fightnight

OK so I guess this is a sort of physics question...

How come when you're driving in a car and there's a fly in the car, it can just fly around, whether you're going 80 MPH, or stopped??? The fly obviously isn't flying 80 mph plus whatever speed/direction he's going. Is it because inside your car is an entire separate environment of sorts??? I thought this would be easy when someone genuinely asked me it, but I can't quite get my mind to come to grips with it. Any assistance?? Thanks!!!

phukraut

Says who? From the point of view of a stationary observer, that fly is speeding by. From your perspective, it's not going that fast because you're both going 80mph in the same direction, plus the the added velocity of the fly since you are sitting still.

It's not really about a separate environment. It's more to do with inertia. The car is going at a constant velocity. If you change velocity enough, the passengers in the car will keep going in the direction the were before unless they somehow stop themselves. Consider if a car suddenly hits the breaks and the passengers go flying towards the windshield.

ObieX

The fly in the air is subject to the motion/momentum of the air in teh car since it's suspended in it and has such a light weight. (or atleast is my udnerstanding). The air in the car may move very little (if at all) when you hit the brakes, maybe some slight pressure changes.. but for the most part it wont move. Now if you have a window open or are in a convertable thats a different story since the air will have someplace to move and the fly will move where the air goes.

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Silvy

I think it's quite easy to picture actually:

Imagine yourself in the car (in the backseat) and throwing a ball forwards, you're suddenly throwing objects in excess of 80mph!
The absolute velocity of the ball is not 0 when you throw it, it's 80 mph. Your power is just adding to that velocity.

Or look at it this way: The fly can fly because it pushes against the air around it right?
That air appears stationary in the car, because it has nowhere to go (compare to a convertible). So the air moves in exactly the same way as the car. The fly pushes against that air, and so it's moving at a normal air-speed, but airspeed + 80mph = absolute speed.

And if it still puzzles: the earth is revolving (at quite high speeds around the equator). But people can walk easily in any direction. It is because we walk relative to our surroundings (which just happen to go very fast through space).

Note: I was told once (don't know if I remember correctly) that if the earth would abrubtly stop spinning we would fly into the wall at an average of 350 mph!

Edit: I hope I'm making sense it's 6 am here

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fightnight

So I had thought of all this, but it doesn't seem to me that if i slam on the brakes the bug will go flying into the windshield at 80... which is where i started having trouble thinking about it. I know I was told once to never leave a box of tissues in the rear window of your car because if you were to get in an accident and hit something stationary at a high speed, the tissue box basically turns into a bullet. Anyways, back to the fly, it doesn't seem to me the fly will continue on at 80 if I slam on the brakes which is where my misunderstanding comes in, am I wrong?

joemc91

The fly has almost no intertia though. The amount of drag the bug has compared to it's weight is huge, so if changes speed with the air almost instantaneously.

ObieX

Gotta think of it in terms of mementum transfer. The more secure something is to another object the easier the momentum with transfer. When the car is stopped the windshild is strong enough to anchor the air, to keep it from moving forward whent he car stops. The fly's wings beating anchors the fly to the air strong enough so that the momentum of the fly moving forward is stopped by the air. If the fly were to stop beating its wings at the exact moment the car stops the fly may very well shoot forward.

If the fly is sitting in the sill of the rear window and the car stops it would also shoot forward the same way that tissue box would unless the fly's feet are able to secure it to the window sill the way the fly's wings would secure it to the air. Just like your seatbelt secures you to the seat, which is secured to the car.

Also keep in mind the fly weighs very very little, so the amount of force that is required to keep it from shooting forward is very very little.

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shakran

And with the box of kleenex, unless you're doing a couple hundred mph I don't really see it hurting you in a wreck - if the collision forces are severe enough that a light box of kleenex can turn into a bullet, you've already been mutilated by whatever you hit anyway.

fightnight

Alright, I supposed I understand now. Thanks for all the help!

Slavakion

Absolute velocity?! A slip of the tongue/fingers?

Oh, ok. I'll let it go.