# Fly in a car

by ycy88
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 P: 3 You are driving a car. A fly is along for the ride (in the car). The fly is not hanging on to anything. Not the window, not the seat. It is flying. If you drive your car fast, faster than the fly to catch up, will the fly hit the back window? Why?
 P: 15,325 :No. The fly is perched on the *air* - which is accelerated with the car.:
 P: 3 If the air is "accelerated", why won't you feel the "wind" when you are driving in the car?
P: 2,163

## Fly in a car

Because you are accelerated too.
 P: n/a The fly will hit the backwindow if your acceleration is high enough.
P: 243
 Quote by DaveC426913 :No. The fly is perched on the *air* - which is accelerated with the car.:
Question:

Wouldn't this depend on whether the windows were open or not? Also, what about inertia?
 P: 606 If the density of the fly is greater than that of the air then he will be moved towards the back window. A helium balloon would move forward in an accellerating car because its density is less than that of the air in the car. My guess is that a fly is denser than air. What I'm really curious about is how Dave made a post with less than 10 characters?
 P: 552 He posted white text to hide his answer. Select to see.
P: 606
 Quote by BicycleTree He posted white text to hide his answer. Select to see.
Oh, very clever. I see limitless possibilities in this!
P: 235
 Quote by Huckleberry If the density of the fly is greater than that of the air then he will be moved towards the back window. A helium balloon would move forward in an accellerating car because its density is less than that of the air in the car. My guess is that a fly is denser than air. What I'm really curious about is how Dave made a post with less than 10 characters?
so the acceleration is simmilar like the one due to gravity isn't it. but if the fly doen't being pull to gravity (acting as same density as air even tough if it don't) would it also smashed to the rear window. i agree with the helium after all
 P: 3 Well, what if it all happen in vacuum? Will the fly smash the rear window? (Assume that you and the fly survive without oxygen)
 P: 2,163 The fly will won't be able to fly.
P: 15,325
 Quote by ycy88 If the air is "accelerated", why won't you feel the "wind" when you are driving in the car?
The air has very little inertia. It is very easy to accelerate it along with the car. You feel nothing. It *does* have *some* inertia, which is why the helium balloon gets pushed forward as the air gets pushed back.

 The fly will hit the backwindow if your acceleration is high enough.
Yep.

 Wouldn't this depend on whether the windows were open or not?
That would be a factor, yes.

 If the density of the fly is greater than that of the air then he will be moved towards the back window. A helium balloon would move forward in an accellerating car because its density is less than that of the air in the car. My guess is that a fly is denser than air.
Yes, but with so little inertia, the fly is affected more by air resistance. Think of dropping a (non-flying) fly off a roof. It is much mroe affected by air resistance than it is by its own mass. Same thing happens in the car under acceleration.

 He posted white text to hide his answer. Select to see. Oh, very clever. I see limitless possibilities in this!
I take no credit. As you peruse this forum, you will see that it is the standard way of providing an answer.

 Well, what if it all happen in vacuum? Will the fly smash the rear window? (Assume that you and the fly survive without oxygen) The fly will won't be able to fly.
Ahah! That's clever! But if it were to hop, or use a rocket pack, then yes it would smash into the back window - just as a feather drops like a stone in a vacuum.
P: 223
 Quote by DaveC426913 Ahah! That's clever! But if it were to hop, or use a rocket pack, then yes it would smash into the back window - just as a feather drops like a stone in a vacuum.
In a vacuum, wouldn't the fly hit the back window very easily? There is no air to keep the fly accelerated... nothing pushing the fly forward. And as mentioned, it can't use it's wings to propel itself forward, just like we can't use our arms to do that in a vacuum.

What about with gravity? Does that mean the fly starts on the floor and most likely would not hit the window because it wouldn't get off the floor?
P: 15,325
 Quote by Rahmuss In a vacuum, wouldn't the fly hit the back window very easily? There is no air to keep the fly accelerated... nothing pushing the fly forward. And as mentioned, it can't use it's wings to propel itself forward, just like we can't use our arms to do that in a vacuum.
That *would* be the case if not for...
 Quote by Rahmuss What about with gravity? Does that mean the fly starts on the floor and most likely would not hit the window because it wouldn't get off the floor?
Correct.10char