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Inertia (AQA GCSE Combined Science: Trilogy): Revision Note

Exam code: 8464

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AQA Combined Science: Trilogy: PhysicsAQA Physics

Inertia

Higher Tier Only

  • The concept of inertia is closely related to motion, it is defined as:

    The tendency of an object to continue in its state of rest, or in uniform motion unless acted upon by an external force

  • In other words, inertia is an object's resistance to a change in motion 

    • If an object is at rest, it will tend to remain at rest

    • If an object is moving at a constant velocity (constant speed in a straight line), it will continue to do so

  • The image below illustrates the concept of inertia using a coin and a cup of water:

Inertia of a small coin, downloadable IGCSE & GCSE Physics revision notes

Demonstrating the inertia of a small coin

Inertia & Mass

Higher Tier Only

  • Inertial mass is the property of an object which describes how difficult it is to change its velocity

  • It is defined as the ratio between the force applied to it and the acceleration it experiences, and can be calculated by:

i n e r t i a l space m a s s space equals space fraction numerator f o r c e over denominator a c c e l e r a t i o n end fraction

m space equals space F over a

  • Where:

    • m = inertial mass in kilograms (kg)

    • F = force in newtons (N)

    • a = acceleration in metres per second squared (m/s2)

  • This equation shows that for a given force, inertial mass is inversely proportional to acceleration

  • This means that:

    • Larger inertial masses are harder to accelerate, so they speed up more slowly

    • Smaller inertial masses are easier to accelerate, so they speed up more quickly

Inertial Mass Examples 1, downloadable IGCSE & GCSE Physics revision notes
Inertial Mass Examples 2, downloadable IGCSE & GCSE Physics revision notes

This collection of objects have a variety of inertial masses. For the same applied force, their accelerations are inversely proportional to their mass

Worked Example

Three objects are used by a physics technician to demonstrate the concept of inertial mass. She applies the same force to each of the three objects and notes that:

  • Object A accelerates at 1.5 m/s2

  • Object B accelerates at 0.7 m/s2

  • Object C accelerates at 2.0 m/s2

Which object has the largest inertial mass?

Answer:

Step 1: State the definition of inertial mass

  • Inertial mass is defined as the ratio of force to acceleration:

i n e r t i a l space m a s s space equals space fraction numerator f o r c e over denominator a c c e l e r a t i o n end fraction

Step 2: Apply the definition to each of the objects in the question

  • Since the same force is applied to all three objects, the inertial mass of each object depends on how much it accelerates

    • An object that accelerates less must have a greater inertial mass.

  • This means the object with the smallest acceleration will have the largest inertial mass.

  • Object B has the smallest acceleration (0.7 m/s²), so it has the largest inertial mass.

Examiner Tips and Tricks

The definition of inertial mass as a ratio between force and acceleration should look similar to Newton's second law. That's because when you calculate the mass of an object using Newton's second law, you are in fact calculating its inertial mass.

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