AQA · GCSE · Physics

    Course version: 8463 · Version from Sept 2016

    Students should be able to use the concept of momentum as a model to:…

    Full specification wording

    Conservation of momentum: Students should be able to use the concept of momentum as a model to: describe and explain examples of momentum in an event, such as a collision (physics only) complete calculations involving an event, such as the collision of two objects.

    Using momentum as a model means writing total momentum before = total momentum after and filling in each term as mass × velocity.

    Checked against the AQA 8463 specification (opens in a new tab) on .

    • Point 3 of 3
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Using momentum as a model means writing total momentum before = total momentum after and filling in each term as mass × velocity. Choose a positive direction first. In a collision where objects join, the after side uses the combined mass. Where they bounce apart, one velocity is likely to be negative. A worked example: a 0.5 kg ball travelling at 4 m/s to the right hits a stationary 1.5 kg ball and stops dead. The total before is 0.5 × 4 = 2 kg m/s, so the second ball must carry 2 kg m/s afterwards, giving 2 ÷ 1.5 = 1.3 m/s to the right. Always finish by stating the direction as well as the number. If an object starts at rest its momentum is zero, but its mass still counts on the other side of the equation once it has been set moving. Check your work by recalculating the total momentum from your final velocities.

    What earns the marks

    • Stating or using total momentum before = total momentum after.
    • Calculating each momentum as mass × velocity with consistent directions.
    • A correct rearrangement to find the unknown mass or velocity.
    • The answer with its unit and its direction stated.

    Common mistakes and how to put them right

    • Using the mass of only one object after two have joined together.

    • Ignoring the sign of a velocity that points the opposite way.

    • Rounding an intermediate momentum so the final velocity is outside the allowed range.

    • Quoting a bare speed when the question asked for a velocity, which needs a direction.

    Exam tips

    • Set out a before and after table with mass and velocity for every object.
    • Keep full accuracy in the working and round only the final answer.
    • If an answer comes out negative, say what that means: the object moves the opposite way.

    Try the lesson

    The start of the Conservation of momentum: Students should be able to use the concept of momentum as a model to: describe and explain examples of momentum in an event, such as a collision (physics only) complete calculations involving an event, such as the collision of two objects. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A 2 kg trolley moving at 3 m/s collides head-on with a stationary 4 kg trolley, and the two trolleys stick together. What is the total momentum of the two trolleys just after the collision?

    Listen

    Using momentum as a model means writing total momentum before = total momentum after and filling in each term as mass × velocity. Choose a positive direction first. In a collision where objects join, the after side uses the combined mass. Where they bounce apart, one velocity is likely to be negative. A worked example: a 0.5 kg ball travelling at 4 m/s to the right hits a stationary 1.5 kg ball and stops dead. The total before is 0.5 × 4 = 2 kg m/s, so the second ball must carry 2 kg m/s afterwards, giving 2 ÷ 1.5 = 1.3 m/s to the right. Always finish by stating the direction as well as the number. If an object starts at rest its momentum is zero, but its mass still counts on the other side of the equation once it has been set moving. Check your work by recalculating the total momentum from your final velocities. A strong answer earns marks for these points. stating or using total momentum before = total momentum after. calculating each momentum as mass × velocity with consistent directions. a correct rearrangement to find the unknown mass or velocity. the answer with its unit and its direction stated. Exam tip: Set out a before and after table with mass and velocity for every object.

    See it

    A top-down flowchart. The first box says choose a positive direction and points to a box stating total momentum before = total momentum after. That points to a box saying calculate each momentum as mass × velocity. This box branches to two boxes: one for objects that join, using the combined mass after, and one for objects that bounce apart, where one velocity is likely negative. Both of these point to a box saying rearrange to find unknown mass or velocity. That points to a box saying state answer with unit and direction, which points to a final box saying check by recalculating total momentum from final velocities.
    Shows the steps for using conservation of momentum in a collision, from choosing a positive direction to checking the answer.

    Key words

    Say what each word means before you turn the card.

    collisions

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

    momentum calculations

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    combined mass

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

    direction and sign

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

    See every definition
    collisions
    Events in which objects interact, and momentum can be described and explained using the concept of momentum as a model.
    momentum calculations
    Calculations involving an event such as the collision of two objects, using total momentum before = total momentum after with each momentum as mass × velocity.
    combined mass
    The mass used on the after side of the equation in a collision where objects join together.
    direction and sign
    Choosing a positive direction first, so that where objects bounce apart one velocity is likely to be negative and the answer states the direction as well as the number.

    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-down flowchart. The first box says choose a positive direction and points to a box stating total momentum before = total momentum after. That points to a box saying calculate each momentum as mass × velocity. This box branches to two boxes: one for objects that join, using the combined mass after, and one for objects that bounce apart, where one velocity is likely negative. Both of these point to a box saying rearrange to find unknown mass or velocity. That points to a box saying state answer with unit and direction, which points to a final box saying check by recalculating total momentum from final velocities.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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    Test your understanding of Conservation of momentum: Students should be able to use the concept of momentum as a model to: describe and explain examples of momentum in an event, such as a collision (physics only) complete calculations involving an event, such as the collision of two objects.

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