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Mass & Weight (Oxford AQA IGCSE Physics)

Revision Note

Leander Oates

Written by: Leander Oates

Reviewed by: Caroline Carroll

Oxford AQA PhysicsOxford AQA Combined Science Double Award: Physics

Mass & Weight

What is the difference between mass and weight?

  • Mass is a measure of how much matter is in an object

    • Mass is measured in kilograms (kg)

  • Weight is the force exerted on an object with mass due to gravity, i.e. the force exerted on an object when it is placed in a gravitational field

    • Weight is measured in newtons (N)

  • The mass of an object is independent of its location

    • A 1 kg bag of sugar will have a mass of 1 kg on Earth and on the Moon

  • The weight of an object is dependent on its location in the Universe

    • A 1 kg bag of sugar has a weight of almost 10 N on Earth, but a weight of 1.6 N on the moon

  • This is because weight depends on the mass of an object and the strength of the gravitational field in which the object is located

The weight equation

  • To calculate weight:

W space equals space m space cross times space g

  • Where:

    • W = weight measured in newtons (N)

    • m = mass measured in kilograms (kg)

    • g = gravitational field strength measured in newtons per kilogram (N/kg)

  • You may need to rearrange this equation in the exam, you can use a formula triangle to help you do this

Weight equation triangle

Formula triangle with weight on the top, mass and gravitational field strength on the bottom
Simply cover up the variable you want to calculate and the formula triangle will show you the correct structure of the equation

Worked Example

A student has a mass of 60 kg. The gravitational field strength on Earth is 9.8 N/kg, and the gravitational field strength on the Moon is 1.6 N/kg.

Calculate the force of weight acting on the student if they were located on Earth and on the Moon.

Answer:

Step 1: List the known quantities

  • Mass, m = 60 kg

  • Gravitational field strength on Earth, gE = 9.8 N/kg

  • Gravitational field strength on the Moon, gM = 1.6 N/kg

Step 2: Write out the equation

W space equals space m space cross times space g

Step 3: Substitute in the known values to calculate the student's weight on Earth

W subscript E space equals space m space cross times space g subscript E

W subscript E space equals space 60 space cross times space 9.8

W subscript E space equals space 588 space straight N

Step 4: Substitute in the known values to calculate the student's weight on the Moon

W subscript M space equals space m space cross times space g subscript M

W subscript M space equals space 60 space cross times space 1.6

W subscript M space equals space 96 space straight N

Examiner Tips and Tricks

The gravitational field strength on Earth is 9.8 N/kg, but this is sometimes rounded to 10 N/kg.

You do not need to remember the value of g as you will be given it in the exam.

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    Author
    Reviewer
    Leander Oates

    Author: Leander Oates

    Expertise: Physics

    Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.

    Caroline Carroll

    Author: Caroline Carroll

    Expertise: Physics Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.