Theory:

Isaac Newton studied Galileo's ideas on force and motion and introduced three fundamental laws that govern motion of the objects. They are called Newton's laws of motion.
An object remains in a state of rest or uniform motion in a straight line unless an external force acts on it.
In other words, all the objects try to resist a change in their state of motion. In a qualitative way, the state of undisturbed objects to stay at rest or to keep moving with the same velocity is called inertia. Hence, the first law of motion is also known as the law of inertia.
Some examples of inertia of rest and motion are given below.
Certain activities that we come across while travelling in a motorcar can be explained based on the law of inertia. We supposed to remain at rest with respect to the seat until the driver applies the brake to stop the car. The car slows down with brakes application, but our body remains to continue in the same state of motion because of its inertia. A sudden application of brakes may cause injury to us by impact or collision with the front part of the car. Safety belts exert a force on our body to make the forward motion slower.
 
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Safety belt holds us when we fall forward.
  
An opposite scenario can be seen while standing on a bus, when it begins to move suddenly. We will fall backwards. This is because of the sudden start of the bus, which causes both the bus and our feet to move since they are in contact. But, the upper portion of the body opposes this motion due to inertia. When a motorcar makes a sharp turn at high speed, we tend to get thrown to one side. When the engine provides an unbalanced force to change the direction of motion of motorcar, we may slip to the other side of the seat due to our body's inertia. It is also based on the law of inertia of direction.
 
We will study about the inertia in detail in upcoming section.