AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    Students should be able to apply Newton’s First Law to explain the motion…

    Full specification wording

    Newton's First Law: Students should be able to apply Newton’s First Law to explain the motion of objects moving with a uniform velocity and objects where the speed and/or direction changes.

    Newton’s First Law says that if the resultant force on an object is zero, the object stays at rest or keeps moving at constant velocity; if the resultant force is not zero, the velocity changes.

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

    • Point 7 of 8
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Newton’s First Law says that if the resultant force on an object is zero, the object stays at rest or keeps moving at constant velocity; if the resultant force is not zero, the velocity changes. Constant velocity means both speed and direction are unchanged, so a car cruising at 20 m/s due north has zero resultant force. When speed and/or direction changes, the object accelerates, so a resultant force must act. A cyclist turning at steady speed is accelerating because direction changes, so a resultant force acts towards the centre of the turn. To apply the law, identify all forces, find the resultant, then link it to the motion: zero resultant gives uniform velocity; non-zero resultant gives changing speed, direction or both.

    What earns the marks

    • State that zero resultant force means the object remains at rest or moves with uniform velocity.
    • Explain that uniform velocity requires both constant speed and constant direction.
    • Identify that a change in speed, direction or both means a non-zero resultant force acts.
    • Apply the law to a named example, such as a car at steady speed on a straight road having zero resultant force.
    • Recognise that an object turning at constant speed is accelerating because its direction changes.
    • Link the direction of the resultant force to the direction of the velocity change.

    Common mistakes and how to put them right

    • Thinking a moving object always has a resultant force acting.

      Put it right: At constant velocity the resultant force is zero.

    • Believing constant speed alone means no acceleration.

      Put it right: A change of direction at constant speed is still acceleration.

    • Assuming a forward force is always larger than resistance for a moving object.

      Put it right: At uniform velocity the forward and resistive forces balance.

    Exam tips

    • Name the forces and state whether they balance before concluding anything about motion.
    • Use the phrase ‘resultant force’ rather than ‘the force’ when explaining changes in motion.
    • For turning objects, state explicitly that direction changes, so velocity changes even if speed is constant.

    Try the lesson

    The start of the Newton's First Law: Students should be able to apply Newton’s First Law to explain the motion of objects moving with a uniform velocity and objects where the speed and/or direction changes. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A cyclist is travelling along a straight, level road at a steady 6 m/s. The forward force from the pedals is 40 N and the total resistive force is 40 N. What is the resultant force on the cyclist, and what happens to the cyclist's motion?

    Listen

    Newton’s First Law says that if the resultant force on an object is zero, the object stays at rest or keeps moving at constant velocity; if the resultant force is not zero, the velocity changes. Constant velocity means both speed and direction are unchanged, so a car cruising at 20 m/s due north has zero resultant force. When speed and/or direction changes, the object accelerates, so a resultant force must act. A cyclist turning at steady speed is accelerating because direction changes, so a resultant force acts towards the centre of the turn. To apply the law, identify all forces, find the resultant, then link it to the motion: zero resultant gives uniform velocity; non-zero resultant gives changing speed, direction or both. A strong answer earns marks for these points. State that zero resultant force means the object remains at rest or moves with uniform velocity. Explain that uniform velocity requires both constant speed and constant direction. Identify that a change in speed, direction or both means a non-zero resultant force acts. Apply the law to a named example, such as a car at steady speed on a straight road having zero resultant force. A common slip: Thinking a moving object always has a resultant force acting. Instead, at constant velocity the resultant force is zero. Exam tip: Name the forces and state whether they balance before concluding anything about motion.

    See it

    A top-down flowchart. The first box says identify all forces acting, and an arrow leads to find the resultant force. From there an arrow leads to a decision box asking whether the resultant force is zero. If yes, an arrow leads to a box saying the object stays at rest or moves with uniform velocity, which leads to a box saying uniform velocity means constant speed and constant direction. If no, an arrow leads to a box saying velocity changes, then to a box saying speed changes, direction changes, or both, then to a box saying the object accelerates.
    Shows how applying Newton's First Law links the resultant force on an object to whether it keeps uniform velocity or its velocity changes.

    Key words

    Say what each word means before you turn the card.

    Uniform velocity

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

    Acceleration

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

    Resultant force and motion

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

    Balanced and unbalanced forces

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

    See every definition
    Uniform velocity
    Both speed and direction are unchanged, so the object keeps moving at constant velocity.
    Acceleration
    When speed and/or direction changes, the object accelerates, so a resultant force must act.
    Resultant force and motion
    Zero resultant gives uniform velocity; non-zero resultant gives changing speed, direction or both.
    Balanced and unbalanced forces
    At uniform velocity the forward and resistive forces balance, so the resultant force 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 top-down flowchart. The first box says identify all forces acting, and an arrow leads to find the resultant force. From there an arrow leads to a decision box asking whether the resultant force is zero. If yes, an arrow leads to a box saying the object stays at rest or moves with uniform velocity, which leads to a box saying uniform velocity means constant speed and constant direction. If no, an arrow leads to a box saying velocity changes, then to a box saying speed changes, direction changes, or both, then to a box saying the object accelerates.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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