Mini Physics
Part of Mini EducationSwitch subject
Search

Planning a Physics investigation

Key idea: Plan an O-Level Physics investigation by defining variables, selecting apparatus, writing a reproducible method, processing data and controlling risks.

  • SEC G3 Physics 2027
On this page

Learning objectives

  • Measurements of length, mass, temperature, time interval, volume of liquids/solids and force (e.g. weight) using appropriate instruments
  • Determination of the density of a liquid, or of a regularly or irregularly shaped solid that sinks in water
  • Determination of the value of the acceleration of free fall
  • Investigation of the effects of balanced and unbalanced forces
  • The principle of moments
  • Determination of the position of the centre of gravity of a plane lamina
  • Investigation of the factors affecting transfer of energy by thermal processes
  • Determination of heat capacities of materials
  • Latent heat of substances
  • The law of reflection
  • Determination of the position and characteristics of an optical image formed by a plane mirror or a thin converging lens
  • The refraction of light through glass blocks
  • The principle of total internal reflection
  • The focal length of lenses
  • Determination of the speed, wavelength and frequency of waves
  • Determination of the resistance of a circuit component
  • Investigation of the magnetic effect of current in a conductor
  • Investigation of the effects of electromagnetic induction

1. Definition

An experimental plan is a reproducible method for answering a stated question with measurements. It identifies what is changed, what is measured, what is controlled, how the evidence is processed and how significant risks are reduced.

2. Key Ideas

  • The independent variable is deliberately changed.
  • The dependent variable is measured or calculated in response.
  • Control variables are influential factors kept constant for a fair comparison.
  • Apparatus must have suitable range, resolution and access for the measurement.
  • The plan must state how results will be used, not stop after data collection.
  • A safety statement pairs a credible hazard with a specific control.

3. Detailed Explanations

Evidence chain for planning a Physics investigationSix connected stages run from the investigation question through variables, method, evidence and conclusion. Safety is shown as a control that applies throughout the method rather than an isolated generic warning.Build a reproducible route from question to conclusionQuestionstate the relationshipVariableschange · measure · controlMethodapparatus · range · repeatsEvidencetable · graph · calculationConclusionanswer from the trendSafety applies throughout the methodcredible hazard → specific control that reduces the risk
Scroll diagram horizontally to read all labels.
A complete plan is an evidence chain: the question determines the variables and method; the measurements are processed into evidence; the evidence supports the conclusion. Name a specific control for each credible risk.

A. Turn the question into variables

For “How does wire length affect resistance?”, change wire length, determine resistance from voltage and current, and control material, diameter and temperature. Do not list quantities that cannot affect the result merely to increase the number of controls.

B. Write a method another candidate could follow

Use an operational sequence:

  1. assemble the apparatus and state where each measuring instrument is connected or positioned;
  2. state the initial value of the independent variable and how it is measured;
  3. measure the dependent quantity using named apparatus;
  4. repeat readings where random variation is significant and calculate a mean;
  5. choose several well-spaced independent-variable values over a safe, feasible range;
  6. keep each named control variable constant;
  7. state the graph, calculation or comparison used to answer the question.

Avoid memorising “five values and three repeats” as an unconditional rule. The number and range must be adequate for the task, available time and apparatus.

C. Select apparatus with a reason

“Use a metre rule” is incomplete when instrument choice is being assessed. State why its range and millimetre divisions suit the expected length. A finer-resolution instrument is not helpful if the object does not fit or the technique introduces a larger error.

D. Connect risk to control

Write “hot water may scald; keep the beaker on a heatproof mat and use eye protection when transferring it,” not “take care.” Do not invent a severe hazard for harmless apparatus.

4. Common Mistakes

  • Changing two independent variables together.
  • Saying “keep everything else constant” without naming relevant controls.
  • Naming apparatus without explaining the measurement technique.
  • Collecting readings without stating how they lead to a conclusion.
  • Giving “repeat and average” as the only improvement to a systematic error.
  • Writing a precaution that does not reduce the named risk.

5. Exam Tips

Use a compact chain: change → measure → control → repeat → process → conclude → protect. Check that every noun in the question appears in a measurable step and that your proposed graph places the independent variable on the horizontal axis.

6. Worked Examples

Modelled example 1

Comparing insulation materials

Core

Problem

Plan an investigation to compare how well different materials reduce the cooling of hot water.
Study the worked solution

  1. Define a fair comparison

    Method

    Change only the insulation material and measure temperature fall.

    Reason

    Material is the independent variable; temperature fall over a fixed time is evidence of cooling.

    Working

    Use identical cups with equal water volumes at the same initial temperature. Wrap each cup with the same thickness and area of a different material.

  2. Collect reproducible readings

    Method

    Measure temperature at equal time intervals using the same thermometer arrangement.

    Reason

    Consistent timing, depth and cup covering prevent the measuring method from becoming another variable.

    Working

    Insert the same type of thermometer to the same depth, cover each cup in the same way, and keep room conditions and cup geometry constant.

  3. Process the evidence and control risk

    Method

    Repeat each material test and compare the mean temperature fall over the same fixed time.

    Reason

    The comparison answers the question, while repeats reduce the effect of random variation.

    Working

    Hot water can scald, so transfer it in a stable heat-resistant container and keep cups away from the bench edge.

