IGCSE Physics / Movement and position
Practical: investigating the motion of everyday objects revision notes
Edexcel International GCSE Physics (4PH1), chapter 1: Movement and position, specification point 1.5. Free lesson.
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Measure an object's average speed by timing it over a measured distance, then dividing distance by mean time. Repeats, longer distances and light gates reduce timing error, while a distance-time graph shows the object's speed through its gradient.
How to measure average speed with a stopwatch
Use fixed marks so the distance and timing are consistent. Release the object in the same way each time, because pushing it or changing its starting point could change its motion. The method works for a toy car or a rolling tennis ball.
How to reduce stopwatch timing error
Your reaction time when starting and stopping a stopwatch adds error to every time. The same reaction-time error has a bigger effect on a short time than on a longer one, so measuring over a longer distance can improve the speed value. For a reaction-time error of about 0.2 s, the percentage error is the error divided by the reading, then multiplied by 100. For a 2 s reading: 0.2/2 × 100 = 10%. For an 8 s reading: 0.2/8 × 100 = 2.5%. The same error is a smaller percentage of the longer time.
How light gates improve the timing
Light gates take the person out of starting and stopping the timer, so there is no reaction-time error. A card on the car interrupts each beam, and a data logger records the time. Run the car along a flat, level bench with a card on top, called an interrupt card. The first gate starts the timer when the card breaks its beam. The second gate stops it. Measure the distance between the gates with a metre rule. With one gate, you can find the speed at a point: card length ÷ time the beam is blocked.
How to find speed from a distance-time graph
Time the car to several marks, rather than measuring just one journey. Plot distance against time and use the mean time for each mark. If the points lie close to a straight line, draw a ruled line of best fit. A straight line shows that the car is moving at a steady speed. Put time on the horizontal axis and distance on the vertical axis. The gradient of the line gives the speed.
How to write a useful improvement
An evaluation answer must say exactly what you would change and why it helps. Words such as “accurate” or “reliable” do not earn marks on their own. Repeat each timing three times and calculate a mean, leaving out any anomalous result, which is one that does not fit the pattern. For the stopwatch method, use light gates and a data logger to remove reaction time, or measure over a longer distance so reaction time is a smaller percentage of each time.