OCR · GCSE · Physics

    Course version: J249 · Version from Sept 2018

    Motion: calculate average speed for non-uniform motion

    Non-uniform motion means the speed changes during a journey, so a single instantaneous speed does not describe the whole trip.

    Checked against the OCR J249 specification (opens in a new tab) on .

    • Point 11 of 14
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Non-uniform motion means the speed changes during a journey, so a single instantaneous speed does not describe the whole trip. Average speed is found by dividing the total distance travelled by the total time taken: average speed = total distance ÷ total time. For example, a cyclist covering 300 m in the first 20 s and 500 m in the next 40 s travels 800 m in 60 s, giving an average speed of about 13.3 m/s. Use the whole journey, not one stage, and convert mixed units such as minutes to seconds and kilometres to metres before dividing. The answer describes overall motion and does not claim the object moved at that speed throughout.

    What earns the marks

    • Adds the distances of each stage to obtain the total distance travelled.
    • Adds the times of each stage to obtain the total time taken.
    • Divides total distance by total time to calculate the average speed.
    • Converts mixed units such as minutes to seconds or kilometres to metres before dividing.

    Common mistakes and how to put them right

    • Averages the separate speeds of each stage.

      Put it right by dividing total distance by total time instead.

    • Uses only one stage's distance or time.

      Put it right by summing every stage of the journey.

    • Mixes units such as minutes and seconds.

      Put it right by converting all quantities to the same units first.

    Exam tips

    • Write down total distance and total time separately before dividing.
    • Convert all times to seconds and distances to metres to keep units consistent.
    • Check the size of the answer against the speeds during the journey for plausibility.
    • Show the division clearly so the method is visible.

    Try the lesson

    The start of the Motion: calculate average speed for non-uniform motion lesson, made from checked teaching notes for this specification point.

    Have a go first

    A bus travels 120 m in 20 s, speeding up and slowing down along the way. What is its average speed?

    Listen

    Non-uniform motion means the speed changes during a journey, so a single instantaneous speed does not describe the whole trip. Average speed is found by dividing the total distance travelled by the total time taken: average speed = total distance ÷ total time. For example, a cyclist covering 300 m in the first 20 s and 500 m in the next 40 s travels 800 m in 60 s, giving an average speed of about 13.3 m/s. Use the whole journey, not one stage, and convert mixed units such as minutes to seconds and kilometres to metres before dividing. The answer describes overall motion and does not claim the object moved at that speed throughout. A strong answer earns marks for these points. Adds the distances of each stage to obtain the total distance travelled. Adds the times of each stage to obtain the total time taken. Divides total distance by total time to calculate the average speed. Converts mixed units such as minutes to seconds or kilometres to metres before dividing. A common slip: Averages the separate speeds of each stage. Instead, dividing total distance by total time instead. Exam tip: Write down total distance and total time separately before dividing.

    See it

    A flowchart running top to bottom. The first box states that non-uniform motion means speed changes during a journey. An arrow leads to a box saying to add the distances of each stage to get the total distance. An arrow leads to a box saying to add the times of each stage to get the total time. An arrow leads to a box saying to convert mixed units, minutes to seconds and kilometres to metres. An arrow leads to a box saying to divide total distance by total time. A final arrow leads to a box giving the average speed for the whole journey.
    It shows the steps for calculating average speed for non-uniform motion: summing distances, summing times, converting units, then dividing total distance by total time.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Add times of each stage to get total time
    2. 2. Add distances of each stage to get total distance
    3. 3. Non-uniform motion: speed changes during journey
    4. 4. Average speed for the whole journey
    5. 5. Divide total distance by total time
    6. 6. Convert mixed units: minutes to seconds, kilometres to metres

    Key words

    Say what each word means before you turn the card.

    Average speed

    Try answering aloud or in your head before turning the card.

    Non-uniform motion

    Try answering aloud or in your head before turning the card.

    Total distance and total time

    Try answering aloud or in your head before turning the card.

    Unit conversion

    Try answering aloud or in your head before turning the card.

    See every definition
    Average speed
    Found by dividing the total distance travelled by the total time taken for the whole journey.
    Non-uniform motion
    Motion where the speed changes during a journey, so a single instantaneous speed does not describe the whole trip.
    Total distance and total time
    The distances of each stage added together and the times of each stage added together, used for the whole journey.
    Unit conversion
    Converting mixed units such as minutes to seconds and kilometres to metres before dividing.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart running top to bottom. The first box states that non-uniform motion means speed changes during a journey. An arrow leads to a box saying to add the distances of each stage to get the total distance. An arrow leads to a box saying to add the times of each stage to get the total time. An arrow leads to a box saying to convert mixed units, minutes to seconds and kilometres to metres. An arrow leads to a box saying to divide total distance by total time. A final arrow leads to a box giving the average speed for the whole journey.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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