OCR · A-Level · Physics

    Course version: H556 · Version from Sept 2015

    Dynamics: (a) net force = mass × acceleration; F = ma

    Newton's second law states that the net force acting on an object equals its mass multiplied by its acceleration, written F = ma.

    Checked against the OCR H556 specification (opens in a new tab) on .

    • Point 1 of 10
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Newton's second law states that the net force acting on an object equals its mass multiplied by its acceleration, written F = ma. The force in the equation is the resultant of all forces acting on the object, not any single applied force, and the acceleration is in the same direction as that resultant. The equation applies instantaneously to constant-mass systems, even if the acceleration is non-uniform, and it links cause and effect: a larger net force gives a larger acceleration, while a larger mass gives a smaller acceleration for the same net force.

    For example: A 2.0 kg trolley pushed by a resultant force of 6.0 N accelerates at 3.0 m s⁻², since 6.0 N ÷ 2.0 kg = 3.0 m s⁻².

    What earns the marks

    • The net force on an object equals its mass multiplied by its acceleration, F = ma.
    • The force in the equation is the resultant of all forces acting on the object, not a single applied force.
    • The acceleration is in the same direction as the net force.
    • For a constant mass, acceleration is directly proportional to the net force and inversely proportional to the mass.
    • The equation applies instantaneously to constant-mass systems, including those with non-uniform acceleration, and the newton is defined so that 1 N = 1 kg m s⁻².

    Common mistakes and how to put them right

    • Substituting a single applied force into F = ma instead of the resultant of all forces acting on the object.

    • Treating mass and weight as the same quantity.

      Put it right: Mass is measured in kilograms, while weight is a force measured in newtons.

    • Assuming F = ma only applies to constant acceleration.

      Put it right by remembering it applies instantaneously to non-uniform acceleration as long as mass is constant.

    Exam tips

    • Draw a free-body diagram and resolve forces before calculating the resultant force to substitute into F = ma.
    • Check that all quantities are in SI units, converting grams to kilograms and so on, before substituting.
    • State the direction of the acceleration alongside its magnitude, since the equation is a vector relationship.

    Try the lesson

    The start of the Dynamics: (a) net force = mass × acceleration; F = ma lesson, made from checked teaching notes for this specification point.

    Listen

    Newton's second law states that the net force acting on an object equals its mass multiplied by its acceleration, written F = ma. The force in the equation is the resultant of all forces acting on the object, not any single applied force, and the acceleration is in the same direction as that resultant. The equation applies instantaneously to constant-mass systems, even if the acceleration is non-uniform, and it links cause and effect: a larger net force gives a larger acceleration, while a larger mass gives a smaller acceleration for the same net force. For example, a 2.0 kg trolley pushed by a resultant force of 6.0 N accelerates at 3.0 m s⁻², since 6.0 N ÷ 2.0 kg = 3.0 m s⁻². A strong answer earns marks for these points. The net force on an object equals its mass multiplied by its acceleration, F = ma. The force in the equation is the resultant of all forces acting on the object, not a single applied force. The acceleration is in the same direction as the net force. For a constant mass, acceleration is directly proportional to the net force and inversely proportional to the mass. A common slip: Assuming F = ma only applies to constant acceleration. Instead, remembering it applies instantaneously to non-uniform acceleration as long as mass is constant. Exam tip: Draw a free-body diagram and resolve forces before calculating the resultant force to substitute into F = ma.

    See it

    A top-down flowchart. At the top, a box labelled 'All forces acting on the object' leads down to a box 'Resolve forces and find the resultant (net) force', which leads to a box 'Net force F'. Separately, a box 'Mass m' and the 'Net force F' box both feed into a central box 'F = ma'. From 'F = ma' an arrow leads to 'Acceleration a', which leads to 'Acceleration is in the same direction as the net force'. Two further arrows branch from the central area: from 'Net force F' to 'Larger net force gives larger acceleration', and from 'Mass m' to 'Larger mass gives smaller acceleration for the same net force'.
    It shows how the resultant of all forces and the mass combine through F = ma to give the acceleration, including its direction and how changing force or mass affects it.

    Key words

    Say what each word means before you turn the card.

    Newton's second law

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

    Resultant force and free-body diagrams

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

    SI units and the newton

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

    See every definition
    Newton's second law
    The net force acting on an object equals its mass multiplied by its acceleration, written F = ma.
    Resultant force and free-body diagrams
    The force in the equation is the resultant of all forces acting on the object, not a single applied force, and a free-body diagram is drawn and forces resolved before calculating it.
    SI units and the newton
    All quantities are checked to be in SI units, converting grams to kilograms and so on, and the newton is defined so that 1 N = 1 kg m s⁻².

    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. At the top, a box labelled 'All forces acting on the object' leads down to a box 'Resolve forces and find the resultant (net) force', which leads to a box 'Net force F'. Separately, a box 'Mass m' and the 'Net force F' box both feed into a central box 'F = ma'. From 'F = ma' an arrow leads to 'Acceleration a', which leads to 'Acceleration is in the same direction as the net force'. Two further arrows branch from the central area: from 'Net force F' to 'Larger net force gives larger acceleration', and from 'Mass m' to 'Larger mass gives smaller acceleration for the same net force'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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