AQA · A-Level · Physics

    Course version: 7408 · Version from Sept 2016

    Projectile motion: Qualitative treatment of friction.

    Friction is a contact force that opposes relative motion or the tendency to move between surfaces.

    Checked against the AQA 7408 specification (opens in a new tab) on .

    • Point 3 of 7
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Friction is a contact force that opposes relative motion or the tendency to move between surfaces. In projectile motion it acts opposite to the velocity, reducing horizontal range and altering the trajectory compared with ideal motion. Qualitatively, friction depends on the nature of the surfaces and the normal contact force: rougher surfaces and larger normal forces give greater friction. For a projectile, air resistance is a form of fluid friction that increases with speed, so the horizontal component of velocity decreases and the path becomes asymmetric, with a steeper descent than ascent. You should describe these effects in words and identify the direction of the frictional force without calculating coefficients.

    What earns the marks

    • States that friction acts in the opposite direction to the relative motion or attempted motion of the projectile through the medium.
    • Explains that friction reduces the horizontal component of velocity, so the range is less than for ideal projectile motion.
    • Describes that friction causes the trajectory to become asymmetric, with a steeper descent than ascent.
    • Identifies that friction depends qualitatively on surface roughness and the normal contact force, and for air resistance increases with speed.
    • Applies the qualitative idea to a named situation, such as a ball thrown through air or sliding on a rough surface, without numerical calculation.

    Common mistakes and how to put them right

    • Treating friction as acting in the direction of motion.

      Put it right by stating that friction opposes relative motion or attempted motion.

    • Assuming friction only affects vertical motion.

      Put it right by explaining that friction also reduces the horizontal velocity component.

    • Believing friction is constant regardless of speed for air resistance.

      Put it right by noting that air resistance increases with speed.

    Exam tips

    • Use the phrase 'opposes relative motion' when defining the direction of friction.
    • Link friction explicitly to the reduced horizontal range and the asymmetric shape of the trajectory.
    • Keep the treatment qualitative: describe effects in words rather than attempting calculations with coefficients.

    Try the lesson

    The start of the Projectile motion: Qualitative treatment of friction. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A box slides across a rough horizontal floor and gradually slows down. What causes the box to slow down?

    Listen

    Friction is a contact force that opposes relative motion or the tendency to move between surfaces. In projectile motion it acts opposite to the velocity, reducing horizontal range and altering the trajectory compared with ideal motion. Qualitatively, friction depends on the nature of the surfaces and the normal contact force: rougher surfaces and larger normal forces give greater friction. For a projectile, air resistance is a form of fluid friction that increases with speed, so the horizontal component of velocity decreases and the path becomes asymmetric, with a steeper descent than ascent. You should describe these effects in words and identify the direction of the frictional force without calculating coefficients. A strong answer earns marks for these points. States that friction acts in the opposite direction to the relative motion or attempted motion of the projectile through the medium. Explains that friction reduces the horizontal component of velocity, so the range is less than for ideal projectile motion. Describes that friction causes the trajectory to become asymmetric, with a steeper descent than ascent. Identifies that friction depends qualitatively on surface roughness and the normal contact force, and for air resistance increases with speed. A common slip: Treating friction as acting in the direction of motion. Instead, stating that friction opposes relative motion or attempted motion. Exam tip: Use the phrase 'opposes relative motion' when defining the direction of friction.

    See it

    A top-down flowchart. The top node reads 'Friction: contact force'. From it, four branches lead down. The first branch goes to 'Opposes relative motion or tendency to move'. The second goes to 'Depends on nature of surfaces', which leads to 'Rougher surfaces give greater friction'. The third goes to 'Depends on normal contact force', which leads to 'Larger normal forces give greater friction'. The fourth goes to 'Air resistance: fluid friction', which leads to 'Increases with speed'. A fifth branch from the top node goes to 'Acts opposite to velocity', which leads to 'Reduces horizontal component of velocity'. That node then splits into two: 'Range less than ideal projectile motion' and 'Trajectory asymmetric: steeper descent than ascent'.
    It shows that friction opposes relative motion, depends on surface roughness and normal contact force, and for a projectile reduces horizontal velocity, range and symmetry of the path.

    Key words

    Say what each word means before you turn the card.

    Friction

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

    Air resistance

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

    Qualitative treatment of friction

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

    See every definition
    Friction
    A contact force that opposes relative motion or the tendency to move between surfaces.
    Air resistance
    A form of fluid friction that increases with speed and acts opposite to the velocity of a projectile.
    Qualitative treatment of friction
    Describing the effects of friction in words and identifying the direction of the frictional force without calculating coefficients.

    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 top node reads 'Friction: contact force'. From it, four branches lead down. The first branch goes to 'Opposes relative motion or tendency to move'. The second goes to 'Depends on nature of surfaces', which leads to 'Rougher surfaces give greater friction'. The third goes to 'Depends on normal contact force', which leads to 'Larger normal forces give greater friction'. The fourth goes to 'Air resistance: fluid friction', which leads to 'Increases with speed'. A fifth branch from the top node goes to 'Acts opposite to velocity', which leads to 'Reduces horizontal component of velocity'. That node then splits into two: 'Range less than ideal projectile motion' and 'Trajectory asymmetric: steeper descent than ascent'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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