How can a fly fly in a car?

Question

From my understanding. if the car is moving at the velocity of 10 m/s and the fly is moving at the velocity of 5 m/s. the fly must move backward at the velocity of 5 m/s. but the fly seem to be able to keep up with the car just fine.

Answer 1

Everything inside the car has been accelerated to 10 m/s with the car. The fly is moving 5 m/s relative to the car, not the ground. Therefore the fly is moving 15 m/s relative to the ground