Why is a fly at rest on air inside on a running bus not being left?it just go with the motion of the bus.

Question

Why is a fly at rest on air inside on a running bus not being left?it just go with the motion of the bus.

Answer 1

The bus is considered a closed system, so all the air inside it moves at the same velocity as the bus.  If the bus didn't have a roof then it would no longer be a closed system and the fly probably wouldn't be able to stay put once the bus picked up speed.  Just like if you have loose papers in your car and you drive with the windows (and roof) up, they don't move.  Open the windows and they might fly around and settle in the back seat.