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Suppose, an observer is on a train which has only platform to stand but no walls. The train is moving with a constant velocity with reference to railroad. While moving, the observer feels strong wind against his face (if he faces towards front). If he puts a light ball on the platform, it automatically goes backwards due to wind. So, $F=ma$ does not work for him and the frame is non inertial to him. But, the train moves in uniform velocity. How come is this possible?

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  • $\begingroup$ According to the observer, there is a wind blowing towards them, whereas from the ground observer the train is moving into the air. $\endgroup$ – John Alexiou May 25 '20 at 18:23
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In both scenarios the wind causes air drag (force $F$) which acts on the ball. For both observers the acceleration of the ball is $$a = \frac{F}{m}$$, but the ball has different initial velocity.

For the train rider the initial velocity is zero, and for the ground observer it equals the speed of the train.

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  • $\begingroup$ i got it.. What a silly question mine was.. thanks. $\endgroup$ – bitan maity May 25 '20 at 21:38
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If the train is moving with a "constant velocity", how is it a "Non-Inertial" reference frame?

Also if there any body moving in a fluid, the drag force is experienced even if the object is moving with constant speed.

The drag force is typically directly proportional to the velocity of the object, so all laws hold.

You can read more about it here.

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