Inertial mass measures resistance to acceleration

1. Inertial mass measures how difficult it is to change an object's velocity, including how difficult it is to start an object moving from rest.
2. The defining equation is $m=\frac{F}{a}$, where $m$ is inertial mass in kilograms, $F$ is resultant force in newtons and $a$ is acceleration in metres per second squared.
3. A large inertial mass means that the same resultant force produces a smaller acceleration because $a=\frac{F}{m}$.
4. A small inertial mass means that the same resultant force produces a larger acceleration.

Resultant force must be used

1. Use the resultant force, not one individual force, because opposing forces may partly or completely cancel.
2. If the resultant force is zero, the acceleration is zero, so the object remains at rest or continues at constant velocity.
3. Inertial mass is a property of the object and does not depend on the strength of the gravitational field.

Comparing objects from data

1. For the same resultant force, compare accelerations using $m=\frac{F}{a}$; the object with the smaller acceleration has the greater inertial mass.
2. For the same acceleration, the object that requires the larger resultant force has the greater inertial mass.
3. On a graph of resultant force against acceleration, $F=ma$, so the gradient equals the inertial mass.

Force and acceleration data reveal mass

1. A straight-line graph through the origin shows that resultant force is directly proportional to acceleration when mass is constant.
2. A steeper $F$ against $a$ graph represents a larger inertial mass because its gradient $\frac{\Delta F}{\Delta a}$ is larger.