Sir Isaac Newton was an English scientist. In the year 1686 he put forwarded three laws about the motion, which are known as Newton’s laws of motion. Here we discuss about the first law of Newton’s laws of motion.

If no outer force is apply on a rigid body then its state (rest or motion) is remain unchanged.

Def: “A body continues to be in its state of rest or of uniform motion in a straight line unless it is compelled by some external force to change that state.”

We know two different things from the newton’s first law.

1. Inertia property of a body: The inherent property of a body by virtue of which it resists any change in its state of rest or uniform motion in a straight line on its own is called Inertia property of a body.

   Inertia property is increase proportionally to it mass. If a heavy body have in rest state then more force need to convert its state to moving state. And also more force need to stop a heavy moveable body. Inertia property of a body is two kinds.

   1. Inertia of rest: The force resists to change the rest state to motion state of a body is known as Inertia of rest.

      Example:

      If a bus suddenly starts then passengers on a bus tends to fall backward. Because the bus and the passenger both are in rest state when the bus stop. When the bus starts suddenly then the lower part of the passenger’s body start moving forward with bus. But upper part of the body tends to remain in the state of rest due to the inertia of rest. As a result, the passengers fall or lean backward.

      
      
   2. Inertia of motion: The force resists to change the motion state to rest state of a body is known as Inertia of motion.
      

      Example:

      If a bus suddenly stop then passengers on a bus tends to fall forward. Because the bus and the passenger both are in motion state when the bus running. When the bus stops suddenly then the lower part of the passenger’s body stop immediately with bus. But upper part of the body tends to remain in the state of motion due to the inertia of motion. As a result, the passengers fall or lean forward.

      
2. Definition of force: from the newton’s 1^st law of motion, we come to know that, if a body is in rest then it remains in rest position. If a body is in motion state then it remains in moving position. This state will be change if and only if we apply some force.