Eli5: How come flies don’t smash into the back window inside a moving vehicle while they’re flying around? Surely they don’t fly “with” the vehicle.

Question

Eli5: How come flies don’t smash into the back window inside a moving vehicle while they’re flying around? Surely they don’t fly “with” the vehicle.

In: Physics

Answer 1

It’s all about relativity….

The fly moves through the air in the car like a fish moves through water. When a car moves forward, so does the air inside (pushed by the windows, doors, etc.) So the fly doesn’t feel any “relative motion” in the air just because the car moves forward, the same way that a fish would not be slammed against its bowl just because the bowl is moving. The air is considered “at rest” relative to the car. This may change for an instant, during hard acceleration or braking, but in general, the air trapped in the car is not moving relative to the car, only relative to the air outside.