What is inertia?

**
Inertia can be defined as the tendency of
an object's motion to remain unchanged**

(That does NOT mean an
object's motion cannot be changed, but a resultant force of >0
must be applied to change to objects motion and inertia is about
the force needed)

If we start by thinking about the implication
of Newtons First Law of motion ...

**... a
resultant force is needed to change the motion of any object**.

In other words, unless acted on by a
resultant force, anything stationary remains at rest (zero velocity), anything
moving keeps moving with the same velocity (same speed and direction).

So we can say
**the tendency of an
object to keep moving with the same velocity is called inertia**.

**Inertial
mass measures an object's resistance to being accelerated by a force (F =
ma)**

**We can measure how difficult it is to change
an object's velocity by calculating its inertial mass.**

An object's inertial mass is a measure of how difficult it is to change its velocity.

We can do this using the equation from
Newton's Second Law of motion (force = mass x acceleration).

F = ma, on rearrangement this
gives:** m = F/a**, and this expression **defines inertial mass**

so**
inertial mass (kg) = applied force (N) /
acceleration (m/s**^{2})

Another way of looking at the equation is to
consider the effect of force on acceleration.

**
a = F/m**, some consequences are ...

as already stated, for a given mass on
object's acceleration is proportional to the force applied, but ...

if the **same force F** is applied to
two different masses, **the smaller mass, with the smaller inertia, will experience the greater
acceleration**,

and, if two objects have the same mass, then
applying the same force to each object will produce the same acceleration.

**
Inertia and moving objects**

As well as looking at inertia from the point
of view of acceleration, think about slowing moving objects down.

If two objects of different masses are
moving at the same speed, the object of greater mass will need a bigger
force to slow it down (decelerate) due to Newton's second law.

Imagine two cars are moving at the same
speed and both drivers take the foot off the accelerator. If the two cars
experience the same air resistance and wheel-road friction forces, the car
of bigger mass would travel on further before coming to a halt. The bigger
the inertial mass, the bigger the force would be needed to bring it to a
halt in the same stopping distance as the car with the smaller mass. F = ma,
force proportional to mass.

Large objects like cargo ships or
high speed trains can take several km to come to a halt.

**
What is the difference between i****nertial
mass and gravitational mass**

**Inertial mass** measures an object's
resistance to acceleration.

Inertial mass = force /
acceleration (**m = F / a**, from F = ma)

**Gravitational mass** determines the
gravitational attractive force it exerts on another object

mass = weight / gravitational
field constant (**m = W / g**, from W = mg).

**BUT,
inertial mass and gravitational
mass are numerical identical.**