Pearson Edexcel · A-Level · Mathematics
Course version: 9MA0 · Version from Mar 2026
Understand and use weight and motion in a straight line under gravity;…
Full specification wording
Forces and Newton’s laws: 8.3 Understand and use weight and motion in a straight line under gravity; gravitational acceleration, g, and its value in S.I. units to varying degrees of accuracy. (The inverse square law for gravitation is not required and g may be assumed to be constant, but students should be aware that g is not a universal constant but depends on location).
Weight is the gravitational force on a mass: W = mg, where m is mass in kg and g is gravitational acceleration in m s⁻².
Checked against the Pearson Edexcel 9MA0 specification (opens in a new tab) on .
- Point 3 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Weight is the gravitational force on a mass: W = mg, where m is mass in kg and g is gravitational acceleration in m s⁻². Near the Earth’s surface, g ≈ 9.8 m s⁻², often used as 9.81 m s⁻² or 9.8 m s⁻² depending on required accuracy. For motion in a straight line under gravity alone, a particle accelerates downwards at g, so equations of constant acceleration (suvat) apply with a = g. For example, a ball dropped from rest falls 4.9 m in 1 s because s = ½gt² = ½ × 9.8 × 1² = 4.9 m. g is not a universal constant: it varies slightly with location and altitude, being about 9.81 m s⁻² in the UK and about 9.78 m s⁻² at the equator. The inverse square law is not required, but you should recognise that g depends on distance from the Earth’s centre. Always use S.I. units: weight in N, mass in kg, g in m s⁻².
What earns the marks
- Calculate weight using W = mg, ensuring mass is in kg and g is in m s⁻².
- Use g = 9.8 m s⁻² or 9.81 m s⁻² as appropriate to the accuracy required, and state the value used.
- Apply the constant acceleration equations with a = g for vertical motion under gravity, taking downwards as positive or negative consistently.
- Recognise that g varies with location and is not a universal constant, though it may be treated as constant near the Earth’s surface.
- Convert between units where necessary, for example grams to kilograms, and keep S.I. units throughout.
Common mistakes and how to put them right
Using g = 10 m s⁻² when the question expects 9.8 m s⁻² or 9.81 m s⁻².
Put it right by reading the required accuracy and using the stated value.
Confusing mass and weight.
Put it right by remembering mass is in kg and weight is a force in N calculated as mg.
Mixing sign conventions in suvat equations.
Put it right by choosing a positive direction and applying it consistently to displacement, velocity and acceleration.
Exam tips
- Write down the value of g you are using at the start of your solution.
- List the suvat variables you know and the one you need before substituting into an equation.
- Check that your final answer has the correct unit and a sensible magnitude for the context.
Try the lesson
The start of the Forces and Newton’s laws: 8.3 Understand and use weight and motion in a straight line under gravity; gravitational acceleration, g, and its value in S.I. units to varying degrees of accuracy. (The inverse square law for gravitation is not required and g may be assumed to be constant, but students should be aware that g is not a universal constant but depends on location). lesson, made from checked teaching notes for this specification point.
Have a go first
An object of mass 8 kg is at rest on a table. Taking g = 9.8 m/s², calculate the weight of the object.
Listen
Weight is the gravitational force on a mass: W = mg, where m is mass in kg and g is gravitational acceleration in m s⁻². Near the Earth’s surface, g ≈ 9.8 m s⁻², often used as 9.81 m s⁻² or 9.8 m s⁻² depending on required accuracy. For motion in a straight line under gravity alone, a particle accelerates downwards at g, so equations of constant acceleration (suvat) apply with a = g. For example, a ball dropped from rest falls 4.9 m in 1 s because s = ½gt² = ½ × 9.8 × 1² = 4.9 m. g is not a universal constant: it varies slightly with location and altitude, being about 9.81 m s⁻² in the UK and about 9.78 m s⁻² at the equator. The inverse square law is not required, but you should recognise that g depends on distance from the Earth’s centre. Always use S.I. units: weight in N, mass in kg, g in m s⁻². A strong answer earns marks for these points. Calculate weight using W = mg, ensuring mass is in kg and g is in m s⁻². Use g = 9.8 m s⁻² or 9.81 m s⁻² as appropriate to the accuracy required, and state the value used. Apply the constant acceleration equations with a = g for vertical motion under gravity, taking downwards as positive or negative consistently. Recognise that g varies with location and is not a universal constant, though it may be treated as constant near the Earth’s surface. A common slip: Using g = 10 m s⁻² when the question expects 9.8 m s⁻² or 9.81 m s⁻². Instead, reading the required accuracy and using the stated value. Exam tip: Write down the value of g you are using at the start of your solution.
See it
Key words
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Weight
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Gravitational acceleration
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Motion under gravity
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S.I. units
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See every definition
- Weight
- The gravitational force on a mass, given by W = mg, where m is mass in kg and g is gravitational acceleration in m s⁻².
- Gravitational acceleration
- The value g, in m s⁻², which near the Earth's surface is approximately 9.8 m s⁻², often used as 9.81 m s⁻² depending on required accuracy.
- Motion under gravity
- Motion in a straight line under gravity alone, where a particle accelerates downwards at g so equations of constant acceleration apply with a = g.
- S.I. units
- The units to always use: weight in N, mass in kg, and g in m s⁻².
Label the diagram
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Test your understanding of Forces and Newton’s laws: 8.3 Understand and use weight and motion in a straight line under gravity; gravitational acceleration, g, and its value in S.I. units to varying degrees of accuracy. (The inverse square law for gravitation is not required and g may be assumed to be constant, but students should be aware that g is not a universal constant but depends on location).
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