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According to Newton's law of inertia (written in book:The Mathematical Principles of Natural Philosophy**) it is said that in all objects having mass, have always inertia included in their property but if a body is in motion well then, would there be inertia in that body.

For instance: A ball is hit by a bat and is in state of motion in air so would there be inertia present in that current state of ball or not?

According to friction law of physics friction is any thing which exists when two or more bodies come in contact with one-another and it is against the direction of force (pushing a body) which is against inertia so would friction be helpful too in supporting inertia of a body consequently?

If yes then could it be said that, inertia is directly proportional to the friction?

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  • $\begingroup$ Of course. There is static inertia, in a stationary body, measured by its mass. And the other kind is dynamic inertia, measured by the momentum of the body. p=mv $\endgroup$ – Self-Made Man Sep 15 '13 at 1:33
  • $\begingroup$ Sounds like you got it man.thank you!Can you tell me one more thing that how would we calculate inertia in statics and dynamics both? $\endgroup$ – Sufyan Shaikh Sep 16 '13 at 17:37
  • $\begingroup$ Also you have not answered me that whether i am right about relation of friction and inertia or not? $\endgroup$ – Sufyan Shaikh Sep 16 '13 at 17:55
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According to Newton, Inertia is the resistance of a body due to which it resists any change in its state of motion. Whether it is in the state of rest or in motion inertia will always be present. That means even is state of motion body will resist any change in its motion. Hence when the ball is in the air, it will resist change in it's motion as well (which is straight line).

Friction is itself a force. And you said that, "it is against the direction of force". That means the friction is a force which will act against the net force acting on the particles, which is incorrect. Friction is not inertia and it will not help inertia.

Friction is a force acting between to bodies against their "relative" motion (to understand the term "relative", see how friction can sometimes increase the velocity of a body in a given frame.) And then you said, "which is against inertia so would friction be helpful too in supporting inertia of a body consequently." I am not what you want to say here but I think this is not a correct statement. Inertia is a property not a force. And $F = \frac{\mathrm{d}(mv)}{\mathrm{d}t}$, rate of change of momentum with time. Now you can see that if force is same and we have a heavier mass and a lighter constant mass, change in velocity ($a = \frac{F}m$) will be less in case of heavier mass. So we can say that heavier mass has more inertia than the lighter mass.

You may want to look into the definition of inertia again. I hope I was helpful. : )

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  • $\begingroup$ sounds like you are surely correct about .Actually what my mistake was that i took inertia as being a force however it is wrong because inertia is just a property of an object that resists change in it's state.Gratefull to you! $\endgroup$ – Sufyan Shaikh Sep 17 '13 at 15:36
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Yes a body always possess inertia. Earlier people thought the natural state of a body was to stay at rest. Because an object always stopped when it was in non accelerated motion. But Galileo through his experiment proved this wrong. He took a minimal friction surface at an angle. Then he rolled down a very smooth marble ball. The ball kept moving, thus he concluded that a body in non-accelerated motion stops only due to friction and even uniform motion along a straight path is a natural state of a body. From this we can say a body in motion also possess inertia, that is, it resists any change in it's state. So the ball you were talking about also possess inertia. As for friction it opposes the motion of the ball and does not help the inertia of the ball. Because friction tries to stop the ball while inertia tries to stay in uniform motion. So, you cannot say that inertia is directly proportional to the friction.

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