First Hand Investigations, students must carry out an experiment and obtain data themselves. In Second Hand Investigations, the experiment is described and the data provided.
The remainder of the assessment is the same, as summarised in the table below.
|Table: First Hand investigations vs Second Hand Investigations|
First Hand Investigation
Second Hand Investigations
|Students are provided with equipment and possibly instructions and must carry out the experiment.||N/A – The experiment may be described in the question.|
|Students must record data using the equipment provided in the course of the experiment.||N/A – Data is provided, usually in a table.|
|Students must analyse the data. Often this involves graphing it so as to produce a straight line graph. The gradient and/or intercept are important.|
|Students must use the data (e.g. the gradient from their graph) and relevant equations to address the aim of the experiment (e.g. calculate an unknown).|
Analysis of method and result
|Students must be able to discuss the variables in the experiment, and assess the method and the result in terms of reliability, accuracy and validity, and suggest improvements.|
Analysis of errors
|Students must be able to discuss the errors in the experiment and suggest improvements.|
Second Hand Investigations may appear in exams, particularly the HSC.
Let’s consider the different aspects of practical assessments. Here are some things that can be done to prepare, and are important to keep in mind.
It’s impossible to predict the experiment in advance, so you should not waste time focusing on that. Instead, you should think through all the possibilities:
Typically the analysis involves drawing a graph which produces a straight line. Sometimes the data needs to be manipulated in order for the graph to produce a straight line so you must understand the underlying governing equations that describe the experiment. This means you must study the theory related to the topic of the experiment.
To apply the theory to the practical assessment you must consider the variables, the relevant equation, and how to apply it. Some additional adjustments may be required: e.g. if the equation gives force, but you measured mass.
For practice, you can go through the equations and consider the dependence of different variables. Make sure you consider how you can produce a straight line for different combinations of variables.
Once you have drawn the graph, you will need to consider the equation that represents the graph, and how that compares to the equations you have already studied. Typically, you will need to identify the meaning of the gradient or the intercept and use it to calculate one of the (control) variables in the experiment or one of the physical constants.
Once again, a good understanding of the underlying theory is important.
Students are often asked to analyse the experiment in terms of:
Finally, practice, practice, practice!
If you know what the experiment will be in advance, you can try and assemble the necessary equipment and practice. If you don’t, then practice anyway on a possible experiment. The important thing is to think through and understand the different aspects of an experiment outlined above. You will then be able to apply them to any experiment.
Here is a simple example: measure the acceleration due to gravity g by dropping a ball.
The independent variable is the height of the ball, and the dependent variable is the time it takes to fall. You set the height and measure it with a tape measure, drop a ball from this height and time how long it takes to fall with a stopwatch or your phone.
You should select a good number of measurements, e.g. five different heights, and measure the time taken from each one five times. For each height, average the five-time measurements.
You must also select a good range for the independent variable (height) that makes the dependent variable (time to fall) able to be measured. For example, if you drop the ball from a height of 5 cm, you cannot measure the time to fall accurately due to your reaction time. On the other hand, heights of 2 m or more are impractical (as you can’t reach). A good range might be from 1 m to 2 m in 20 cm increments.
Once you have your data, consider how to analyse it. You must start with the governing equation. In this case, it is:
Δr = ut + ½at2
Taking this into account gives:
Δr = ½gt2.
To proceed with the analysis of the data, a graph is used in order to reduce the effect of errors. Some tips on drawing scientific graphs can be found in this blog, and this blog explains why drawing graphs reduces some errors.
Typically we plot the independent variable on the x-axis and the dependent variable on the y-axis. If we did that we would have x = Δr and y = t and the equation describing the graph would be:
y = (2x/g)½.
This is not a straight line, so g will be difficult to determine.
Instead, the data must be manipulated. By looking at Δr = ½gt2 we see that Δr is proportional to t2, so we must calculate t2 and plot a graph with x = Δr and y = t2. The equation describing the graph will be:
y = 2x/g.
This is a straight line with a gradient of 2/g. Drawing a line of best fit and measuring its gradient will allow you to determine g:
g = 2/(gradient).
Now that you have your value, what can you analyse?
You may need to suggest improvements. For example, a small heavy object will be less affected by air resistance in this experiment. A small metal ball or a marble would be more suitable than a scrunched up piece of paper or a ping-pong ball. Dropping a feather, on the other hand, would make the experiment invalid.
You may need to suggest alternative experiments. For example, the Year 12 physics syllabus requires students to measure g using the motion of a pendulum.
Want more practice? Examples of Second Hand Investigations can be found in past HSC papers, or you can download some additional practice First Hand Investigations from this page.