Inertia is a property of matter, as described in Newton's first law of motion, that basically says if not external force is applied to an object, the object will continue with the motion it already has. That means that objects at rest stay at rest, and objects in motion stay in motion. The greater the mass, the greater the inertia. That is why it takes so long, for example, for a large ship or train to stop or change direction versus stopping or changing the direction of a toy car. You also experience inertia when you are driving in a car and change speeds suddenly. Your body wants to keep moving at the same rate as before and so your seat belt may suddenly engage as your body tries to keep moving forward. Newton talked about inertia more than mass when describing the relationship between the force applied to objects and the resultant acceleration the object would undergo.
Inertia is a property. It is not force. The force is responsible for the change of the state of motion.Inertia is the property that is described by the Newton's first law: Every body continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise.A body in uniform motion does not change its state of motion due to inertia.So it cannot come to rest without a an external force. The body at rest continuing in rest or in uniform motion without acceleration or zero acceleration continuing in that state is the property of all matters called inertia. Thus the inertia is the property of matter and it is not a force.
Yes I will agree with my friend that inertia keeps things in their state of rest or uniform motion. However, I will not describe inertia as as a force. As a matter of fact inertia is measured in terms of mass. By definition inertia is a property of matter due to which any body continues to be in a state of rest or uniform motion unless acted upon by some external force.
The concept of inertia was first described by Sir Issac Newton. He introduced this idea in his First Law of Motion published in 1687.