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# Fly Hits Car

08/31/2007 11:53 AM

This is an old one which came up in the pub again and I had difficulty in explaining it in an easy to understand way then the booze took over and I gave up. Simple explinations please.

A car is travelling along the road and a fly is flying at it in the opposite direction. When the fly hits the windscreen it sticks on and travels in the opposite direction it came from. If the fly changed direction it must have come to a complete stop at some point before moving in the other direction so does the car also come to a complete stop at that moment in time.

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#1

### Re: Fly hits car

08/31/2007 12:35 PM

No the fly comes to a complete stop relative to the car's windscreen which continues to move forward (unless its a huge fly!!) the energy used to reverse the fly's velocity causes a slight loss in forward momentum of the car, related to the speeds of the individual components of the car and fly and the mass of both...

John.

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#24

### Re: Fly hits car

09/02/2007 12:14 PM

Yes the fly comes to a stop relative to the car, as it is stationary on the windscreen. The fly will also be stationary relative to the road for a moment in time as it has to reversed direction of travel.

So the point is if the fly is stationary relative to the road and the car at that point in time - what is the car doing.

I think at that answer is that at the point of impact the fly and car are not connected (yet) so they do not have to be doing the same thing.

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#25

### Re: Fly hits car

09/02/2007 2:53 PM

agreed guest, maybe on the microscopic scale parts of the windscreen change velocity, molecules etc... but the fact that the fly's body is 'plastic' at that moment the fly's body will be doing all the velocity changing and not the windscreen?

John.

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#27

### Re: Fly hits car

09/03/2007 3:18 AM

You're right in what you say. The whole point is that the fly must be stationary relative to the road momentarily, and if it is in contact with the car at that moment, it suggests the car is stationary too. My view is that a few atoms of the car windscreen at the point of impact will indeed be brought to a halt, but the effect is miniscule. Maybe it's easier to visualise what's happening if we consider the car windscreen to be made of plasticine and the fly to be, say, a small ball bearing.

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#29

### Re: Fly hits car

09/03/2007 11:25 AM

"if we consider the car windscreen to be made of plasticine and the fly to be, say, a small ball bearing."

Okay, lets look at this a little deeper. When your ball bearing (fly) contacts the plasticine windscreen the molecules in the windscreen, at the point of impact, are pushed backward to the limit of the materials elasticity. Then they spring forward to their original position in the windscreen. During their forward travel, this portion of the windscreen must exceed the speed of the car!

Speed of windscreen (at point of impact) = (speed of car + speed of molecules returning to original position). Wahlah!

It is this additional velocity that causes the fly's demise.

-John

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#33

### Re: Fly hits car

09/04/2007 3:04 PM

The fly changes direction, thus, it must have zero velocity for a very brief moment relative to the road.

The car never changes direction, thus, it never has zero velocity relative to anything, except the fly after impact

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#36

### Re: Fly hits car

09/06/2007 3:46 AM

Yes, the car as a whole doesn't stop, but a few molecules of windscreen glass must match the velocity of the fly at some instant i.e. be stationary. I think Johnjohn's seen the point of my BB/plasticine analogy.

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#38

### Re: Fly hits car

09/06/2007 9:09 AM

but a few molecules of windscreen glass must match the velocity of the fly at some instant i.e. be stationary

I disagree with that assertion, though it may be true. There is really no objective evidence from which you can make that statement. What you say is pure speculation and educated guess work. I do agree that the glass must match the velocity of the fly, but I say that that instant is after the fly comes to rest on the windshield, and that velocity is not zero, it is whatever speed the vehicle is moving

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#39

### Re: Fly hits car

09/06/2007 11:46 AM

Agreed, a lot of this is speculation. I think the arguable point is whether the fly, or the windscreen, actually "stops".

If the car is going, say 30 mph, and lets call the direction positive. Now, the poor old fly is traveling along at, say, 5 mph in a negative direction (relative to the car). In order for the fly to wind up going 30 mph positive, from 5 mph negative, it must have passed through zero. So, the question is, can an object change velocity 180Âº without passing through zero?

I'm sure the old fly didn't ponder it though... (maybe there's "hocus-pocus" going on when the fly molecules "marry" the windscreen molecules).

-John

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#2

### Re: Fly hits car

08/31/2007 1:01 PM

John is exactly right. Works the same with meteors and the earth as well. Energy is exchanged, and the car is imperceptably slowed, but the fly gives his all.

