7.8 - Planning an Investigation
What a planning question looks like
- In an exam, you may be asked to plan an investigation to find out how one variable affects another.
- A typical planning question reads: "A student wants to find out how the length of a pendulum affects the time it takes to complete one swing. Plan an investigation the student could carry out."
- There's no single "right" experiment. What's judged is whether your plan contains each key ingredient:
- The variable you change
- The variable you measure
- The variables you keep the same
- The apparatus
- The method
- The repeats
- A safety precaution
A typical question
A student wants to find out how the length of a pendulum affects the time it takes to complete one swing. Plan an investigation the student could carry out.
The ingredients of a plan
A full plan works through each key ingredient. Here they are for an investigation into how pendulum length affects the time for one swing:
| Ingredient | What it means | Worked example (a pendulum) |
|---|---|---|
| The variable you change | The one thing you change, given a range of about five values in even steps | Length of the string: 20, 40, 60, 80, and 100 cm |
| The variable you measure | The thing you measure, and how you measure it | Time for one swing, found by timing twenty swings with a stopwatch and dividing by twenty |
| Controlled variables | The things kept the same to make it a fair test, each named specifically | Same mass on the string; same angle of release; same clamp stand, so nothing moves between runs |
| Apparatus | The right instrument for each variable measured or controlled | A metre stick for the string, a stopwatch for the swings, a clamp stand to hold the pendulum |
| Method | Enough detail to follow, including how each reading is taken | Set the first length, pull the mass to a fixed small angle, release, time twenty swings, divide by twenty, repeat for each length |
| Repeats | Repeat at each value and take a mean, for reliability | Three sets of twenty swings at each length, then the mean time |
| Safety | A hazard together with a way to reduce it | The mass could swing into someone, so keep the area clear and use a small angle of release |
What makes the test fair and reliable
- Keeping the controlled variables the same ensures that any change in the measured variable is caused only by the variable you changed - making it a fair test with a valid result.
- Repeating and taking a mean reduces the effect of anomalies and makes the result reliable.
Choosing the right apparatus
Always name the specific instrument needed to measure or control each variable.
| Job to do | The right apparatus | Example in an experiment |
|---|---|---|
| Measure a length or distance | Metre stick, or a ruler for shorter lengths | Length of the string on a pendulum |
| Measure a small thickness or diameter | Micrometer | Diameter of a resistance wire |
| Measure a time | Stopwatch, or light gates and a timer for very short times | Timing twenty swings of a pendulum |
| Measure a current | Ammeter, connected in series | Current through a resistor |
| Measure a potential difference | Voltmeter, connected in parallel | Potential difference across a resistor |
| Measure a temperature | Thermometer | Temperature of water as it cools |
| Measure a mass | Balance | Mass hung on a pendulum |
| Measure a force | Newton balance | Force needed to pull a block along a bench |
Key points for apparatus questions
- Name a specific instrument - "an ammeter" rather than "a meter".
- Match the instrument to the precision required - A micrometer measures the diameter of a wire, because a metre stick can't resolve a fraction of a millimetre.
Improving a method with better apparatus
- If single swings are timed by hand, time twenty swings and divide by twenty, or use light gates to remove reaction-time error.
- If a length is judged by eye, measure it using a metre stick or ruler.
Common exam questions
Question 1
A student plans an investigation into how the length of a wire affects its resistance. Name the variable to change, one variable to control for validity, and say how to make the results reliable.
The variable to change is the length of the wire, given a range of values. A variable to control for validity is any factor kept the same, such as the thickness of the wire, the material it is made from, or the supply voltage. To make the results reliable, repeat at each length and take a mean.
Question 2
Name the kinds of thing a good plan should include.
The variable to change with a range of values; a measurable variable to measure; the variables kept constant to make it a fair test; the apparatus and method; repeats and a mean for reliability; and a safety precaution.
Question 3
Why is "use different lengths" a weak way to describe the variable you change in a plan?
Because it gives no range; a good answer states several specific values in even steps, such as 20, 40, 60, 80, and 100 cm.
Question 4
A student writes "keep everything else the same" for the controlled variables. Why does this not score?
The controlled variables must be named specifically, such as the same mass on the string and the same angle of release; a general phrase does not identify any of them.
Question 5
Name a piece of apparatus a student could use to measure the diameter of a thin wire.
A micrometer. A ruler or metre stick cannot resolve a fraction of a millimetre, so it does not give a usable reading for a wire.
Question 6
Why would a plan time twenty swings of a pendulum rather than one?
Because a single swing is short enough that reaction time makes up a large share of the reading; timing twenty and dividing by twenty spreads that error across all of them.
Question 7
A plan says "the mass is heavy" as its safety point. What is missing?
A way of reducing the hazard; the hazard alone scores nothing, so it needs something like "keep the area around the stand clear and release the mass from a small angle".