Guided practice 2

Wire length and resistance

About 8 min

Problem

Outline a plan to investigate how the length of a wire affects its resistance. Identify what to change, measure and control, then state how the readings answer the question.

Draft the plan before opening support

Hints

Hint 1: separate measurements from the result
Resistance is determined from a potential-difference reading and a current reading; it is not read directly from wire length.
Hint 2: protect the fair comparison
Keep wire material and diameter constant, and limit heating so temperature does not change substantially.
Hint 3: finish with evidence
Use several well-spaced lengths, repeat readings where useful, calculate R = V/I, and state what graph or comparison will show the relationship.
View solution step by step

  1. Choose variables and apparatus

    Method

    Change the measured wire length and determine resistance from voltage and current.

    Reason

    This isolates the required cause-and-effect comparison.

    Working

    Connect an ammeter in series and a voltmeter across the selected length of resistance wire. Measure the length with a metre rule.

  2. Control heating and wire properties

    Method

    Use the same wire material and diameter, a small current, and open the switch between readings.

    Reason

    Resistance also depends on material, cross-sectional area and temperature.

    Working

    Keep material and diameter fixed; switch off between readings so the wire stays near the same temperature.

  3. Process the readings

    Method

    For each of several lengths, record V and I, calculate R = V/I, and plot resistance against length.

    Reason

    The graph reveals the relationship across a range instead of relying on one pair of values.

    Working

    Use a results table with columns for length, V, I and calculated R, then draw the graph.

Common misconception 3

Changing two variables together

Find and correct the mistake

Learner response

A learner investigates how pendulum length affects period. For each longer string, the learner also uses a heavier bob, times one oscillation once, and concludes that length caused the change. Locate the first design error and repair the plan.

Diagnose before viewing the repair

First design error

View solution step by step

  1. Find the first invalid comparison

    Method

    Reject changing bob mass together with length.

    Reason

    If the period changes, the learner cannot attribute it to length because two independent variables changed.

    Working

    Keep bob mass fixed while changing only the measured pendulum length.

  2. Repair the method

    Method

    Use the same bob and release angle for every measured length.

    Reason

    Keeping the other influential conditions constant makes length the only deliberate change.

    Working

    Time several oscillations, divide by their number to obtain the period, repeat and compare mean periods across several lengths.

Examiner practice 4

Force and spring extension

6 marks

Examination question

Plan an investigation to determine how the extension of a spring depends on the applied force. Include measurements, controls and processing. [6 marks]

Write your plan before viewing the mark scheme

View solution step by step

  1. Measure force and extension

    3 marks

    Method

    Measure the unloaded spring length, apply a known force, and measure the new length at eye level.

    Reason

    Extension is the loaded length minus the unloaded length.

    Working

    Use a fixed pointer beside a millimetre scale and repeat for several well-spaced forces without exceeding the elastic limit.

  2. Keep the comparison controlled

    1 mark

    Method

    Use the same spring and arrangement throughout.

    Reason

    Changing the spring would introduce different stiffness as another variable.

    Working

    Keep the spring, support, scale position and reading method unchanged.

  3. Process and conclude

    2 marks

    Method

    Repeat readings where useful, calculate mean extension, and plot extension against force.

    Reason

    A straight best-fit line through the origin within scatter supports direct proportionality over the tested range.

    Working

    Plot extension on the vertical axis against force on the horizontal axis and judge the best-fit line.

Challenge 5

Ramp angle and travel time

Minimal support

New context

Plan an investigation into how the angle of a ramp affects the time taken for a trolley to travel a fixed distance. State a reproducible release method, controls, processing and one specific risk control.

Attempt the less-supported plan

Hints

Hint 1: make the release reproducible
Release the same trolley from rest at a marked line without pushing it.
Hint 2: connect readings to a conclusion
Measure several angles, repeat each timing, calculate means and compare or plot mean time against angle.
View solution step by step

  1. Set the independent variable

    Method

    Measure the ramp angle with a protractor and use several safe, well-spaced angles.

    Reason

    Angle is the deliberate change, so ramp length is not changed at the same time.

    Working

    Record each angle before releasing the trolley, while leaving the ramp length fixed.

  2. Measure travel time consistently

    Method

    Mark one release line and a fixed finish line, release the same trolley from rest without a push, and measure the travel time.

    Reason

    The same trolley, travel distance, ramp surface and release method control other influences.

    Working

    Start timing as the trolley crosses the release line and stop at the same fixed finish line for every angle.

  3. Process evidence and reduce risk

    Method

    Repeat each angle, calculate mean time, and plot mean time against ramp angle.

    Reason

    The range shows the trend and repeats reduce random timing variation.

    Working

    Place a stop block or tray beyond the finish so the trolley cannot fall from the bench or strike someone.

7. Mind Stretchers

Mind stretcher 1: Why is a wide range useful?Extension

Explain why a wide range of the independent variable can strengthen an investigation, and state one limit on that choice.

Show Answer

A wider safe range makes a trend easier to distinguish from measurement scatter and reduces the percentage effect of reading resolution. It does not justify exceeding apparatus ratings or leaving the regime being investigated.

8. Practice and next step

Draft a plan from a familiar topic without looking at notes, then check it against the seven-part chain above. Continue to Manipulation, Measurement and Observation.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027