The key is relative velocity, if the fly was hovering, and the car hit him, the result is essentially the same, also if the fly is moseying along a 5 mph in the same direction as the car and the car overtakes at 70 mph, the effect is still the same, the fly goes splat. In each case the car is imperceptably slowed, until the engine brings it back to speed.

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#14

### Re: Fly hits car

09/01/2007 11:40 AM

What about the situation where the fly is approaching the car at a speed such that his momentum is equal to that of the car? Obviously the windshield would smash, but beyond that?

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#3

### Re: Fly hits car

08/31/2007 3:40 PM

OK so what is the last thing to go throught the fly's mind

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#4

### Re: Fly hits car

08/31/2007 4:18 PM

Why the windshield of course!

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#5

### Re: Fly hits car

08/31/2007 4:56 PM

Nope his butt!!

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#6

### Re: Fly hits car

08/31/2007 7:36 PM

Assuming that he is flying in the same direction as the car is driving. If he is flying toward the car, the the last thing to pass through his butt is his mind...

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#37

### Re: Fly hits car

09/06/2007 5:19 AM

Well, that's relativity for you.

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#16

### Re: Fly hits car

09/01/2007 1:35 PM

No no, just before that

"Hope I've guts enough to stick"..........but not enough nto do it again!

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#8

### Re: Fly hits car

09/01/2007 9:35 AM

Depends on the fly's direction of velocity relative to the car. In any case the fly's last thought might be "Beats the ---- out of me".

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#7

### Re: Fly Hits Car

09/01/2007 8:10 AM

no ,car momentum will be disturbed , will depend on difference in masses , if both masses are equal than there will be impact to both on moment of time

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#9

### Re: Fly Hits Car

09/01/2007 10:26 AM

Del, lunch is served.

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#10

### Re: Fly Hits Car

09/01/2007 10:33 AM

I've often been told that motorcycle drivers would carry pennies to toss up in the air and onto windsheilds of obnoxious drivers.

Assuming both vehicles are traveling at about 55 MPH, what would the impact be?

Remember the basis is that the coins were thrown up into the air and decended back onto the car...

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#11

### Re: Fly Hits Car

09/01/2007 10:47 AM

Great idea, poke the bear.

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#12

### Re: Fly Hits Car

09/01/2007 11:14 AM

Chewing tobacco produces a nice car slowing effect when riding. Takes a little practice.

Oddly it seems to have a secondary effect of making the middle finger rise as well.

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#31

### Re: Fly Hits Car

09/04/2007 8:13 AM

It's difficult to calculate the specific velocity of a penny at the time it smacks the windshield. I'd imagine resistance would slow it down quite a bit before the trailing car hits it.

Why waste good currency? Keep a pocket full of bearings!

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#13

### Re: Fly Hits Car

09/01/2007 11:35 AM

If the fly has the same mass and weight as your car, both objects would essentially come to a halt at the same instant and the sums of the energy released would cause both objects to be substantially destroyed. That is what happens in a commonplace head-on auto accident. Your example, in reverse, would be as though your small car were hit by an 18-wheeler truck. The truck would keep going and your car would momentarily stop and reverse direction and the result would be the energy would cause your car to go "splat".

Happy Laborless Day Weekend

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#15

### Re: Fly Hits Car

09/01/2007 1:15 PM

At that exact point in time, nothing is moving at all! It takes time for something to move. Which only goes to prove that time does not exist! There will come a point in time when time no longer is needed and thus will no longer be. If time doesn't exist then, it can't exist now.

Sorry, that's all the time I have right now.

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#18

### Re: Fly Hits Car

09/01/2007 2:55 PM

Explain that to a tennis ball .....after aracket encounter.....................

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#17

### Re: Fly Hits Car

09/01/2007 2:17 PM

My father was driving down the road and passed a school bus going in the opposite direction. Him 60mph .. them probably 50. Some kid thought it would be funny to throw his nearly empty 22 ounce or maybe half liter of soda out the window at his truck (with the cap on). The result was a plastic soda bottle projecting thru the windshield. About half on either side and him covered in soda and glass.

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#28

### Re: Fly Hits Car

09/03/2007 7:40 AM

I hope the can got fingerprinted and the kid got his butt kicked in, that could have killed or maimed someone!!

Laminated screens are better in such areas in my experience.....

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#19

### Re: Fly Hits Car

09/01/2007 3:45 PM

The fly never stops moving, hence the splat. The car produces a shock wave in the fly greater than the strength of the fly's exoskeleton.

