AQA · GCSE · Physics

    Course version: 8463 · Version from Sept 2016

    The weight of an object can be calculated using the equation: weight =…

    Full specification wording

    Gravity: The weight of an object can be calculated using the equation: weight = mass × gravitational field strength W = m g weight, W, in newtons, N mass, m, in kilograms, kg gravitational field strength, g, in newtons per kilogram, N/kg (In any calculation the value of the gravitational field strength (g) will be given.) The weight of an object may be considered to act at a single point referred to as the object’s ‘centre of mass’.

    Weight is calculated from W = m g, with weight W in newtons, mass m in kilograms and gravitational field strength g in N/kg.

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

    • Point 3 of 5
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Weight is calculated from W = m g, with weight W in newtons, mass m in kilograms and gravitational field strength g in N/kg. Write the equation first, substitute, then evaluate: a 4.0 kg bag where g = 9.8 N/kg has a weight of 4.0 × 9.8 = 39.2 N. The value of g is always given, so use the one in the question. Mass supplied in grams must be converted to kilograms first, so 250 g becomes 0.250 kg. The weight of an object may be treated as acting at a single point called the centre of mass, the point from which the weight may be considered to act, or equivalently the point where the mass appears to be concentrated; on a diagram the weight arrow is drawn from there.

    What earns the marks

    • Writing W = m g before substituting.
    • A correct substitution with the mass in kilograms and the value of g given in the question.
    • The answer in newtons.
    • Defining centre of mass as the point from which the weight may be considered to act, or where the mass appears to be concentrated.

    Common mistakes and how to put them right

    • Leaving the mass in grams, so the weight comes out a thousand times too large.

    • Using a remembered value of g instead of the one supplied.

    • Quoting the weight in kilograms.

    • Describing the centre of mass as the middle of the object, which fails for irregular shapes.

    • Drawing the weight arrow from the base of the object rather than from its centre of mass.

    Exam tips

    • Set out equation, substitution and answer on three lines; the first two carry marks even if the arithmetic slips.
    • Convert grams to kilograms as your first line of working, in view of the examiner.
    • For centre of mass, 'the point through which the line of action of the weight acts' is also accepted.

    Try the lesson

    The start of the Gravity: The weight of an object can be calculated using the equation: weight = mass × gravitational field strength W = m g weight, W, in newtons, N mass, m, in kilograms, kg gravitational field strength, g, in newtons per kilogram, N/kg (In any calculation the value of the gravitational field strength (g) will be given.) The weight of an object may be considered to act at a single point referred to as the object’s ‘centre of mass’. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A box has a mass of 6 kg. The gravitational field strength is 10 N/kg. What is the weight of the box?

    Listen

    Weight is calculated from W = m g, with weight W in newtons, mass m in kilograms and gravitational field strength g in N/kg. Write the equation first, substitute, then evaluate: a 4.0 kg bag where g = 9.8 N/kg has a weight of 4.0 × 9.8 = 39.2 N. The value of g is always given, so use the one in the question. Mass supplied in grams must be converted to kilograms first, so 250 g becomes 0.250 kg. The weight of an object may be treated as acting at a single point called the centre of mass, the point from which the weight may be considered to act, or equivalently the point where the mass appears to be concentrated; on a diagram the weight arrow is drawn from there. A strong answer earns marks for these points. writing W = m g before substituting. a correct substitution with the mass in kilograms and the value of g given in the question. the answer in newtons. defining centre of mass as the point from which the weight may be considered to act, or where the mass appears to be concentrated. Exam tip: Set out equation, substitution and answer on three lines; the first two carry marks even if the arithmetic slips.

    See it

    A top-to-bottom flowchart. The first box says start with the mass and g given in the question, and an arrow leads to a box about converting mass from grams to kilograms if needed. An arrow leads to a box saying write the equation W = m g, then to a box saying substitute mass in kg and the supplied value of g, then to a box saying evaluate the answer, and finally to a box saying give weight in newtons, N. Separately, a box defining the centre of mass as the point from which the weight may be considered to act has an arrow leading to a box saying draw the weight arrow from the centre of mass.
    The steps for calculating weight from W = m g, including converting grams to kilograms and giving the answer in newtons, plus where the weight arrow is drawn from the centre of mass.

    Key words

    Say what each word means before you turn the card.

    W = m g

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

    units N, kg and N/kg

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

    unit conversion

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

    centre of mass

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

    See every definition
    W = m g
    The equation used to calculate the weight of an object, where weight = mass × gravitational field strength.
    units N, kg and N/kg
    Weight, W, is in newtons, N; mass, m, is in kilograms, kg; and gravitational field strength, g, is in newtons per kilogram, N/kg.
    unit conversion
    Mass supplied in grams must be converted to kilograms first, so 250 g becomes 0.250 kg.
    centre of mass
    The point from which the weight may be considered to act, or where the mass appears to be concentrated.

    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-to-bottom flowchart. The first box says start with the mass and g given in the question, and an arrow leads to a box about converting mass from grams to kilograms if needed. An arrow leads to a box saying write the equation W = m g, then to a box saying substitute mass in kg and the supplied value of g, then to a box saying evaluate the answer, and finally to a box saying give weight in newtons, N. Separately, a box defining the centre of mass as the point from which the weight may be considered to act has an arrow leading to a box saying draw the weight arrow from the centre of mass.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

    There is more to this lesson

    The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.

    Carry on with the full lesson

    A week of Premium from your first lesson · Then the free plan continues · No card, no charge

    Test your understanding of Gravity: The weight of an object can be calculated using the equation: weight = mass × gravitational field strength W = m g weight, W, in newtons, N mass, m, in kilograms, kg gravitational field strength, g, in newtons per kilogram, N/kg (In any calculation the value of the gravitational field strength (g) will be given.) The weight of an object may be considered to act at a single point referred to as the object’s ‘centre of mass’.

    Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.

    Take topic quiz

    A week of Premium from your first lesson · Then the free plan continues · No card, no charge

    Start free

    Cookies on MasteryMind

    Essential cookies keep you signed in.

    Analytics and advertising cookies stay off unless you accept.

    Read our Privacy Policy.