Section 1 of 6Inertia (HT only)
Definition
- Definition Inertia is the tendency of an object to stay at rest, or keep moving at a constant velocity, unless a resultant force acts on it.
- It is the same idea as Newton's first law of motion.
At rest
- At rest A stationary object stays stationary until a force makes it move.
- Pull a tablecloth quickly and the crockery stays almost where it was.
In motion
- In motion A moving object keeps the same speed in a straight line.
- Passengers lurch forward when a car brakes because their bodies tend to keep moving.
Section 2 of 6Inertial mass (HT only)
Inertial mass, m = F ÷ a
Meaning
- Meaning Inertial mass measures how hard it is to change an object's velocity.
Equation
- Equation inertial mass (kg) = force (N) ÷ acceleration (m/s2).
Large mass
- Large mass For a given force, a large inertial mass gives a small acceleration.
Small mass
- Small mass For the same force, a small inertial mass gives a large acceleration.
Link to F = ma
- Link to F = ma Rearranging F = m × a gives the same relationship, so calculating mass from force and acceleration finds the inertial mass.
Section 3 of 6Comparing objects
| Same force applied to | Acceleration | Inertial mass |
|---|---|---|
| Object A | 1.5 m/s2 | Medium |
| Object B | 0.7 m/s2 | Largest (smallest acceleration) |
| Object C | 2.0 m/s2 | Smallest (largest acceleration) |
Section 4 of 6Using the equation
1
Write down the force and the acceleration, checking both are in base units (newtons and m/s2).
2
State the equation m = F ÷ a.
3
Substitute the values: a 12 N force producing 4 m/s2 gives m = 12 ÷ 4.
4
Calculate and include the unit: m = 3 kg.
5
If asked which object has the greater inertial mass, argue from the equal-force comparison: the one with the smaller acceleration.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Inertia | The tendency of an object to resist any change in its state of rest or uniform motion. |
| Inertial mass | The ratio of the resultant force applied to an object to the acceleration it produces. |
| Resultant force | The single force that has the same effect as all the forces acting on an object. |
| Acceleration | The rate of change of velocity, measured in m/s2. |
| Uniform motion | Motion at constant speed in a straight line. |
Section 6 of 6Exam technique
1
Saying inertia is a forceit is a property of matter, not a push or pull.
2
Thinking a moving object needs a force to keep it moving: a force is only needed to change its velocity.
3
Concluding the object with the largest acceleration has the largest inertial mass: it has the smallest.
4
Confusing mass with weightmass is in kg and does not change, weight is a force in newtons.