AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Newton's Second Law: acceleration, a, in metres per second squared, m/s²
Acceleration is the rate of change of velocity and is measured in metres per second squared (m/s²).
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 8 of 13
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Acceleration is the rate of change of velocity and is measured in metres per second squared (m/s²). In Newton's second law, F = m × a, acceleration is directly proportional to the resultant force when mass is constant, and inversely proportional to mass when force is constant. For example, a resultant force of 10 N acting on a 2 kg mass gives a = F ÷ m = 10 N ÷ 2 kg = 5 m/s². Acceleration can be positive (speeding up) or negative (slowing down, often called deceleration). Always use the resultant force, not any single force, and ensure mass is in kilograms before dividing. A common method is to write the equation, rearrange to a = F ÷ m, substitute values with units, then calculate and state m/s².
What earns the marks
- State that acceleration is measured in metres per second squared (m/s²) and is the rate of change of velocity.
- Rearrange F = m × a correctly to a = F ÷ m when acceleration is the unknown.
- Use the resultant force, found by combining all forces acting along the line of motion, before calculating acceleration.
- Substitute mass in kilograms and force in newtons to obtain acceleration in m/s².
- Interpret acceleration as directly proportional to resultant force for constant mass.
- Interpret acceleration as inversely proportional to mass for constant resultant force.
- Recognise negative acceleration as deceleration when the resultant force opposes motion.
Common mistakes and how to put them right
Using a single applied force instead of the resultant force.
Put it right by subtracting opposing forces such as friction before dividing by mass.
Forgetting to convert grams to kilograms, which gives an acceleration 1000 times too small.
Put it right by converting mass to kg first.
Writing the unit as m/s instead of m/s².
Put it right by remembering acceleration is the change in velocity per second, so the unit is metres per second squared.
Exam tips
- Underline the resultant force in the question and calculate it before using a = F ÷ m.
- Show the rearranged equation, substitution and answer with unit m/s² to gain method and accuracy credit.
- For graph questions, recall that acceleration is the gradient of a velocity–time graph, so read values carefully.
Try the lesson
The start of the Newton's Second Law: acceleration, a, in metres per second squared, m/s² lesson, made from checked teaching notes for this specification point.
Have a go first
A resultant force of 20 N acts on a block with a mass of 4 kg. Using the equation F = m × a, what is the acceleration of the block?
Listen
Acceleration is the rate of change of velocity and is measured in metres per second squared (m/s²). In Newton's second law, F = m × a, acceleration is directly proportional to the resultant force when mass is constant, and inversely proportional to mass when force is constant. For example, a resultant force of 10 N acting on a 2 kg mass gives a = F ÷ m = 10 N ÷ 2 kg = 5 m/s². Acceleration can be positive (speeding up) or negative (slowing down, often called deceleration). Always use the resultant force, not any single force, and ensure mass is in kilograms before dividing. A common method is to write the equation, rearrange to a = F ÷ m, substitute values with units, then calculate and state m/s². A strong answer earns marks for these points. State that acceleration is measured in metres per second squared (m/s²) and is the rate of change of velocity. Rearrange F = m × a correctly to a = F ÷ m when acceleration is the unknown. Use the resultant force, found by combining all forces acting along the line of motion, before calculating acceleration. Substitute mass in kilograms and force in newtons to obtain acceleration in m/s². A common slip: Using a single applied force instead of the resultant force. Instead, subtracting opposing forces such as friction before dividing by mass. Exam tip: Underline the resultant force in the question and calculate it before using a = F ÷ m.
See it
Put it in order
Put these steps in the order they happen.
- 1. State the answer in m/s²
- 2. Calculate
- 3. Find the resultant force by combining all forces along the line of motion
- 4. Substitute values with units
- 5. Write the equation F = m × a
- 6. Rearrange to a = F ÷ m
- 7. Ensure mass is in kilograms
Key words
Say what each word means before you turn the card.
Acceleration
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Newton's second law
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Direct and inverse proportionality
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Resultant force and deceleration
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See every definition
- Acceleration
- The rate of change of velocity, measured in metres per second squared (m/s²).
- Newton's second law
- F = m × a, where acceleration is directly proportional to the resultant force when mass is constant and inversely proportional to mass when force is constant.
- Direct and inverse proportionality
- Acceleration is directly proportional to the resultant force when mass is constant, and inversely proportional to mass when force is constant.
- Resultant force and deceleration
- The resultant force is found by combining all forces acting along the line of motion, and negative acceleration is deceleration when the resultant force opposes motion.
Label the diagram
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1Box 1
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3Box 3
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