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

Imagine you’re holding a ball in a moving vehicle. If you throw that ball in the air, it goes straight up relative to you. This is because by holding it, you’ve given it your momentum, your momentum having come from the car. Momentum lasts until something stops it. When a fly takes off in a car, it keeps the momentum of the car. There’s nothing that causes the fly to lose that momentum, so it can just fly and around relative to the car.