Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Explain that inertial mass is a measure of how difficult it is to change…
Full specification wording
Topic 2 – Motion and forces: 2.22 Explain that inertial mass is a measure of how difficult it is to change the velocity of an object (including from rest) and know that it is defined as the ratio of force over acceleration
Inertial mass measures how resistant an object is to a change in velocity.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 21 of 32
Study notes
From the checked teaching notes for this specification point.
The idea
Inertial mass measures how resistant an object is to a change in velocity. A larger inertial mass needs a larger resultant force to produce the same acceleration, whether the object is already moving or starting from rest. The definition follows from Newton's second law, F = m × a, rearranged to give m = F ÷ a, so inertial mass is the ratio of force over acceleration. For example, a resultant force of 12 N producing an acceleration of 3 m/s² gives an inertial mass of 12 N ÷ 3 m/s² = 4 kg. Inertial mass is a property of the object and does not depend on its location, unlike weight, which depends on gravitational field strength.
What earns the marks
- States that inertial mass measures how difficult it is to change an object's velocity.
- Applies the idea that a larger inertial mass requires a larger force for the same acceleration, including when starting from rest.
- Defines inertial mass as the ratio of force over acceleration, m = F ÷ a.
- Calculates inertial mass correctly from a given force and acceleration, keeping units consistent.
Common mistakes and how to put them right
Confuses inertial mass with weight.
Put it right: Distinguish mass in kilograms from weight as a force in newtons.
Divides acceleration by force instead of force by acceleration; use m = F ÷ a.
Assumes inertial mass applies only to moving objects.
Put it right: Apply it equally to objects starting from rest.
Exam tips
- Quote the definition as force divided by acceleration and show the rearrangement from F = m × a.
- Substitute values with units and check that the answer is in kilograms.
- Explain difficulty of changing velocity in terms of the force needed for a given acceleration.
Try the lesson
The start of the Topic 2 – Motion and forces: 2.22 Explain that inertial mass is a measure of how difficult it is to change the velocity of an object (including from rest) and know that it is defined as the ratio of force over acceleration lesson, made from checked teaching notes for this specification point.
Have a go first
A resultant force of 12 N gives a trolley an acceleration of 3 m/s². Using inertial mass = force ÷ acceleration, what is the inertial mass of the trolley?
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Topic 2 – Motion and forces: 2.22 Explain that inertial mass is a measure of how difficult it is to change the velocity of an object (including from rest) and know that it is defined as the ratio of force over acceleration
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge