Pearson International A Level Physics Core Practical 2: Determine liquid viscosity using a falling ball

This core practical is part of Pearson's practical-learning programme. The method below is one defensible implementation, not a confidential instruction sheet. Centres may adapt equipment while preserving the scientific purpose, controls, measurements and safety requirements.

Aim and scientific principle

Infer viscosity from terminal falling speed after allowing the ball to reach steady motion away from container walls.

Begin by translating the title into a testable question. Identify what is deliberately changed, what is measured directly, what is calculated and what must remain controlled. Predict a relationship only after naming the mechanism. A prediction without a reason is weak; a reason without measurable variables cannot guide a practical design.

Apparatus and setup

Use transparent tube, viscous liquid, small spheres, micrometer, timing marks, timer or video, thermometer and density data. Select every measuring instrument by range and resolution, not by name alone. Draw a labelled scientific setup that shows the working geometry and exactly where readings are taken. Include a control or blank when it distinguishes the target effect from drift, contamination or background response.

Before collecting results, check zero readings, calibration, leaks, alignment or sterility as relevant. Record the instrument resolution beside the first raw-data table. If a digital device displays more digits than its underlying accuracy supports, do not treat every digit as equally certain.

Method

1. Measure sphere diameter and mark a timing region below the release where terminal speed is expected.
2. Release centrally without bubbles and time travel between marks.
3. Repeat with the same sphere or a planned radius range while recording temperature.
4. Calculate terminal speed and apply the specified force balance or Stokes relationship within its assumptions.

Plan the analysis before starting. This reveals whether an additional measurement is needed for a derived quantity and prevents a completed experiment from lacking a mass, length, concentration, background or calibration value. A method should be detailed enough for another trained student to reproduce, while avoiding decorative steps that do not affect validity.

Variables and controls

• Independent variable: sphere radius or liquid temperature.
• Dependent variable: terminal speed and derived viscosity.
• Important controls: