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Speeds (distance /time) Not What You Thought...

09/27/2006 3:39 PM

We are now talking physics, not medicine! and here we go:

A car drives at a constant speed of 100Km/hr, on a strait line, due north.

Opposite (180 Deg.) there is a fly, flying at 15Km/hr exactly south.

And than, uuuuch, they collide. You probably know what happened but the question is this:

The remaining of the fly, now spread on the car's windshield, are now heading north, at the car's constant speed of 100Km/hr.

The car NEVER stopped, changed speeds or direction, NOR did the fly.

Conclusion: the fly HAD to pass through 0 (zero) speed before (or during) the change of direction,going from 15Km/hr, to 0 Km/hr and than to 100Km/h, Isn't it?

So what's going on? How can that be?

Wangito.

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#1

Re: Speeds (distance /time) Not what you thought...

09/27/2006 3:53 PM

Yes, but so does a basketball when it bounces back the same vector that it came from.

Actually, the conservation of energy still applies. Some of the KE of the fly slows down the car, but not enough to be noticeable.

So, what is the question you really want to ask? Can you be more specific?

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#2
In reply to #1

Re: Speeds (distance /time) Not what you thought...

09/27/2006 4:13 PM

No, this is a different case

The ball WENT through 0 speed on the bounce, as the ring is stationary. (that is if it really hit the ring,) and if not, than gravity took control. and again it went through 0 on it's peak. In our case gravity is playing no role.

But the car is NOT stationary in any given moment, and the bouncing is against a moving object.

Wangito

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#3

Re: Speeds (distance /time) Not What You Thought...

09/27/2006 4:36 PM

Just consider the following vector analysis of the problem, whereas the fly is going from a magnitude of (+)15km/hr southerly to a magnitude of (-)100km/hr southerly; or from (-)15km/hr northerly to (+)100 km/hr northerly. In either case, graphically speaking, the fly goes through (although instantaneously) a 0 km/hr point in the transition from positive to negative magnitude.

Does that make sense?

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#4
In reply to #3

Re: Speeds (distance /time) Not What You Thought...

09/27/2006 6:34 PM

Good vector analysis, but,

Whatever the time laps is , and as short as it may be, zero speed is not possible, because the bouncing is done at a speed greater than zero.

Wangito.

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#5
In reply to #4

Re: Speeds (distance /time) Not What You Thought...

09/27/2006 11:29 PM

Dear Wangito,

Your FLY never vent through O (zero) Km/h speed because actually his speed was raised from 15Km/h to 100Km/h just simply changing its direction even you admit or not.

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#6
In reply to #4

Re: Speeds (distance /time) Not What You Thought...

09/27/2006 11:39 PM

I think if the fly were perfectly elastic, like a steel BB, then it would deform as it hit the windshield, then recover its shape as it changes direction and bounces off in the direction of the car's movement. I think its during that initial deformation when its velocity goes through zero as it rapidly decelerates and is momentarily motionless as the windshield begins to move through its position. Of course, the bug isn't so elastic, so its deceleration is more of a "squish" !

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#7
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Re: Speeds (distance /time) Not What You Thought...

09/27/2006 11:56 PM

And also as I said, if the FLY never went through O (zero) Km/h because actually his speed was raised from 15Km/h to 100Km/h just simply changing its direction, otherwise we have to speak about a misterious back in time traveler of -100Km/h.

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#8
In reply to #7

Re: Speeds (distance /time) Not What You Thought...

09/28/2006 12:08 AM

The point is here that speed has a relative meaning. 15 km/h and 100 km/h are both in relation to an observer which is on the ground and in relation to the observer at a moment the fly reaches to 0 km/h and it is the moment of collision. bye

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#9
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Re: Speeds (distance /time) Not What You Thought...

09/28/2006 12:14 AM

I think we know that the fly will no longer be a fly - its molecules and atoms are spread over the windscreen. So each fly atom had to come to a stop (yes) relative to the ground and then change direction.

Normally the question is: since the car's and the fly's atoms are in contact with each other, did it mean that the car also momentarily stopped? If I remember my Applied Math prof's words correctly, yes, some atoms on the windshield also stopped momentarily due to microscopic elastic deformations and then bounced back again!

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#10

Re: Speeds (distance /time) Not What You Thought...

09/28/2006 4:11 AM

is this chap a troll?

I think everybody knows what happens here, don't they?

buzz buzz zoom squish. end of chat.

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#11
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Re: Speeds (distance /time) Not What You Thought...

09/28/2006 8:45 AM

I think Jorrie will actually agree with me here, but I think the speed of the fly relative to the ground is irrelevant. You could just as well say that the car is travelling at 115 and the fly is stationary, the effect is the same. The change in relative velocity between the two masses means that the fly's vector changes in a manner that catastrophically alters the state of the fly, and must convert some energy into heat. If you like, the vector changes to all intents and purposes instantaneously, but not an exact reversal as some particles splash outward in a radial pattern from the centre of impact. The rate of acceleration is the same relative to the car whether the fly is moving relative to the ground or not.

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#12
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Re: Speeds (distance /time) Not What You Thought...

09/28/2006 9:08 AM

So far, no good guys,

To simply say that it didn't go through 0, doesn't stand the vector analysis.

In our case there is a change in magnitudes of 180 degrees, (-15 to + 100) therefore must pass a certain "zero" meaning the time when the vector is missing one of it's defining components , the stroke, becoming a scalar. (as one said here, "for a brief moment."). But brief moment is NOT a mathematical definition. (is it a nanosec or an hour?) so we will have to ignore it for now.

The fly is (still) in it's molecular state, and the reason I didn't choose a solid object instead is because my car's insurance doesn't cover glass.

But one of you guys came quite close, but not close enough...

Wangito.

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#13
In reply to #12

Re: Speeds (distance /time) Not What You Thought...

09/28/2006 11:28 AM

I would think that some of the "molecular pieces" of the fly have a probablility of having a "zero" velocity for a brief moment. However, the majority of the fly would not, as many of its molecules would have an outward motion applied as the spread across the windshield (messy). Those molecules would then display a parobolic type motion if viewd from an independant position. I do not think that the vector analysis can hold as the fly is definately an elastic concotion of carbon based material. Which reminds me of the proverbial saying "It is the stopping that hurts"

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#14

Re: Speeds (distance /time) Not What You Thought...

09/28/2006 11:30 AM

I think it's time everybody goes back to work...

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#15

Re: Speeds (distance /time) Not What You Thought...

09/28/2006 2:44 PM

Well I'm not a real physicist, but I'll take a shot at this since it doesn't seem to be a real physics problem either…

I think a part of the question is, if you'll pardon the pun, being lost in translation.

You state that the fly isn't like a ball bouncing off the ground because the ground isn't in motion, like the car is, so it doesn't have the speed -> 0 -> -speed relationship. I believe the question that you are trying to ask is this…

The fly is traveling at 15k/hr, the car is traveling towards the fly at 100 k/hr. When they hit, the fly experiences negative acceleration of 100 k/hr, and you can calculate how long it takes to reach 0 k/hr if you're so inclined, but the interesting part is this, after the fly reaches 0 k/hr from the deceleration, it is now traveling at 100 k/hr without ever accelerating again. I think this is what is being asked about how can that be? How can it be traveling at 15 k/hr, decelerate to 0, but then be traveling at 100, without accelerating again?

Like most riddles, it has less to do with the physics and more to do with the misdirection of your focus. To revisit the original example about the ball bouncing off the earth, there is a flaw there as well. The argument was the car and earth are different because the earth isn't in motion and the car is. Really? The earth is rotating at 900 mph. It's orbiting the sun at 90 mi/sec. Sounds like it's moving to me.

It all comes down to your inertial frame of reference, and I think that's the trick here as well. Think of it this way… from the inertial frame of the fly, it's falling towards the car. When it reaches the car, the fly experiences deceleration trauma and comes to rest (in pieces) on the windshield at 0 k/hr. That's it, the fly never changes speed after it hits the car. Its speed is always 0 k/hr after impact. If you shift your inertial frame of reference, then the fly is traveling 100 k/hr on the car. Or 90 miles a second on the earth. Or 1 million miles a day in our galaxy, and so on…

So if the trick to the riddle is how does the fly go from 15 to 0 to 100 without accelerating again? It doesn't. The fly just goes from 15 to 0 and if you the fly and the car are all there together looking at the situation, nothing is moving any more. Jump off the car and the fly and car are traveling 100 k/hr. Jump off the planet and …

QED ;-)

But then again, what do I know? I'm not a real physics guy.

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#16
In reply to #15

Re: Speeds (distance /time) Not What You Thought...

09/28/2006 3:42 PM

If you reffer to the fly's ID, you could be right, otherwise its atomic structures described by you "in peces" still are travelling as well as the place where he rests (on the unwashed windshield, car, earth, etc.) at accordingly modified speed of the car.

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#17

Re: Speeds (distance /time) Not What You Thought...

