An object offer resistance to change in its state of motion. This was studied by Newton in detail and he presented his findings in the form of three fundamental laws that govern the motion of objects.
Newton’s first law of motion is stated as:
"Every body continues in its state of rest or of uniform motion in a straight line until unless it is compelled by some unbalanced force to change that state."
Newton's first law of motion tells that all bodies resist a change in their state of motion. This property of bodies is called inertia. That is why, Newton’s first law of motion is also known as the law of inertia.
First law of motion has many applications in daily life.
Why do the passengers standing in a bus fall in the backward direction when the stationary bus begins to move suddenly?
This observation can be explained on the basis of first law of motion. The feet of passengers are in contact with the bus. When the bus starts suddenly, the feet start moving with the bus. But the upper part of the passengers tries to remain at rest due to inertia and tends to fall in the backward direction.
What happen when the moving bus stops suddenly?
In this case the passengers standing in the bus fall in the forward direction.