Inertial mass is a measure of how difficult it is to change the velocity…

The idea

Inertial mass describes an object's resistance to a change in velocity. A larger inertial mass means that for a given resultant force, the acceleration is smaller, so the velocity changes more slowly. Alternatively, to produce the same acceleration, a larger resultant force is needed. You can investigate this by applying the same resultant force to trolleys of different masses and measuring acceleration; the more massive trolley accelerates less. Inertial mass is measured in kilograms and is defined through the equation F = m a, where m is the inertial mass. It is not the same as gravitational mass, although in everyday situations the two have the same value. Explaining this idea means linking mass, resultant force and acceleration quantitatively and qualitatively.

What earns the marks

• Define inertial mass as a measure of how difficult it is to change an object's velocity, or its resistance to acceleration.
• State that inertial mass is measured in kilograms and is the m in the equation F = m a.
• Explain that for a given resultant force, a larger inertial mass produces a smaller acceleration, so velocity changes more slowly.
• Explain that to give two objects the same acceleration, the object with greater inertial mass needs the greater resultant force.
• Use a practical example, such as comparing a loaded and an unloaded trolley pushed with the same force, to show the effect of inertial mass.
• Rearrange F = m a to m = F ÷ a and describe how this relationship defines inertial mass from force and acceleration measurements.

Common mistakes and how to put them right

• Confusing inertial mass with weight.

  Put it right by stating that weight is a force in newtons caused by gravity, while inertial mass is a property in kilograms resisting velocity change.

• Thinking that a more massive object cannot accelerate.

  Put it right by explaining that it can accelerate, but a greater resultant force is needed for the same acceleration.

• Believing that inertial mass changes with location.

  Put it right by stating that inertial mass is constant for an object, whereas weight changes with gravitational field strength.

Exam tips

• Use the phrase 'resistance to change in velocity' or 'difficulty in changing velocity' to show you understand inertial mass rather than just quoting the word mass.
• When comparing objects, refer explicitly to a constant resultant force or a constant acceleration so your proportional reasoning is clear.
• Support explanations with the equation F = m a, identifying each symbol and its unit, to connect the definition to quantitative work.