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Measurement (Cambridge O Level Physics)
Revision Note
Measuring Length & Volume
- Rulers can be used to measure small distances of a few centimetres (cm).
- They are able to measure to the nearest millimetre (mm)
Measuring with a ruler
A ruler can measure small distances to the nearest mm
- When measuring larger distances (of a few metres) a tape measure is more appropriate or, when measuring even larger distances, a trundle wheel
A trundle wheel
Trundle wheels can be used to measure large distances
- Measuring cylinders can be used to measure the volume of liquids
- By measuring the change in volume, a measuring cylinder can also be used to find the volume of an irregular shape
Measuring cylinders to measure volumes
Measuring cylinders can be used to determine the volume of a liquid or an irregular shaped solid
Worked example
The diagram shows four identical ball-bearings placed between two blocks on a steel ruler.
Calculate the diameter of one ball-bearing.
Answer:
Step 1: Measure the length of all four ball-bearings
- The blocks mark the edges of the first and last ball bearings
- The blocks make it easier to measure the length of all four ball-bearings
Total length = 12 cm − 4 cm = 8 cm
Step 2: Divide the total length by the number of ball-bearings
Diameter = total length ÷ number of ball-bearings
Diameter = 8 ÷ 4
Diameter = 2 cm
Measuring Time
- Stop-clocks and stopwatches can be used to measure time intervals
- An important factor when measuring time intervals is human reaction time. This can have a significant impact upon measurements when the measurements involved are very short (less than a second)
Worked example
A stopwatch is used to measure the time taken for a runner to complete a lap of a 400 m track.
The images below give the readings on the stopwatch at the start and the end of the lap.
Calculate how long it took the runner to complete the lap. Give your answer in seconds.
Answer:
Step 1: Identify the start time for the lap
- The stopwatch was already at 0:55:10 when the runner started the lap
- Start time = 55.10 seconds (s)
Step 2: Identify the finish time for the lap
- The stopwatch reads 1:45:10 at the end of the lap
- Finish time = 1 minute and 45.10 s
Step 3: Convert the finish time into seconds
1 minute = 60 seconds
Finish time = 60 s + 45.10 s
Finish time = 105.10 s
Step 4: Calculate the time taken to complete the lap
The time taken to complete the lap = finish time − start time
Time taken to complete lap = 105.10 s − 55.10 s
Time taken to complete lap = 50 s
Examiner Tip
You will sometimes find that information is given in the question that is not actually needed in the calculation.
In this worked example, you were told that the track the runner is running on is 400 m. This had nothing to do with the calculation the question asked you to perform.
This is a common method for making a question seem more difficult. Don't let it catch you out!
Multiple Readings
Multiple readings scenarios
Examples of measurements where multiple readings are needed
- Suppose you have to measure the thickness of a sheet of paper
- The thing that you are trying to measure is so small that it would be very difficult to get an accurate answer
- If, however, you measure the thickness of 100 sheets of paper you can do so much more accurately
- Dividing your answer by 100 will then give an accurate figure for the thickness of one sheet
- This process of taking a reading of a large number of values and then dividing by the number, is a good way of getting accurate values for small figures, including (for example) the time period of a pendulum
- Measure the time taken for 10 swings and then divide that time by 10 to find the average
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