Practical Activity: Planning, Measuring and Evaluating
What practical work demands
The practical component assesses whether you can plan a valid experiment, take reliable readings, process data and evaluate uncertainty. You should be able to identify independent, dependent and control variables, choose suitable apparatus, and describe a method that another student could repeat. Familiar contexts include investigating Ohm's law, Hooke's law, acceleration down a ramp, the period of a pendulum, and the refraction of light through a block.
Concepts you must understand
Know the difference between precision and accuracy, and between random and systematic error. Understand resolution of instruments: a metre rule to 1 mm, a vernier calliper to 0.1 mm, a micrometer to 0.01 mm, a stopwatch to 0.01 s. Know that reaction time causes random error in timing and can be reduced by timing multiple oscillations and dividing. Recognise that a zero error is systematic and can be removed by subtracting it. Be able to plot a graph, draw a line of best fit, and use its gradient to determine a physical quantity.
Traps to avoid
- Failing to repeat readings and average them.
- Plotting the wrong variable on each axis, so the gradient has no physical meaning.
- Forcing the line of best fit through the origin when the data do not support it.
- Reporting a percentage uncertainty without showing the calculation.
- Describing an improvement vaguely, such as 'be more careful', rather than naming a specific change.
Rule of thumb and worked example
Percentage uncertainty = (uncertainty ÷ measured value) × 100. Example: a pendulum's 20 oscillations take 40.0 s with a stopwatch uncertainty of 0.2 s. The period is 2.00 s. The percentage uncertainty in the total time is (0.2 ÷ 40.0) × 100 = 0.5%, and this carries through to the period. Increasing the number of oscillations timed reduces the percentage uncertainty, because the fixed timing uncertainty is shared over a larger measured value.