Why do flies explode on impact with my windshield on the outside of the car but are able to fly around inside my car while it’s in motion without exploding all over the inside of my rear window?
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If the fly is in your car from the start, then it accelerates with the car. Even the interior air it’s flying through if it starts buzzing around is mostly coming along with car.
So the fly is moving along at the same speed as the car and if it bumps into the rear window it is bumping into with only whatever DIFFERENCE in speed from the car it made with its wings.
If the fly is OUTSIDE the car, then it wasn’t already moving at the same speed as the car. It is smacked with the full difference of whatever speed and direction the car was traveling relative to the ground vs whatever speed and direction it was traveling relative to the ground. So now instead of a difference of maybe a couple miles and hour, it’s experiencing a difference of say… 60 miles an hour.
It’s not speed that imparts force, it’s suddenly changing speed. Otherwise known as acceleration. And force is mass times acceleration.
To rephrase that: if a fly is in a car going at 60mph, it’s traveling at 60 mph in the same direction just by being in the car. So if it flies towards the back of the car, maybe it gets up to 2mph in the opposite direction relative to the car. What it’s really done is slow itself (relative to the ground) to 58 mph. When it hits the rear window, it gets a speed change of 2mph (back up to the 60 the window is moving at) over the time the impact takes.
If it’s traveling at 2mph relative to the GROUND and your car hits it going 60mph, then the fly experiences a change of 58 mph in roughly the same amount of time. WAY more force.
… And yes I assumed or handwaved several things for ELI5 here, let’s roll with it for an example.
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