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Practical Skills » Planning Investigations

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Study time: 30 minutes

Edexcel spec: Experimental skills, Suggested practical investigations (Appendix 6)

Plan an investigation in a clear order, from the question to the measurements
Choose equipment and a safe, skilful technique that gives precise readings
Set out a results table with headings and units
Use four of the spec's suggested investigations as planning practice

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Planning starts with a question

In the exam you may be asked to plan an investigation you have never done. That is fine. Every good plan answers the same questions in the same order, and you can use your physics to choose the method. The spec says you may be tested on your ability to devise and plan investigations, describe methods, make measurements with suitable precision and record them methodically. It also says you may have to solve problems set in a practical context.

Use this checklist every time:

Step	Question to answer
1. Aim	What am I trying to find out? Write it as "how X affects Y".
2. Variables	What will I change, what will I measure, and what must I keep the same?
3. Equipment	Which instrument measures each quantity, and is it precise enough?
4. Method	What do I do, in numbered steps, so someone else could repeat it?
5. Safety	What could go wrong, and how do I stop it?
6. Results	How will I record the readings?

You met the independent, dependent and control variables in the lesson Practical: Investigating Motion. In a plan, name all three clearly. Naming a control variable is not enough: say how you will keep it the same.

Key terms:

Precision: how finely an instrument can measure. A ruler marked in millimetres is more precise than one marked only in centimetres.
Risk assessment: spotting a hazard, saying what harm it could cause, and saying how you will reduce the risk.

Choosing equipment and technique

📏 Measure precisely

Choose the instrument whose smallest division suits the size of the thing you measure. A metre rule read to the nearest millimetre suits a rebound height of 40 cm. Read at eye level so the scale is not misread. Quote readings to the same number of decimal places, for example 24.5 cm and 26.0 cm, not 24.5 cm and 26 cm.

⏱️ Measure skilfully

Start the stopwatch and the event together, and time a longer event where you can, (the reaction time from Practical: Investigating Motion matters less). Zero a newton meter before use. Hold rulers vertical for heights. Take more than one reading of each setting and use the mean.

Remember from Practical: Refractive Index of Glass, the range is the lowest to highest values of the independent variable. Pick a sensible range: at least five values of the independent variable, spread evenly, such as drop heights of 20, 40, 60, 80 and 100 cm. Too few values make a pattern hard to see.

Safety in a plan

Name one real hazard and one action. Examples: hot lamps (let them cool before touching), falling masses (keep feet clear), hot water (use a tray and a heatproof mat), flying or snapping objects (wear eye protection). "Be careful" earns nothing.

Recording results

Draw the results table before you start. The independent variable goes in the first column, then one column for each repeat, then a column for the mean. Put the quantity and the unit in the column heading and only numbers in the cells.

Drop height (cm)	Rebound 1 (cm)	Rebound 2 (cm)	Rebound 3 (cm)	Mean rebound (cm)
20
40

Results are then presented in a suitable way, usually a graph with the independent variable on the horizontal axis. How to analyse and evaluate them is the next lesson, Analysing and Evaluating Experimental Data.

The suggested investigations

The spec ends with a list of suggested practical investigations (Appendix 6). They are extras that show the scientific process. They are suggestions, not required practicals, but they make excellent planning practice. They are:

power consumption of low-voltage electrical items
factors affecting the generation of current by induction
how the nature of a surface affects energy radiated or absorbed
models to show refraction, such as toy cars travelling into a region of sand
areas beyond the visible spectrum, as found by Herschel and Ritter (infrared and ultraviolet)
the relationship between voltage, current and resistance, and for a component whose resistance varies with temperature, light intensity or pressure
the relationship between force, mass and acceleration
forces needed to slide blocks along different surfaces
how crumple zones reduce forces in collisions
forces between charges
the motion of falling
momentum during collisions
power by running up stairs or lifting objects of different weights
the critical angle for Perspex/air, glass/air or water/air boundaries
factors affecting the height of rebound of bouncing balls
the temperature and volume relationship, and the volume and pressure relationship, for a gas
absorption of light by translucent materials

Four of them, planned in outline:

Investigation	Independent	Dependent	Control variables	Instrument
Rebound height of a ball	Drop height	Rebound height	Same ball, same hard floor, same way of releasing the ball	Metre rule
Sliding a block along surfaces	Type of surface	Force to keep the block moving steadily	Same block, same speed of pull, same pulling direction	Newton meter
Power running up stairs	Mass of the person (or height of stairs)	Power	Same stairs for each person, same way of timing	Bathroom scales, ruler, stopwatch
Absorption of light by tracing paper	Number of layers	Light reaching a sensor	Same lamp, same distances, room darkened	Light sensor, or LDR with ammeter

For the stairs, the energy you transfer is your weight × the height climbed, and power is that divided by the time. So you measure your mass (weight = mass × 10 N/kg), the vertical height of the stairs and the time. A person of mass 50 kg climbing 3.0 m in 4.0 s transfers 50 × 10 × 3.0 = 1500 J, so the power is 1500 / 4.0 = 375 W. Choose a clear, dry staircase with a handrail.

For the sliding block, pull it with the newton meter at a slow, steady speed. The reading while it moves steadily equals the friction force. You could also tilt a board covered with each surface and measure how steep it must be before the block starts to slip.

A model 6 mark plan

Question: Describe how you would investigate how the drop height of a ball affects its rebound height. [6 marks]

Model answer

Place a metre rule upright next to a hard, flat floor and hold it steady (1).
Drop one ball from a height of 20 cm, measured from the bottom of the ball to the floor, releasing it without pushing it (1).
Watch from eye level, or film it, and record how high the bottom of the ball rises on the first bounce (1).
Repeat three times for this height and find the mean, then repeat for at least five drop heights, such as 40, 60, 80 and 100 cm, keeping the same ball and floor (1).
Record the readings in a table with headings and units (1).
Keep the area clear so the ball does not hit anyone (1).

Notice the pattern: the independent variable, the instrument, the way of reading it, repeats, a range, controls, a table and safety. Real mark schemes differ, but good plans have all of these.

Common mistakes

Saying "keep it a fair test" without saying what is kept the same.
Changing two things at once, such as using a different ball and a different height.
Choosing an instrument that is too imprecise, such as timing a 0.3 s fall with a hand stopwatch.
Putting units in every cell instead of in the column heading.
Giving only one reading for each setting.
Vague safety: "be careful" instead of a named hazard and a named action.

Exam-style question

A student plans to find out how the number of layers of tracing paper affects the amount of light that passes through it. A lamp and a light sensor are available.

(a) State the independent variable and the dependent variable. [2 marks]

(b) Describe one control variable and how you would keep it the same. [2 marks]

Model answer

(a) Independent variable: the number of layers of paper (1). Dependent variable: the light reading from the sensor (1).

(b) Any one: the distance from the lamp to the sensor (1), kept the same by fixing both in clamps and measuring with a ruler (1). Or the room light (1), kept the same by darkening the room for every reading (1).

(c) A table with the number of layers in the first column and the light reading in the next columns, with repeat readings and a mean (1). Every column heading has a quantity and a unit (1).

Exam tip

For any "plan" question, say what you change, what you measure, what you keep the same, and how you measure it. Four answers, four chances of marks.

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