AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Gravity: mass, m, in kilograms, kg
Mass measures how much matter an object contains and is measured in kilograms (kg).
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 7 of 11
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Mass measures how much matter an object contains and is measured in kilograms (kg). In gravity calculations you must use mass in kg, so convert grams to kilograms by dividing by 1000: 2500 g = 2.5 kg. Mass is a scalar and stays the same everywhere in the universe, unlike weight, which is a force in newtons and changes with gravitational field strength. In the equation W = m × g, m is the mass in kg, g is in N/kg and W comes out in N. For example, a 3 kg bag has W = 3 × 9.8 = 29.4 N on Earth. Always label the quantity and unit clearly, and check that the mass is in kg before substituting into any formula.
What earns the marks
- State that mass is measured in kilograms (kg) and is a scalar quantity.
- Convert masses given in grams to kilograms by dividing by 1000, for example 4500 g = 4.5 kg.
- Use mass in kg when substituting into W = m × g so that the weight is calculated in newtons.
- Distinguish mass (kg, constant everywhere) from weight (N, depends on gravitational field strength).
- Read values from tables or diagrams and record mass with the correct unit, not as a bare number.
Common mistakes and how to put them right
Using grams directly in W = m × g, which gives a weight 1000 times too large.
Put it right by converting to kg first.
Writing the unit as 'KG' or 'Kg'.
Put it right: The correct symbol is kg, lower case.
Confusing mass with weight and giving mass in newtons.
Put it right by stating mass in kg and weight in N.
Exam tips
- Underline the mass value and its unit in the question before starting any calculation.
- If a mass is given in grams, convert to kg in a clearly shown step so the examiner can award the conversion.
- Give the final answer with the correct unit and, where asked, to an appropriate number of significant figures.
Try the lesson
The start of the Gravity: mass, m, in kilograms, kg lesson, made from checked teaching notes for this specification point.
Have a go first
A pupil records the mass of a school bag as 4 kilograms. Which unit has the pupil used for this mass?
Listen
Mass measures how much matter an object contains and is measured in kilograms (kg). In gravity calculations you must use mass in kg, so convert grams to kilograms by dividing by 1000: 2500 g = 2.5 kg. Mass is a scalar and stays the same everywhere in the universe, unlike weight, which is a force in newtons and changes with gravitational field strength. In the equation W = m × g, m is the mass in kg, g is in N/kg and W comes out in N. For example, a 3 kg bag has W = 3 × 9.8 = 29.4 N on Earth. Always label the quantity and unit clearly, and check that the mass is in kg before substituting into any formula. A strong answer earns marks for these points. State that mass is measured in kilograms (kg) and is a scalar quantity. Convert masses given in grams to kilograms by dividing by 1000, for example 4500 g = 4.5 kg. Use mass in kg when substituting into W = m × g so that the weight is calculated in newtons. Distinguish mass (kg, constant everywhere) from weight (N, depends on gravitational field strength). A common slip: Using grams directly in W = m × g, which gives a weight 1000 times too large. Instead, converting to kg first. Exam tip: Underline the mass value and its unit in the question before starting any calculation.
See it
Key words
Say what each word means before you turn the card.
Mass
Try answering aloud or in your head before turning the card.
Weight
Try answering aloud or in your head before turning the card.
See every definition
- Mass
- A scalar quantity, symbol m, measured in kilograms (kg) that stays the same everywhere in the universe.
- Weight
- A force in newtons that changes with gravitational field strength, unlike mass.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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