OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Explain that inertia is a measure of how difficult it is to change the…
Full specification wording
Newton's laws: explain that inertia is a measure of how difficult it is to change the velocity of an object and that the inertial mass is defined as the ratio of force over acceleration (Higher tier only)
Inertia describes how strongly an object resists any change in its velocity, whether that change is a speeding up, a slowing down or a change of direction.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 13 of 20
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Inertia describes how strongly an object resists any change in its velocity, whether that change is a speeding up, a slowing down or a change of direction. A loaded trolley is harder to start, stop or turn than an empty one because it has more inertia. Inertial mass puts a number on this: it is defined as the ratio of force over acceleration, so m = F ÷ a. A larger inertial mass means a smaller acceleration for the same resultant force. The definition is a ratio, so the force must be the resultant force acting on the object and the acceleration must be measured in the same direction. Units follow from the ratio: newtons divided by metres per second squared give kilograms.
What earns the marks
- States that inertia measures how difficult it is to change an object's velocity.
- Identifies that a greater inertial mass produces a smaller acceleration for the same resultant force.
- Defines inertial mass as the ratio of force over acceleration, m = F ÷ a.
- Recognises that the force in the definition is the resultant force acting on the object.
- Applies the ratio to a numerical case, for example a resultant force of 12 N giving an acceleration of 3 m/s² implies an inertial mass of 4 kg.
Common mistakes and how to put them right
Confuses inertia with weight.
Put it right: Keep inertia as resistance to a change in velocity and weight as the force of gravity on a mass.
Uses total applied force rather than resultant force in the ratio.
Put it right: Subtract opposing forces first.
Inverts the ratio and divides acceleration by force.
Put it right: Write m = F ÷ a and substitute force over acceleration.
Treats inertia as a force with a direction.
Put it right: Describe it as a property of the object's mass instead.
Exam tips
- Write the definition as a ratio in words and as an equation before substituting numbers.
- Check that the force is the resultant force and that acceleration is in the same direction.
- Use unit analysis: N ÷ (m/s²) gives kg, which confirms the ratio is arranged correctly.
Try the lesson
The start of the Newton's laws: explain that inertia is a measure of how difficult it is to change the velocity of an object and that the inertial mass is defined as the ratio of force over acceleration (Higher tier only) lesson, made from checked teaching notes for this specification point.
Have a go first
A resultant force of 60 N acts on a crate and produces an acceleration of 4 m/s². What is the inertial mass of the crate?
Listen
Inertia describes how strongly an object resists any change in its velocity, whether that change is a speeding up, a slowing down or a change of direction. A loaded trolley is harder to start, stop or turn than an empty one because it has more inertia. Inertial mass puts a number on this: it is defined as the ratio of force over acceleration, so m = F ÷ a. A larger inertial mass means a smaller acceleration for the same resultant force. The definition is a ratio, so the force must be the resultant force acting on the object and the acceleration must be measured in the same direction. Units follow from the ratio: newtons divided by metres per second squared give kilograms. A strong answer earns marks for these points. States that inertia measures how difficult it is to change an object's velocity. Identifies that a greater inertial mass produces a smaller acceleration for the same resultant force. Defines inertial mass as the ratio of force over acceleration, m = F ÷ a. Recognises that the force in the definition is the resultant force acting on the object. Exam tip: Write the definition as a ratio in words and as an equation before substituting numbers.
Key words
Say what each word means before you turn the card.
Inertia
Try answering aloud or in your head before turning the card.
Inertial mass
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See every definition
- Inertia
- A measure of how difficult it is to change the velocity of an object.
- Inertial mass
- The ratio of force over acceleration, so m = F ÷ a.
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Test your understanding of Newton's laws: explain that inertia is a measure of how difficult it is to change the velocity of an object and that the inertial mass is defined as the ratio of force over acceleration (Higher tier only)
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