Mass & Weight (Oxford AQA IGCSE Combined Science Double Award)

Revision Note

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 Tip

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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