AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Inertial mass is defined as the ratio of force over acceleration.
Full specification wording
Newton's Second Law: inertial mass is defined as the ratio of force over acceleration.
Inertial mass measures how strongly an object resists a change in its velocity.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 10 of 13
Study notes
From the checked teaching notes for this specification point.
The idea
Inertial mass measures how strongly an object resists a change in its velocity. It is defined as the ratio of force over acceleration, so m = F ÷ a, where F is the resultant force in newtons and a is the acceleration in metres per second squared. A larger inertial mass gives a smaller acceleration for the same resultant force. For example, a 1500 kg car pushed by a resultant force of 4500 N accelerates at 4500 N ÷ 1500 kg = 3 m/s². Rearranging gives F = m × a and a = F ÷ m. The ratio definition shows that mass is not the same as weight: weight is a force measured in newtons, while inertial mass is measured in kilograms.
What earns the marks
- State that inertial mass is the ratio of force to acceleration, written as m = F ÷ a.
- Identify F as the resultant force in newtons and a as the acceleration in metres per second squared.
- Explain that a larger inertial mass produces a smaller acceleration for the same resultant force.
- Substitute values into m = F ÷ a and give the unit of mass as kilograms.
- Rearrange the relationship to find force using F = m × a or acceleration using a = F ÷ m.
- Distinguish inertial mass in kilograms from weight, which is a force in newtons.
Common mistakes and how to put them right
Using weight in newtons in place of mass in kilograms.
Put it right by dividing weight by gravitational field strength, for example 600 N ÷ 10 N/kg = 60 kg, before using m = F ÷ a.
Dividing acceleration by force instead of force by acceleration.
Put it right by writing m = F ÷ a and checking that a larger force gives a larger mass for the same acceleration.
Forgetting to use the resultant force when several forces act.
Put it right by adding or subtracting forces along the line of motion first, then dividing by acceleration.
Exam tips
- Write the equation m = F ÷ a before substituting numbers so the examiner can see the relationship you are using.
- Check the unit of every quantity: force in newtons, acceleration in metres per second squared and mass in kilograms.
- When a question gives a velocity change and a time, calculate acceleration first using a = Δv ÷ t, then use m = F ÷ a.
Try the lesson
The start of the Newton's Second Law: inertial mass is defined as the ratio of force over acceleration. lesson, made from checked teaching notes for this specification point.
Have a go first
A cyclist and bicycle have a total inertial mass of 80 kg. A resultant force of 200 N acts on them. Using inertial mass = force ÷ acceleration, what acceleration is produced?
Listen
Inertial mass measures how strongly an object resists a change in its velocity. It is defined as the ratio of force over acceleration, so m = F ÷ a, where F is the resultant force in newtons and a is the acceleration in metres per second squared. A larger inertial mass gives a smaller acceleration for the same resultant force. For example, a 1500 kg car pushed by a resultant force of 4500 N accelerates at 4500 N ÷ 1500 kg = 3 m/s². Rearranging gives F = m × a and a = F ÷ m. The ratio definition shows that mass is not the same as weight: weight is a force measured in newtons, while inertial mass is measured in kilograms. A strong answer earns marks for these points. State that inertial mass is the ratio of force to acceleration, written as m = F ÷ a. Identify F as the resultant force in newtons and a as the acceleration in metres per second squared. Explain that a larger inertial mass produces a smaller acceleration for the same resultant force. Substitute values into m = F ÷ a and give the unit of mass as kilograms. A common slip: Using weight in newtons in place of mass in kilograms. Instead, dividing weight by gravitational field strength, for example 600 N ÷ 10 N/kg = 60 kg, before using m = F ÷ a. Exam tip: Write the equation m = F ÷ a before substituting numbers so the examiner can see the relationship you are using.
See it
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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