OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Describe how to measure distance and time and use these to calculate speed…
Full specification wording
Motion: describe how to measure distance and time and use these to calculate speed (to include: from graphs)
Speed is distance travelled divided by time taken, so a measurement needs both a length and a timing.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 6 of 14
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Speed is distance travelled divided by time taken, so a measurement needs both a length and a timing. Measure distance with a metre rule or tape measure, choosing a straight run between two clear marks, and measure time with a stopwatch started and stopped at those marks. Repeat and average to reduce random error. On a distance–time graph, speed is the gradient: pick two points on a straight section, read the change in distance and the change in time, then divide. A steeper line means greater speed; a horizontal line means the object is stationary. On a speed–time graph, distance travelled is the area under the line. Convert units before dividing so the answer is in m/s.
What earns the marks
- Selects a metre rule or tape measure for distance and a stopwatch for time, reading each to the smallest division available.
- Measures the distance along a straight run between two fixed marks and times the motion between the same two marks.
- Repeats the measurement and averages the results to reduce the effect of random error.
- Calculates speed by dividing the measured distance by the measured time, converting units first so the answer is in m/s.
- Finds speed from a distance–time graph by taking the gradient between two points on a straight section.
- Recognises that a steeper line on a distance–time graph represents a greater speed and a horizontal line represents zero speed.
Common mistakes and how to put them right
Divides time by distance instead of distance by time.
Put it right: Keep speed equal to distance divided by time.
Mixes centimetres and seconds without converting.
Put it right: Convert the distance to metres before dividing.
Reads the gradient from a single point rather than the change between two points.
Put it right: Use the change in distance divided by the change in time.
Treats a horizontal line on a distance–time graph as fast motion.
Put it right: Recognise it as stationary.
Exam tips
- Write the equation speed = distance ÷ time before substituting numbers so the rearrangement is clear.
- Show the unit conversion explicitly, for example 250 cm = 2.5 m, to secure the unit mark.
- When reading a graph, state the two coordinates you used so the gradient method is visible.
Try the lesson
The start of the Motion: describe how to measure distance and time and use these to calculate speed (to include: from graphs) lesson, made from checked teaching notes for this specification point.
Have a go first
A cyclist covers 100 m in 20 s. Using speed = distance ÷ time, what is the cyclist's average speed?
Listen
Speed is distance travelled divided by time taken, so a measurement needs both a length and a timing. Measure distance with a metre rule or tape measure, choosing a straight run between two clear marks, and measure time with a stopwatch started and stopped at those marks. Repeat and average to reduce random error. On a distance–time graph, speed is the gradient: pick two points on a straight section, read the change in distance and the change in time, then divide. A steeper line means greater speed; a horizontal line means the object is stationary. On a speed–time graph, distance travelled is the area under the line. Convert units before dividing so the answer is in m/s. A strong answer earns marks for these points. Selects a metre rule or tape measure for distance and a stopwatch for time, reading each to the smallest division available. Measures the distance along a straight run between two fixed marks and times the motion between the same two marks. Repeats the measurement and averages the results to reduce the effect of random error. Calculates speed by dividing the measured distance by the measured time, converting units first so the answer is in m/s. Exam tip: Write the equation speed = distance ÷ time before substituting numbers so the rearrangement is clear.
See it
Key words
Say what each word means before you turn the card.
speed
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gradient
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random error
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unit conversion
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See every definition
- speed
- Distance travelled divided by time taken, found by dividing the measured distance by the measured time.
- gradient
- On a distance–time graph, found by taking two points on a straight section and dividing the change in distance by the change in time.
- random error
- Reduced by repeating the measurement and averaging the results.
- unit conversion
- Done before dividing so the answer is in m/s, for example converting the distance to metres.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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