OCR · GCSE · Physics

    Course version: J249 · Version from Sept 2018

    Estimate, for everyday road transport, the speed, accelerations and forces…

    Full specification wording

    Physics on the move: estimate, for everyday road transport, the speed, accelerations and forces involved in large accelerations (content found only in the physics separate science qualification)

    This separate-science criterion asks you to make order-of-magnitude estimates for everyday road transport, covering three linked quantities: speed, acceleration and the force needed for a large…

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

    • Point 8 of 8
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    This separate-science criterion asks you to make order-of-magnitude estimates for everyday road transport, covering three linked quantities: speed, acceleration and the force needed for a large acceleration. Start from familiar speeds: a car on a motorway travels at roughly 30 m/s (about 70 mph), and a town speed of 30 mph is about 13 m/s. For a large acceleration, take a car reaching 27 m/s from rest in about 9 s, giving a ≈ 3 m/s². Then use F = ma with a mass near 1200 kg to get F ≈ 3600 N, roughly 4 kN. Check units: m/s for speed, m/s² for acceleration, N for force. Round to one significant figure and state your assumptions, because an estimate is judged on sensible values and clear reasoning, not on a single exact answer.

    What earns the marks

    • Selects a realistic everyday road speed such as about 13 m/s in town or about 30 m/s on a motorway.
    • Calculates acceleration from a chosen change in speed and time using a = Δv ÷ t with consistent units.
    • Applies F = ma with a sensible vehicle mass to estimate the force for a large acceleration.
    • Rounds estimates to one or two significant figures and states the assumptions behind them.
    • Keeps units consistent, quoting speed in m/s, acceleration in m/s² and force in N or kN.

    Common mistakes and how to put them right

    • Uses a speed in mph directly in a calculation.

      Put it right: Convert to m/s first, for example 30 mph ≈ 13 m/s.

    • Forgets to square or misapplies units for acceleration.

      Put it right: Acceleration is measured in m/s², not m/s.

    • Omits the vehicle mass when estimating force.

      Put it right: State a mass such as 1200 kg before applying F = ma.

    Exam tips

    • Write down your assumed values before calculating so the examiner can follow your reasoning.
    • Round to one significant figure for an estimate and say 'approximately' with the unit.
    • Sanity-check the answer: a few kN is a reasonable force for a family car accelerating hard.

    Try the lesson

    The start of the Physics on the move: estimate, for everyday road transport, the speed, accelerations and forces involved in large accelerations (content found only in the physics separate science qualification) lesson, made from checked teaching notes for this specification point.

    Have a go first

    A car on a public road speeds up from 0 m/s to 20 m/s in 5 s. Calculate its acceleration.

    Listen

    This separate-science criterion asks you to make order-of-magnitude estimates for everyday road transport, covering three linked quantities: speed, acceleration and the force needed for a large acceleration. Start from familiar speeds: a car on a motorway travels at roughly 30 m/s (about 70 mph), and a town speed of 30 mph is about 13 m/s. For a large acceleration, take a car reaching 27 m/s from rest in about 9 s, giving a ≈ 3 m/s². Then use F = ma with a mass near 1200 kg to get F ≈ 3600 N, roughly 4 kN. Check units: m/s for speed, m/s² for acceleration, N for force. Round to one significant figure and state your assumptions, because an estimate is judged on sensible values and clear reasoning, not on a single exact answer. A strong answer earns marks for these points. Selects a realistic everyday road speed such as about 13 m/s in town or about 30 m/s on a motorway. Calculates acceleration from a chosen change in speed and time using a = Δv ÷ t with consistent units. Applies F = ma with a sensible vehicle mass to estimate the force for a large acceleration. Rounds estimates to one or two significant figures and states the assumptions behind them. Exam tip: Write down your assumed values before calculating so the examiner can follow your reasoning.

    See it

    A top-to-bottom flowchart with seven boxes connected by arrows. The first box says choose a speed in m/s. An arrow leads to a box saying choose a change in speed and time. An arrow leads to a box saying calculate acceleration a = Δv ÷ t. An arrow leads to a box saying state a vehicle mass. An arrow leads to a box saying estimate force F = ma. An arrow leads to a box saying round to one significant figure and state assumptions. An arrow leads to a final box saying check units: m/s, m/s², N or kN.
    It shows the sequence of steps for estimating speed, acceleration and force in everyday road transport.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Check units: m/s, m/s², N or kN
    2. 2. Round to one significant figure and state assumptions
    3. 3. Calculate acceleration a = Δv ÷ t
    4. 4. Estimate force F = ma
    5. 5. Choose a speed in m/s
    6. 6. Choose a change in speed and time
    7. 7. State a vehicle mass

    Key words

    Say what each word means before you turn the card.

    speed

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

    acceleration

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

    force

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

    significant figures

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

    See every definition
    speed
    A quantity measured in m/s, such as about 13 m/s in town or about 30 m/s on a motorway.
    acceleration
    Found from a chosen change in speed and time using a = Δv ÷ t, and measured in m/s².
    force
    Estimated by applying F = ma with a sensible vehicle mass, and quoted in N or kN.
    significant figures
    Estimates are rounded to one or two of these, with the assumptions behind them stated.

    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 top-to-bottom flowchart with seven boxes connected by arrows. The first box says choose a speed in m/s. An arrow leads to a box saying choose a change in speed and time. An arrow leads to a box saying calculate acceleration a = Δv ÷ t. An arrow leads to a box saying state a vehicle mass. An arrow leads to a box saying estimate force F = ma. An arrow leads to a box saying round to one significant figure and state assumptions. An arrow leads to a final box saying check units: m/s, m/s², N or kN.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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