The Ol' fly on a windshield conumdrum

[Circa89]

Its an old drunken bar game to see if you can get your mates to buy you a beer.

Heres the set up if you have never heard this before.

pt 1. Do you think a fly can stop a car (or a train)? NO, of course not.

pt.2 Do you agree that if something is travelling in a straight line and then reverses it had to stop at somepoint? Yes, I agree

pt.3 So then if a fly in travelling due south and hits a windsheild of a car travelling due north. The fly changes direction right? yes
Therefore the fly stopped right? Yes
If the fly stopped then the car had to have stopped right?
Therefore the fly stopped the car.

I have heard this more than once and at one point I knew the right answer. However since this question is usually only asked in bars and since I am always drunk in bars I have forgotten the logic to solve this dilemma.

Now I'm mad and I turn to you my wise and all knowing internet brethren for the correct solution.

What is the physics behind the answer? I know its easy but I told my friend I would have the "real" answer for him in 2 days.

[Prototype]

I don't get it... the car stops the fly. But the car doesn't change direction. It stops the fly due to it moving forward. There's a flaw in your logic...

[iggypop]

Yeah why would the car have had to stop just because the fly stopped?

[BlackArcher101]

Originally Posted by Circa89

What is the physics behind the answer? I know its easy but I told my friend I would have the "real" answer for him in 2 days.

The answer is easy... you are drunk and what you said is entirely wrong and goes against all theories/laws of physics.

[Circa89]

Its not my logic. Its drunken Irish dude logic.
The premise states that ifthe fly stopped, the car stopped, because it is impossible for the fly to have "stopped" if the car kept moving. Get it?
The fly had to stop to change directions, but when did the fly stop if the car was moving at a constant speed.
We already agreed the fly stopped to change directions.

[metallicat]

http://answers.yahoo.com/question/in...2092713AAl1VoK

I've never heard of this before. Maybe I need to spend more time in bars.

[BlackArcher101]

Could you imagine the acceleration (gee's) if a train stopped within ms (actually, what you are saying is way less than that, basically instantly).... I hope you aren't serious.

Read this on collisions:

http://www.brainycreatures.org/physics/collision.asp

[Circa89]

Originally Posted by BlackArcher101

The answer is easy... you are drunk and what you said is entirely wrong and goes against all theories/laws of physics.

Haha I have been drinking but thats beside the point.

Of course the car never stopped. But, The real question is how did the fly change directions if he didn't stop. And if the fly did indeed stop (which it didn't) the car had to have stopped at the same point that the fly stopped.


Stupid but it's irritating when a beer is on the line and you can explain it.


Here's another good one to get your buddies on.

What is greater in length. The cicumference of a really tall Pilsner glass or the height?

Its always the circumferece. I like getting free beers that way.

[ken0042]

I'm not sure the fly actually stops. If it flies in head first, the head would change direction before the butt actually changes direction. (As is the old joke of "what's the last thing to go thru a fly's mind when he hits a windshield.) Buy the time the butt hits the windshield, the remains of the head are now going in another direction.

[Circa89]

Thanks Oilerfan,
I'll have to print that off. I don't think I'll be able to quote that verbatim

[Temporary_User]

Sounds like one of Zeno's paradox's regarding motion.

Originally Posted by Wikipedia

Finally, in the arrow paradox, we imagine an arrow in flight. At every moment in time, the arrow is located at a specific position. If the moment is just a single instant, then the arrow does not have time to move and is at rest at that instant. Now, in following instants, it then must also be at rest for the same reason. The arrow is always at rest and cannot move: motion is impossible.

[Circa89]

Believe it or not I tried to use that (zeno's paradox) to explain away the guts getting squished sideways arguement. I said that the guts go sideways but then realized that the guts have to stop at some point to move in the opposite direction.

Edit - Not sure if this is the same arguement regarding the arrow but it is argued that the arrow can never reach the target. If you take the half way point from the launch to the target you can continually divide in half. Forever!!!!!

[coolsurfer]

Using some simple physics:

The fly has the momentum m1v1.
The cars momentum is m2v2.
They them collide and thus m1v1+m2v2=(m1+m2)v0
If you used real numbers then since the two objects are travelling in
opposit directions you have to assign one a positive velocity and the other a negative velocity. For example the car goes from 60 km/h to 59.99999 km/h.
The fly on the otherhand goes from -10 km/h to 59.99999. Although the acceleration was extremely fast at one point the fly was stopped. The car
on the other hand only went from 60 to 59.99999. It was at some point going every velocity between those 2 values. The fly at some point was travelling at some point every velocity between -10 and 59.99999 at some point.
The key to this problem is that velocity is a vectored value. it's basically a
simple non-elastic momentum question. Also, i highly exagerated the velocities, but they wouldn't be hard to figure out if you used the mass and velocities of each object.

[SpitFire40]

I think coolsurfer need to change his name to nerdsurfer.. I kid

[icarus]

Originally Posted by Circa89

And if the fly did indeed stop (which it didn't) the car had to have stopped at the same point that the fly stopped.

I'm confused... why does this have to be true?

[Circa89]

Originally Posted by coolsurfer

Using some simple physics:
Although the acceleration was extremely fast at one point the fly was stopped.
.

Sorry, I'm an idiot. So if the fly was actually stopped the car never did because the difference in 60 versus 59.9999999999999 was a great enough difference to make up the time for the fly to stop and the car to slow just 1/1billionth of a second right?

[coolsurfer]

Originally Posted by Circa89

Sorry, I'm an idiot. So if the fly was actually stopped the car never did because the difference in 60 versus 59.9999999999999 was a great enough difference to make up the time for the fly to stop and the car to slow just 1/1billionth of a second right?

Well the fly went from -10 to 59.99999999. 0 Is between those two numbers.
So at some point is was stopped. The car went from 60 to 59.9999999. 0 is not between those, so it was never stopped. The key to the question is the vector properties of velocity. if they are travelling in opposite directions then one of them in travelling at a negative velocity.

[Circa89]

Thanks dude. I like that answer alot!

[BlackArcher101]

Here I thought this was all about the car stopping, not about whether or not the fly did.

I forgot my dynamics notes at work, so no acceleration calcs from me right now. However, keep in mind the above is a 2d solution, and the amount of time that the fly was at zero velocity (absolute) is so small that it's negligible. If the fly didn't impact perfectly perpendicular to the windshield surface (otherwise if the winshield isn't flat and the fly impacted at an angle), then the flies velocity never reaches zero as the velocity vector just changes during the impact and at least one doesn't equal zero. (Fly might be moving in Y direction but not in X, and a little in Z)

[Circa89]

Originally Posted by BlackArcher101

Here I thought this was all about the car stopping, not about whether or not the fly did.

My Bad, It is about the car/train and not the fly and the dilema posed is if the car continues forward at a constant rate how did the Fly "stop" if the fly was on the moving windshield when the Fly "stopped".

I forgot my dynamics notes at work, so no acceleration calcs from me right now. However, keep in mind the above is a 2d solution, and the amount of time that the fly was at zero velocity (absolute) is so small that it's negligible. If the fly didn't impact perfectly perpendicular to the windshield surface (otherwise if the winshield isn't flat and the fly impacted at an angle), then the flies velocity never reaches zero as the velocity vector just changes during the impact and at least one doesn't equal zero. (Fly might be moving in Y direction but not in X, and a little in Z)

This isn't a real situation its more of a possiblity. What if the fly went 100% headfirst into a train / car with a 100% flat window travelling exactly headfirst into the fly.

But the hockey game is now over and I'm going to bed. My brain is all sore from this question and the Keiths.