It is doubtful if you could even measure the slowing of the car because the friction of resistance to rolling is proportionally much greater than the force imparted by the fly's impact. You may be able to extrapolate the force by the vibration imparted to the windshield though.

Easier just to weigh the fly and the car, then do the math.

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#20

### Re: Fly Hits Car - Car Accelerates momentarily

09/01/2007 8:30 PM

Imagine an ultra high speed motion picture (1 M frames per second) of the impact, but now in slow motion with extreme instruments to measure the car's momentum, speed, acceleration, vibration, and whatever else Steve S. wants to measure . . . uhemm . . .

Now then: Let's look at instruments and the video frame by frame.

1.0 As the fly approaches, at the last 2.4 CM before impact, all of his/her compound eyes cross.

2.0 At 3 mm, the 'z' of "Jeeeeeezzzzz" can be seen by a FBI fly lip reader.

3.0 At 1.2 mm a tiny amount of fly poop can be seen coming out of the back side of the fly

4.0 Just 2 frames after impact, the compound eyes go blew. One blew left . . . the other right.

4.1 Note: The fly is now classified as a 'ride'

5.0 Following is the progression of the next 20 frames on our Shimadzu camera (consequently also the technical name the gunky stuff around the top of the ketchup bottle): We notice the car slows down as we see the body of the fly begin to compress, thus causing the almost non-compressible F-Squared (F^2 - fly fluids) to become excited, and molecular impacts increase on the interior of the elastic exoskeleton, delta T increases until the internal pressure exceeds the yield of the exoskeleton. Leakage of F^2 is observed.

6.0 As the F^2 deposits on the windscreen, and the exoskeleton become an inso-exo-andalloversoskelton the car leaps forward in the next 3 frames as the surface roughness of the windscreen is increased causing a boundary layer of air to build up over the surface of the F^2 reducing the drag of the car.

7.0 Unknown, but assumed, the rolling ball mechanism inside the air bag deployment device in the car begins first to cause the ball to roll towards the detonator upon METRYOFE (Molecular Excitation Temperature Rise Yield Of Fly Exoskeleton), however violently reversing back to its resting place caused by the acceleration indicated in 6.0. A Hans device is indicated for HSE compliance purposes.

So . . . I recommend a pint to be consumed before reading each point above (pint per point) in the pub and then have a discussion whilst explaining that the car actually leaps forward just after impact. 'Leaps' is critical in the explanation.

"Theeenk quue Dr. PetroPower"

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#22

### Re: Fly Hits Car - Car Accelerates momentarily

09/01/2007 9:09 PM

Great one Petro. Hahahahahahahahaha LMAO!

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#23

### Re: Fly Hits Car - Car Accelerates momentarily

09/01/2007 9:26 PM

Bravo! LMAO ditto

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#26

### Re: Fly Hits Car - Car Accelerates momentarily

09/02/2007 5:39 PM

Nice to find you are not at a loss for words..........

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#30

### Re: Fly Hits Car - Car Accelerates momentarily

09/03/2007 1:18 PM

Bored at night . . . . .

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#32

### Re: Fly Hits Car - Car Accelerates momentarily

09/04/2007 2:09 PM

That was the absolute most brilliant description I have ever read to any post in here.

I have one question tho.

Is the FBI sure the fly was saying "Jeeeeeezzzzz" or possibly some other word that has the same ounding final syllable.

HMMMMMM. People all over the shop are wondering why I am holding my side and tears are rolling down my cheeks. Better not let them on that I am actually having a laugh.

Great stuff Petro absolutly GREAT stuff!

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#34

### Re: Fly Hits Car - Car Accelerates momentarily

09/04/2007 6:44 PM

Compressor shop in Broken Arrow?

Not sure about the Z sound. And I won't question the FBI again because when the agent contacted me and said "I'm special agent Eugene Magillicutty with the FBI . . . " I stopped him and said "OK . . how do you spell that?" . . and he became frustrated (I guess everyone asks him) so he slooooowly said "M A G I ....."

I stopped him again and said "No no . . . not that . . FBI ?"

I'll be out next week for good behaviour!

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#35

### Re: Fly Hits Car - Car Accelerates momentarily

09/05/2007 2:13 PM

No print shop in Broken arrow.

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#21

### Re: Fly Hits Car

09/01/2007 9:03 PM

Hi MACA,

This one had been hashed and rehashed here.

-John

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