AS practical skills

Learning objectives

• Identify independent, dependent and controlled variables.
• Record units and appropriate precision.
• Use graphs and uncertainties to evaluate a relationship.

The big idea

Change one independent variable systematically while measuring the dependent variable and controlling other influences. Choose a safe range, repeat readings and record raw data clearly before calculating derived quantities.

A graph tests a model rather than proving it absolutely. Use a suitable scale and a best-fit line, then interpret its gradient and intercept physically. Uncertainty and anomalous readings should be discussed using evidence rather than removed simply to improve agreement.

1. Plan a measurement, not just a picture

State which variable you change, which response you measure and which conditions should remain fixed. Choose a range broad enough to reveal the relationship but safe for the apparatus. For a spring investigation, increase load in small steps, measure extension rather than final length and keep the instrument aligned. Specify how the load is supported and how falling masses will be contained.

2. Record evidence before processing

Give raw readings in a table with units in headings and consistent precision appropriate to the instrument. Record repeats rather than only a final average. A zero check can expose systematic error before collecting a large data set. Avoid changing inconvenient readings after seeing a graph; repeat them and document a reason if an instrument problem is found. Repeats alone do not remove bias or poor calibration.

3. Make the graph answer a question

Select axes so the model predicts a meaningful relationship. Use a scale covering a substantial part of the plotting area and draw a best-fit line or curve rather than mechanically joining every point. Estimate a gradient with well-separated points on the best-fit line, not necessarily original readings. State its units and use the physical model to convert the gradient into the desired quantity. An intercept may reveal a real offset or a limitation in the model.

4. Evaluate specific limitations

Compare uncertainty with the size of the measured effect. Timing ten oscillations can reduce the fractional effect of reaction time compared with timing one. A pointer and fixed reference can reduce ambiguity when measuring spring extension. Explain how a proposed change addresses the identified limitation. More readings over the same narrow range do not automatically resolve an unknown systematic error or make an unsuitable model correct.

A method you can reuse

1. Define variables, control conditions and safe limits.
2. Check instrument zero and collect labelled raw readings with repeats.
3. Process quantities without premature rounding.
4. Plot an appropriate best-fit relationship and calculate its gradient with units.
5. Identify the limiting uncertainty and a concrete targeted improvement.

Equations and relationships

Investigate and explain

Plan and carry out a teacher-approved force–extension or low-voltage circuit investigation. Include a risk assessment and uncertainty estimate.

Use simulation data to practise variables, graphs and model tests. Real practical competence also requires instrument handling, calibration, uncertainty estimation and safe work with apparatus; a noiseless model cannot replace those skills.

Open the interactive investigation

Check your understanding