# 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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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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The same reason you can sit inside your car comfortably but if you were to be hit by a car you would be splattered.

The fly is moving at the same speed as the air (more or less), and the air inside your car is going the same speed as your car, so the difference in speed between the fly and your car is close to zero.

From the flies point of view, there’s no difference between being inside a parked car or a moving car

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.

Relative velocity.

If the fly is doing 5mph and you hit it at 50mph, that’s somewhere between a 45mph and a 55mph relative velocity (depending on which way you were both travelling).

If the fly is inside the car, it’s already doing 50mph relative to the Earth, the same as the car, but it’s only going 5mph relative to the car itself.

You’re currently travelling over 1000mph. Earth is rotating you at that speed. You don’t know and don’t splat into things because everything else you see is also doing 1000mph in the same direction. So, relatively, you are doing the same speed.

You’re also travelling around the sun, the sun is travelling around the galaxy, the galaxy is spinning at ridiculous speeds.

None of it matters if you’re ALL moving at the same speed. The only thing that matters is the *difference* in speed between you and the thing you hit.

It’s why hitting a stationary car at 40mph is exactly the same as hitting a 30mph car at 70mph. The physics are basically identical. What matters is the relative velocity, not the actual velocity (because, for a start, you have no idea what velocity you’re doing right now – because of all those other things that are also moving you, that I mentioned above).