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Author Topic: Do moving objects stop before reversing direction?  (Read 8267 times)

Offline Jackalope

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I was wondering if anyone knows....when a wheel, or anything moving in one direction, quickly moves in the opposite direction.....no matter how quickly, is there an instant where it actually stops ?


 

Offline weebrain

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Re: Do moving objects stop before reversing direction?
« Reply #1 on: 03/08/2005 05:17:25 »
There would be a point when the forces come into equilibrium, but it would be impossible to get the actual time of this event.


p.s. welcome Jackalope


sir loony
 

Offline Jackalope

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Re: Do moving objects stop before reversing direction?
« Reply #2 on: 03/08/2005 05:27:32 »
Actual time is no concern, I was just wondering if motion actually stops, or if objects can go one way to another without stopping. Hope this makes sense.
 

Offline chimera

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Re: Do moving objects stop before reversing direction?
« Reply #3 on: 03/08/2005 07:56:26 »
Cool question. Light bouncing off a mirror (doing essentially what's asked) does not stop and scratch its head to see whereto next, either, if you see my point.

Time is a great teacher, but unfortunately it kills all its pupils - Hector Louis Berlioz
 

Offline neilep

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Re: Do moving objects stop before reversing direction?
« Reply #4 on: 03/08/2005 13:00:31 »
...great question...I want to know too !...certainly got me thinking !!...i would say the wheel has to stop for a really small time else how could it start reversing ?

Men are the same as women.... just inside out !!
 

Offline Jackalope

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Re: Do moving objects stop before reversing direction?
« Reply #5 on: 03/08/2005 16:57:44 »
That's what I was thinking, if only for a fraction of a second. I've done experiments w/ my bike, and it doesn't seem like the wheel stops, but it must, right ? I've wondered about this for years.
 

Offline chris

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Re: Do moving objects stop before reversing direction?
« Reply #6 on: 03/08/2005 17:18:44 »
The wheel changes velocity because it accelerates in the opposite direction to that in which it was initially travelling.

This means that it will slow down, instantaneously have zero velocity, and then increase in velocity in the opposite direction.

The same can be said for a ball thrown into the air. As it ascends it is being 'acclerated' downwards again by gravity, so it slows down at the rate of 10 m/s/s. It will therefore reach a point where it stops moving for an instant, before falling back to earth. But again, this is only an instant in time. Imagine cutting a cake into smaller and smaller wedges - the cake wedges are the speed of the ball - this is what the ball is doing as it approaches its maximum height above the ground.

Chris

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Offline finchbeak

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Re: Do moving objects stop before reversing direction?
« Reply #7 on: 03/08/2005 17:58:54 »
quote:
Originally posted by chris

The wheel changes velocity because it accelerates in the opposite direction to that in which it was initially travelling.

This means that it will slow down, instantaneously have zero velocity, and then increase in velocity in the opposite direction.

The same can be said for a ball thrown into the air. As it ascends it is being 'acclerated' downwards again by gravity, so it slows down at the rate of 10 m/s/s. It will therefore reach a point where it stops moving for an instant, before falling back to earth. But again, this is only an instant in time. Imagine cutting a cake into smaller and smaller wedges - the cake wedges are the speed of the ball - this is what the ball is doing as it approaches its maximum height above the ground.

Chris

"I never forget a face, but in your case I'll make an exception"
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But then you would need to have negative cake when the ball begins moving the opposite direction...;)

It is an excellent question, one that is related to the ancient Zeno paradoxes.  Essentially, yes, there is a moment - but it's infinitessimally small.  In other words, it comes down to the question of what does it mean for something to be infinitessimally small.  Great recent book on the topic:
http://www.orionbooks.co.uk/MP-32470/Everything-and-More.htm [nofollow]
(Everything and More, by David Foster Wallace)
 

Offline gsmollin

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Re: Do moving objects stop before reversing direction?
« Reply #8 on: 03/08/2005 21:58:10 »
This question is applicable to any kind of motion that reverses direction. Just waving your hand around, a pendulum, a ball in the air, or a vibrating air column. There are many more examples, and the answer would always be, "Yes, it come to rest just as it reverses its direction of motion." Any particle with mass has to do this.

A light beam is an exception. The photons do not come to rest, then reverse direction as they are reflected from a mirror. They always travel at c. Photons are massless and are bosons, so they can reverse direction without stopping, and they can travel back through other photons without crashing into them.
 

