OCR · A-Level · Physics

    Course version: H556 · Version from Sept 2015

    (a) Système Internationale (S.I.) base quantities and their units – mass

    Full specification wording

    S.I. units: (a) Système Internationale (S.I.) base quantities and their units – mass

    The Système Internationale defines seven base quantities, each with its own base unit.

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

    • Point 1 of 16
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    The Système Internationale defines seven base quantities, each with its own base unit. Mass is one of them, and its base unit is the kilogram (kg). The kilogram is unusual because it already contains a prefix, but it is still the base unit, so derived units such as the newton are built from kg rather than from a gram. In calculations, convert grams to kilograms by dividing by 1000, for example 250 g = 0.25 kg. Mass measures the quantity of matter and is not the same as weight, which is a force measured in newtons.

    What earns the marks

    • Identifies mass as one of the seven S.I. base quantities.
    • States that the S.I. base unit of mass is the kilogram (kg).
    • Recognises that the kilogram is the base unit even though it carries a prefix.
    • Converts a mass given in grams to kilograms by dividing by 1000 before substituting into an equation.

    Common mistakes and how to put them right

    • Treating the gram as the base unit.

      Put it right: The base unit is the kilogram (kg).

    • Confusing mass with weight.

      Put it right: Mass is measured in kg while weight is a force measured in N.

    • Substituting grams directly into an equation that requires kilograms.

      Put it right: Divide the value in grams by 1000 first.

    Exam tips

    • Check the unit of every mass in a calculation and convert to kg before substituting.
    • Remember that the kilogram is the only base unit that already includes a prefix.
    • If an answer comes out with an odd power of ten, check whether a gram-to-kilogram conversion was missed.

    Try the lesson

    The start of the S.I. units: (a) Système Internationale (S.I.) base quantities and their units – mass lesson, made from checked teaching notes for this specification point.

    Listen

    The Système Internationale defines seven base quantities, each with its own base unit. Mass is one of them, and its base unit is the kilogram (kg). The kilogram is unusual because it already contains a prefix, but it is still the base unit, so derived units such as the newton are built from kg rather than from a gram. In calculations, convert grams to kilograms by dividing by 1000, for example 250 g = 0.25 kg. Mass measures the quantity of matter and is not the same as weight, which is a force measured in newtons. A strong answer earns marks for these points. Identifies mass as one of the seven S.I. base quantities. States that the S.I. base unit of mass is the kilogram (kg). Recognises that the kilogram is the base unit even though it carries a prefix. Converts a mass given in grams to kilograms by dividing by 1000 before substituting into an equation. A common slip: Treating the gram as the base unit. Instead, the base unit is the kilogram (kg). Exam tip: Check the unit of every mass in a calculation and convert to kg before substituting.

    See it

    A top-down flowchart. The top node reads 'Mass is an S.I. base quantity' and branches to two nodes. The first branch leads to 'Base unit: kilogram (kg)', which in turn leads to three nodes: 'Kilogram already contains a prefix but is still the base unit', 'Derived units such as the newton are built from kg', and 'Convert grams to kilograms by dividing by 1000'. That last node leads to '250 g = 0.25 kg'. The second branch from the top node leads to 'Mass measures quantity of matter', which leads to 'Not the same as weight, a force measured in newtons'.
    Shows mass as an S.I. base quantity with the kilogram as its base unit, how it differs from weight, and the gram-to-kilogram conversion.

    Key words

    Say what each word means before you turn the card.

    S.I. base quantities

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

    Base units

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

    Mass and the kilogram

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

    Unit conversion

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

    See every definition
    S.I. base quantities
    The Système Internationale defines seven base quantities, each with its own base unit, and mass is one of them.
    Base units
    Each of the seven S.I. base quantities has its own base unit, and derived units are built from these.
    Mass and the kilogram
    Mass measures the quantity of matter, and its S.I. base unit is the kilogram (kg), which is unusual because it already contains a prefix.
    Unit conversion
    In calculations, convert grams to kilograms by dividing by 1000, for example 250 g = 0.25 kg.

    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 'Mass is an S.I. base quantity' and branches to two nodes. The first branch leads to 'Base unit: kilogram (kg)', which in turn leads to three nodes: 'Kilogram already contains a prefix but is still the base unit', 'Derived units such as the newton are built from kg', and 'Convert grams to kilograms by dividing by 1000'. That last node leads to '250 g = 0.25 kg'. The second branch from the top node leads to 'Mass measures quantity of matter', which leads to 'Not the same as weight, a force measured in newtons'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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