09/28/2006 7:19 PM

We know that the car did not stop,and the fly did reverse direction.So how did it happen without the fly crossing thru zero?The fly struck the winshield one molecule at a time, and one molecule at a time, the polymers in the winshiled stretched a microscopic amount, as the car contined on.This happens for every molecule of the fly.When the molecules of the windshiled quit stretching, the fly has stopped, relative to the windshield, but not relative to the car.One by one, the fly molecules bounce back, and for a brief instant, they are traveling a little faster than the car, (relative to the car) and due to the sticky nature of the fly's molecular structure, and the prevailing wind pressure the fly does not leave the windshiled, until the driver hits his cleaner fluid and wipers.The only part of the car that stopped was the microscopic part of the windshield that contacted the fly.Oh yes, the point of contact on the windshield became a little hotter because of the energy produced by the collision.This may even contribute a little to global warming.Of course, if we factor in the heat produced by the fly flapping it's wings during a normal lifetime(without hitting a windshiled) and the carbon dioxide and methane produced by it's bodily functions (Do flies flatulate?) it is probably a wash, unless you really want to split hairs and calculate the effect on gas milage due to the extra weight and wind resistance of the fly.(Don't forget to subtract for the loss of bodily fluid as the fly dehydrates and becomes lighter.There are many more variables here, too many to mention, including the butterfly effect on the weather in China, which will ultimately affect the price of eggs there, but that's another problem altogether.

Does this answer your question?

Obenay N Pebvk

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#18
In reply to #17

Re: Speeds (distance /time) Not What You Thought...

09/28/2006 11:42 PM

In effect, this is what I also said in post #9, but it seems like Wangito isn't buying it!

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#19
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Re: Speeds (distance /time) Not What You Thought...

09/29/2006 9:02 AM

OK Guys.

Assuming that the answers given by Jorrie, QED, Pebvk, et al, are correct, (and I didn't say they are not,) let's put them to the test.

Let's change 2 of the main parameters, and replace the car and the fly for two equal materials, having zero elasticity. All other variables remain the same. If we now have an answer different, from the proposed ones, that means that we are going to have a different answer for each degree of elasticity, meaning a question with infinitive number of answers. That is literature, not physics...

What do you think?

Wangito

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"Two things are infinitesimals, Space and the stupidity of mankind. But I am not so sure about space."

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#20
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Re: Speeds (distance /time) Not What You Thought...

09/29/2006 9:23 AM

Quoting Wangito: " two equal materials, having zero elasticity."

This very condition of yours makes it sci-fi and not physics! Let us stay more or less with physics or engineering.

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#22
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Re: Speeds (distance /time) Not What You Thought...

10/02/2006 10:59 AM

Here we go again.

The problem with this kind of questions is that you take it for granted that the question has only relevant correct and directive parameters.

well :

Car speed is irrelevant, but only for the first part of the question, as it has already been pointed out. as is the velocity of our planet in space, Our point of reference for all of the sppeds given, is the ground speed. In other words, the distance in which the vehicle (or the fly) is advancing over the ground, per time units. that is also true for the fly, or any other material. (not sci-fi) for that matter.Vector analysis will show that one of the arms of this movement vector, at the moment of impact, will decrease in value until reaching zero, will change direction (180deg.) and is now increasing its magnitude (accelerating,) until reaching a constant velocity of 100Km/h.

Yes Sir,The fly, (or whatever remained of it,) will reach ZERO speed.

"Don't confuse me with the facts" (R. Reagan)

So next time don't let me do this. and there is one more small point.

A good friend of mine a doctor once told me, "I shouldn't have learnt medicine. Every small headache is a brain tumor, and every time I forget where I put my car's keys, it is the begining of Alzheimer I guess I know too much..."

You understand what I mean.... the simplest solution is always the correct one...

Yours truly,

"wangito"

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#21

Re: Speeds (distance /time) Not What You Thought...

10/02/2006 9:57 AM

The reference frame is stated, true? For both objects, you've given speeds in reference to the ground. So then, this is a simple collision like any other. The physics are just like the collision between two cars of different mass. The lighter one decelerates to zero ground speed, then accelerates to the new velocity of the larger vehicle. (The large car is the bat. The small car is the ball.)

The premise that the car never changed speed is simply untrue: of course it did. It has to have slowed, just as it would in a collision with an equal mass car (to zero) a smaller car (a lot), a person (noticeably), a watermellon (by .03 Km/Hr) or a dragon fly (barely detectable). In none of these cases is the velocity change instantaneous.

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