Galileo Galilei (1564 - 1642): a premier scientist in the $17^{th}$ century. He developed the concept of inertia. The law of inertia is the basis of the physics of the $17^{th}$ century. This law is also true according to modern physics.
"The natural tendency of an object to remain at rest unless acted upon by an external force."
The concepts of inertial in Galileo's writing would later come to be modified by Isaac Newton as the first of his Laws of Motion.
Laws of Inertia:
Galileo thought that, a moving objects eventually stop because of a force called friction. In experiment, using a pair of inclined planes facing each other, as shown in figure. Let us imagined this motion:
Galileo imagined a ball roll down from a left inclined plane and roll up to same height of the right inclined plane. If smoother plane were used, the ball would roll up closure to the original height of the opposite plane. If the rolling ball did not reach the original height of the opposite plane(i.e. difference between initial and final height), there presence some external force called Friction. Galileo conclude that: "If friction could be entirely eliminated then, the ball would reach exactly the same height."
Galileo demonstrated that: if the slope of right inclined plane is slightly reduced, then ball would roll for an even longer time on the right side plane before coming to a stop. Again if the slope of right inclined plane were reduced to $0 - degree$ then, ..... a ball in motion would continue in its state of motion..... moving at a constant speed in a straight line. This property of matter is called inertia.
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Inertia is the tendency of an object to continue in its original state. A body at rest moves only when a force acts on it. Velocity of a uniformly moving body will change only when a force acts on it.
1. Inertia of rest: It is the tendency of a body to remain in its state of rest.
Example:a) Passengers in a bus fall backward if the bus suddenly starts to move due to inertia of rest.
b) Place a coin on a paper. Pull the paper out suddenly. the coin remains in its original position due to inertia of rest.
2. Inertia of motion: It is the tendency of a body to remain in its uniform motion.
Example:a) Passengers in a bus fall forward when the bus suddenly stops.
b) To jump a long jump an athlete has to run a long distance before taking a jump.
Inertial Mass:
Inertial frames:
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