Edexcel IGCSE Physics · Spec 1.4
Investigating Motion Required Practical
Required practical: investigating the motion of everyday objects.
Physics revision video
Investigating Motion Required Practical
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Explained
Measuring motion, and what counts as improving the method
Every way of measuring a speed comes down to the same two quantities, a distance and the time taken to cover it. The methods differ only in how precisely they measure the time.
Three methods, in order of precision
A metre rule and a stopwatch is the simplest. It works, but human reaction time introduces an error of a couple of tenths of a second at each end, which matters a great deal over a short run and hardly at all over a long one.
Ticker tape gives you a record rather than a single reading. A timer prints dots at a fixed rate, usually fifty per second, so the spacing between dots shows how fast the trolley was moving at each moment. Dots close together mean slow, dots spreading out mean accelerating.
Light gates connected to a data logger are the most precise. The gate times the object automatically as it passes, with no human reaction involved, and two gates a known distance apart give you the speed at each point and therefore the acceleration.
Measuring acceleration with a trolley
Set the trolley on a track with two light gates a measured distance apart. Release it, and the data logger gives the speed at each gate and the time between them.
Acceleration is the change in speed divided by that time. Repeat the run several times and take a mean.
Friction is the main systematic error, and the standard compensation is to tilt the track very slightly until the trolley moves at a steady speed when given a gentle push. At that angle the component of the weight down the slope balances the friction, so any further force you apply produces the acceleration you are actually measuring.
What the mark scheme accepts and rejects
Edexcel International GCSE Physics mark schemes for method questions reserve a mark for what one calls further experimental detail relating to accuracy. On a volume measurement question, the examples it gives are avoiding splashing, reading the measuring cylinder at eye level, placing it on a flat surface, and making sure no air is trapped. These are the details that show you have actually done the experiment rather than read about it.
A separate mark scheme is precise about what a repeat means. It awards a mark for repeating the experiment and calculating the mean, then rejects the answer if the method has been modified, giving the example of testing additional metals. Doing a different version of the experiment is not a repeat, however useful it might be.
It also allows identifying or ignoring anomalies as an alternative to calculating a mean, which tells you what repeats are for. They are not there to make the experiment longer; they are there to reveal results that do not fit.
On plotting, the same mark scheme rejects axis scales that go up in multiples of 30, 45, 60 or 80 for ten small squares, because those cannot be read reliably between the marked values. It allows a false origin. Scale choice is marked, and awkward scales lose the mark before any point is plotted.
Precision, accuracy and repeats
These three words are marked separately and mean different things.
Precision is about how finely you can measure. Light gates are more precise than a stopwatch because they resolve much smaller intervals of time.
Accuracy is about how close the reading is to the true value. A stopwatch started late is inaccurate no matter how precisely it displays the time.
Repeating and taking a mean reduces the effect of random error, such as reaction time varying from run to run. It does nothing at all about systematic error, such as friction or a stopwatch that always starts late, because that error is the same size every time. Saying which kind of error your improvement addresses is what separates a full answer from a partial one.
Spec 1.4
What you need to know
- Describe ways to measure speed
- Measure the acceleration of a trolley
- Improve the accuracy of your results
Active recall
Quick check
Answer each question before opening the answer.
What equipment can measure speed and acceleration accurately in the lab?
Light gates connected to a data logger (or ticker tape and timer).
How do you find speed from a single light gate?
Divide the length of the card by the time it takes to pass through the gate.
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