I have faced a scenario mentioned in this Question.
There was a fly inside my car and I was travelling so fast. shutters are opened in the car. I observe that when I slow down and accelerate, there was no unusual movement of the fly. it is fling so normal when I took hardly press the brake paddle.
this is way more similar to this question and also this. but it has old mechanical physics answers. please can someone clarify it with relative physics?
The fly is flying in the air in your car. The air in your car is moving at the same speed as your car, more or less. When you brake or accelerate the change in the speed of your car is small compared with the mean speed of the molecules of the atmosphere in your car, so there is no gross tidal movement of the air as a consequence. The fly therefore moves relative to the air in the car, which moves, more or less, in unison with the car that encloses it, which means the fly moves relative to the car as if the car were stationary. If you were to remove your windscreen and rear window, the air would no longer move with the car, and the fly would no longer remain in the cabin with you.
There is no physics explanation here. The fly can accelerate much faster than your car, has a fast reaction time, and is guided by its stimulus response, not free body physics. Insofar as a fly is capable of wanting, the fly doesn't behave like a free body because it doesn't want to. It is capable of forces much larger than the momentum pseudoforce that is tending to "push" the fly opposite the car's acceleration and reacts quickly to stimuli, so it does what it wants.
There are no relativistic effects in this system.
The fly is not tied to the air (nor to the car). It is a relative heavy object in comparison to an air molecule. If you drop a dead fly, it will drop to the floor not stay at the same height. Likewise, if you had a dead fly fastened by a weightless string to the roof inside your car, it will swing to the back when you accelerate. It would take an active action by the fly to stay at the same spot in the car/air (as much as it takes active action to stay in the air in the first place). The fly apparently does not like to 'fall' towards the back/front of the car when the car is accelerating/decelerating, so it compensates for it by applying its own acceleration in order to stay at rest relatively to the air inside the car.
