AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    Momentum is a property of moving objects: momentum = mass × velocity

    Momentum is a property of every moving object, calculated as momentum = mass × velocity.

    Checked against the AQA 8464 specification (opens in a new tab) on .

    • Point 2 of 6
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Momentum is a property of every moving object, calculated as momentum = mass × velocity. Mass is measured in kilograms (kg) and velocity in metres per second (m/s), so momentum has units kg m/s. Velocity is a vector, so momentum has direction as well as size. A 2 kg ball moving at 3 m/s has momentum 6 kg m/s in the direction of motion. If the ball moves in the opposite direction, its momentum is −6 kg m/s. A stationary object has zero momentum because its velocity is zero. Doubling the mass doubles the momentum if velocity is unchanged; doubling the velocity doubles the momentum if mass is unchanged. Momentum explains why a heavy lorry is harder to stop than a small car at the same speed, and why a fast-moving object is harder to stop than a slow one of the same mass.

    What earns the marks

    • State the equation momentum = mass × velocity and identify momentum, mass and velocity with their units kg m/s, kg and m/s.
    • Substitute values correctly into p = m v, including converting grams to kilograms and km/h to m/s where needed.
    • Recognise that velocity is a vector, so momentum has direction; use positive and negative values for opposite directions.
    • Explain that a stationary object has zero momentum and that changing mass or velocity changes momentum proportionally.
    • Calculate momentum for a moving object and interpret the result in terms of how difficult it is to stop.

    Common mistakes and how to put them right

    • Using grams instead of kilograms.

      Put it right: Convert mass to kg by dividing by 1000 before multiplying.

    • Treating velocity as a scalar and ignoring direction.

      Put it right: State the direction or use a sign for momentum in one-dimensional problems.

    • Confusing momentum with kinetic energy.

      Put it right: Momentum is m v, while kinetic energy is ½ m v², so the values and units differ.

    Exam tips

    • Write the equation, substitute the numbers with units, then give the unit kg m/s in your answer.
    • Check that mass is in kg and velocity is in m/s before calculating; convert first if necessary.
    • For direction questions, choose a positive direction and give momentum a sign, explaining what the sign means.

    Try the lesson

    The start of the Momentum is a property of moving objects: momentum = mass × velocity lesson, made from checked teaching notes for this specification point.

    Have a go first

    A cyclist of mass 80 kg rides at a velocity of 5 m/s. Using momentum = mass × velocity, what is the momentum of the cyclist?

    Listen

    Momentum is a property of every moving object, calculated as momentum = mass × velocity. Mass is measured in kilograms (kg) and velocity in metres per second (m/s), so momentum has units kg m/s. Velocity is a vector, so momentum has direction as well as size. A 2 kg ball moving at 3 m/s has momentum 6 kg m/s in the direction of motion. If the ball moves in the opposite direction, its momentum is −6 kg m/s. A stationary object has zero momentum because its velocity is zero. Doubling the mass doubles the momentum if velocity is unchanged; doubling the velocity doubles the momentum if mass is unchanged. Momentum explains why a heavy lorry is harder to stop than a small car at the same speed, and why a fast-moving object is harder to stop than a slow one of the same mass. A strong answer earns marks for these points. State the equation momentum = mass × velocity and identify momentum, mass and velocity with their units kg m/s, kg and m/s. Substitute values correctly into p = m v, including converting grams to kilograms and km/h to m/s where needed. Recognise that velocity is a vector, so momentum has direction; use positive and negative values for opposite directions. Explain that a stationary object has zero momentum and that changing mass or velocity changes momentum proportionally. Exam tip: Write the equation, substitute the numbers with units, then give the unit kg m/s in your answer.

    See it

    A flowchart. Two boxes at the top, 'Mass in kg' and 'Velocity in m/s', both point down to a box reading 'momentum = mass × velocity'. That box points to 'Momentum in kg m/s'. From there, one arrow leads to 'Momentum has direction', which splits into two boxes: 'Positive value: one direction' and 'Negative value: opposite direction'. A second arrow from 'Momentum in kg m/s' leads to 'Stationary object: velocity zero, so momentum zero'.
    It shows how mass and velocity combine to give momentum in kg m/s, that momentum has direction with positive and negative values, and that a stationary object has zero momentum.

    Key words

    Say what each word means before you turn the card.

    Momentum

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

    Velocity

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

    Stationary object

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

    See every definition
    Momentum
    A property of every moving object, calculated as mass multiplied by velocity, with units kg m/s.
    Velocity
    A vector measured in metres per second, so momentum has direction as well as size.
    Stationary object
    An object with zero momentum because its velocity is zero.

    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. Two boxes at the top, 'Mass in kg' and 'Velocity in m/s', both point down to a box reading 'momentum = mass × velocity'. That box points to 'Momentum in kg m/s'. From there, one arrow leads to 'Momentum has direction', which splits into two boxes: 'Positive value: one direction' and 'Negative value: opposite direction'. A second arrow from 'Momentum in kg m/s' leads to 'Stationary object: velocity zero, so momentum zero'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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