Mass & Inertia (Edexcel GCSE Physics)

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:

Demonstrating the inertia of a small coin

• 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:

• 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
  • Larger inertial masses will experience small accelerations
  • Smaller inertial masses will experience large accelerations

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

Step 1: State the definition of inertial mass

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

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

• • Because each object experiences the same force, inertial mass is inversely proportional to acceleration
  • This means that the object with the largest inertial mass will experience the smallest acceleration
  • Object B has the smallest acceleration, so it has the largest inertial mass