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## Clarification regarding Newon's first law.

 Quote by Doc Al Here's how I would describe those two cases: (1) A box is at rest with no external forces acting. It just sits there, so Newton's first law holds. (2) That same box has no external forces acting yet is seen to accelerate. So Newton's first law does not hold.
But is that example correct ? I think that the teacher while giving that example , is confusing frame of references and relative force.

 Not clear what you mean. If you are viewing things from a non-inertial frame, Newton's first law will not hold.
But what I am asking is this :

Earth is practically regarded as inertial frame of reference , right ? Suppose the ship is in acceleration with respect to earth. So ship is a non inertial frame. Now an observer in the ship cannot apply Newton's first law by observing object inside ship , or also cannot apply Newton's first law outside the ship ?

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 Quote by sankalpmittal But is that example correct ? I think that the teacher while giving that example , is confusing frame of references and relative force.
The examples, properly explained, are fine. The point is that from a non-inertial frame Newton's first law doesn't hold without modification.
 But what I am asking is this : Earth is practically regarded as inertial frame of reference , right ? Suppose the ship is in acceleration with respect to earth. So ship is a non inertial frame. Now an observer in the ship cannot apply Newton's first law by observing object inside ship , or also cannot apply Newton's first law outside the ship ?
Once again: If you view things from a non-inertial frame, such as your ship, then Newton's first law will not apply. It doesn't matter if you are viewing something that happens inside or outside the ship, as long as you are describing them from the non-inertial frame of the ship then Newton's first law will not apply.

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 Quote by sankalpmittal Ok , so my first question has been well answered however second question has not , for I've mistyped it. ... Please do not leave any question unanswered now, otherwise I'll have to re-post it.
It doesn't make any sense to get snippy about your question being unanswered when you mistyped it (rather significantly). You expect us to do you the courtesy of taking the effort to re-answer a question that we already answered (which I will do), so please also do us the courtesy of being polite about your request.

 Suppose a block is kept at rest on a floor. An observer observes this standing beside the block and thus concludes that the block is at rest. So |a| =0 Or |F| =0. So he says that Newton's first law holds. Another observer in a car is moving with an acceleration towards the block w.r.t ground and observes the acceleration in the block as -a.
OK we have two definitions of Newtons first law which we would like to apply to this situation:

A) "Every object remains in its original state of rest or uniform motion unless it is acted upon by a net external force."

B) "If observation is being made from an inertial or non accelerating frame of reference , then every object remains in its original state of rest or uniform motion unless it is acted upon by a net external force."

In all reference frames the block is affected by the following two real forces: the weight of the block directed vertically down, and the normal force from the floor directed vertically up. These two forces cancel each other out in all reference frames, for a net force of 0. In the car's reference frame the block is accelerating horizontally at a rate of -a.

Definition A fails because the block is not staying at rest even though it is not acted upon by a net external force.

Definition B succeeds because the observation is being made from an accelerating frame, so the remainder of the definition does not apply.

 Recognitions: Gold Member Oh ! Forgot to say thanks !! :) Thanks for the efforts .....