Offline David Sparkman

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Re: Do moving objects stop before reversing direction?
« Reply #9 on: 04/08/2005 03:29:15 »
Ah well here is the answer for a wheel: No the wheel never actually comes to rest because you have choosed an object that is subject to torson (sp). I assume the force is being asserted though the axel (doesn't matter). The Axel will first undergo a hugh reversal force which could be applied though friction (assume some slippage) or though gears (assume some compression of the gear faces), then the axel will begin rotation in the opposite direction while the wheel is still rotating in the old direction! The spokes of the wheel flex, and then the outer wheel stops and reverses. Nothing is absolutly stiff, and if you reverse something with enough momemtum you may find that not everything reverses. In a race in the early 1900's a stanley steamer race driver reversed his car to avoid a wreck, and his car came apart: top kept going forward, and motor and carriage went backwards.

Now I would like to understand how the light reverses off of a mirror. An electron adsorbing and reemmitting a photon would be a posability, but the photon typically is broken into two photons: one having a quantum energy level charastic of the element and the other having the remainder. And the backscattering would be random, so forget that theory.

David
 

Offline gsmollin

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Re: Do moving objects stop before reversing direction?
« Reply #10 on: 04/08/2005 04:00:59 »
You are writing about an ensemble of objects. The original question was about "a" object. Now while we all understand that real objects are not rigid, and are also not at absolute zero, so in a sense they are never at rest, the question is more fundamental, at least IMHO. The poster wants to know if a massive object (read rigid body) can reverse direction of motion without stopping. The answer is no, it must first stop. If you complicate the motion of the object by adding deformation, then you must refocus your question to a part of the wheel, say a spot on the rim.

Of course, if you continue this process, you get to the fundamental particle level. Of couse, at the fundamental particle level we encounter HUP, so we cannot say the particle's position and velocity simultaneously to infinite precision, although we can always choose an error bound and show the particle slowing below the error bound at a given position.
 

Offline finchbeak

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Re: Do moving objects stop before reversing direction?
« Reply #11 on: 04/08/2005 13:51:12 »
HUP always wins.  Just like Entropy.
In fact, they're kind of the same thing, aren't they?
I'm ignorant of the technical connection between Heisenberg and Entropy... what is it?
 

Offline simeonie

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Re: Do moving objects stop before reversing direction?
« Reply #12 on: 04/08/2005 23:33:01 »
i think it is qutie clear that the wheel would have to stop. I think if the car or w/e it is has like a lot of power and can accelerate well then it would be for a ver short time but it has to stop b4 it can go in the oposite direction.

B4 i say what I am going to say... velocity is speed given in a given direction right? If not then excuse me coz I cant think of the word. But as I was saying......

Times the number by 100 to make it faster and it is mph. when it goes to negative then it is reversing

+5
+4
+3
+2
+1
0
-1
-2
-3
-4
-5

The decreasing positive numbers represent the de-acceleration. And the  decreasing  negative numbers represent speeding up reversing.

Do you get me? Coz I am not good at explaing things. If anyone does understand what I am saying then please reword it





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Time is like a knife..... slowly and relentlesly.... cuts the cored of life.

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« Last Edit: 04/08/2005 23:34:57 by simeonie »
 

Offline chimera

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Re: Do moving objects stop before reversing direction?
« Reply #13 on: 05/08/2005 11:45:56 »
Funny thing is, with massless things, like light, it does not seem to apply. Light never stops or even slows down (in a vacuum).
« Last Edit: 05/08/2005 13:03:48 by chimera »
 

Offline David Sparkman

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Re: Do moving objects stop before reversing direction?
« Reply #14 on: 07/08/2005 03:48:51 »
Simeonie, sure if we slice time smaller and smaler, we should be able to find a point in time where the velocity of a point is zero. But that is in theory, and rarely fits the practical world except in a large crude way.

As far as light bouncing back from a silvered mirror, some photons will completely penetrate the mirror, some will be adsorbed and heat the mirror, and most will be reflected back and show us how beautiful we are. The funny thing is that the photons may be reflected back from the second, third or even fourth layer of silver atoms, not just the first level. Infact they could be reflected from a lot deeper. My proffessor in college was doing some surface work on platitum down to about 4 atomic layers and could only find the element Carbon. The carbon, an impurity, had diffused out to the surface, and we couldn't visually see such a thin layer because the light was reacting with deeper levels and still bouncing back from the platium. (I was the lab tech).

Im still looking for a quantium physics level explaination of reflection of light.

David
 

Offline finchbeak

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Re: Do moving objects stop before reversing direction?
« Reply #15 on: 07/08/2005 15:16:35 »
quote:
Originally posted by David Sparkman



Im still looking for a quantium physics level explaination of reflection of light.

David



Feynman addresses this question in the classic "QED".
http://www.oberlin.edu/physics/dstyer/TeachQM/QED.html [nofollow]

I can't say I remember it (or even understand it) very well.  But it's a marvelous book nonetheless.
 

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Re: Do moving objects stop before reversing direction?
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