Ask a Teacher

Inertia is  the property of matter by which it retains its state of rest or its motion  along a straight line so long as it is not acted upon by an external force.

                           An object with mass requires force to change its motion, or to start moving from a stationary position. Once the force is applied, the object will move at a constant velocity until affected by another force.ie inertia is proportional to the mass of any object - the greater the mass, the greater will be the inertia. 

example:

 If a bus suddenly stops and you aren't holding onto a support, you will be pushed to the front of the bus. This is due to inertia of rest.