# Interesting Physics Questions

Discussion in 'Physics' started by wes, Sep 18, 2011.

1. ### wes Thread Starter Active Member

Aug 24, 2007
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So me and a friend were talking and a question came up, so here it is.

So what would happen if there was a object connected by a string to say a crane in a gravitational field. (happens everyday, lol)

-------------------------- <crane
-
- < string
-
----
- - <object
----

now the interesting part, if you were to cut the string then would the object fall instantly? or would it fall after the Information that the string was cut and no longer supporting it be what happens.

We are not sure, lol. It is in a gravitational field so there is always a force acting on it, so when the string is cut it should fall, but is it instantaneous.

I think it would take a certain amount of time before it fell because the information that there is no longer a supporting force can't travel faster then light. So if the string was 1 ft long, then if cut at the top, it would take 1 Nanosecond before it started to fall.

So any ideas on what actually happens, lol

2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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Are u related to Darwin ?

3. ### steveb Senior Member

Jul 3, 2008
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Yes, you are right. Otherwise, you would have developed a form of instantaneous communications called the "Wes String-Cut One-Bit Telegraph".

However, I must point out that if the object is Wile E. Coyote and the string is personally cut by the Road Runner, the propagation time is considerably longer than 1 nsec/ft, and typically takes about 2 seconds, irrespective of the length of rope used.

4. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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Huh!!!!

I must really be stupid.

5. ### steveb Senior Member

Jul 3, 2008
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Oh sorry! I should have pointed out that the crane needs to be made by ACME.

6. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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I know road runner and Acme.

What I don't get is what the heck are those doing here.

7. ### shortbus AAC Fanatic!

Sep 30, 2009
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It's an excerpt from steveb's new book "Physics for Dummies"

8. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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Physics + Coyote + Road Runner = Bonkers.

Better yet. " Physics for Morons."

9. ### Papabravo Expert

Feb 24, 2006
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1,790
There is no need for information to travel anywhere. All the action takes place at the point of the cut. The string is in tension across the plane of the cut producing a normal stress. As the cut begins the cross sectional area supporting the normal stress is reduced thus increasing the normal stress, At some point in time the cut is completed or what is more likely the normal stress exceeds the breaking strength of the string. In either case we are interested in knowing if the process is continuous and happens in finite time, or is discontinuous and happens in zero time?

I vote for a continuous process in finite time. In my experience, discontinuous jumps in physical processes happen only in Quantum Mechanics. In the macro world they may take place on exceedingly short time scales that cannot be adequately resolved but that does make them instantaneous.

10. ### steveb Senior Member

Jul 3, 2008
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It's just an attempt at humor, not an official book.

Of course, my first instructions in physics was from the road runner show at age 4. Some of this I needed to "un-learn" as I got older and learned the real laws of physics.

The physics answer is that nothing capable of communicating information can go faster than the speed of light. Since, cutting a wire is a form of 1-bit communications, the object can not begin to fall before a time of 1 nsec/foot has elapsed. The road runner may defy many laws of physics, but I never saw him violate this one.

11. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
Alright yea, that is pretty much what we eventually figured out. That it had to be a delay otherwise we would already have instant communications.

The crazy thing we thought of though was that after the string is cut, there will be a short time period where the object is actually floating in a gravitational field with no actual supporting force because the string has already been cut. So the only thing holding it up is a time delay (it still thinks it should stay there so it does, lol)

12. ### steveb Senior Member

Jul 3, 2008
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It's definitely one of those tricky questions. In reality, the string tension is still present on the object when the string is cut. The string has mass, and none of the pieces that make up the string near the object can begin to accelerate instantaneously after the cutting. The physics is dictated by Einstein's equations, not Newton's equations, and these are not easy to calculate, or even to visualize. But, the principle of non-instantaneous communications saves us here, and no calculations are needed to answer the question.

13. ### Wendy Moderator

Mar 24, 2008
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2,536
I don't remember the setups, but I remember you put a button or bead in the middle of a string. If you pulled on the string which side of the button it broke on was somewhat dependent whether the button was moving or not.

In other words, the mass and inertia of the button was enough to dictate where the stress went to on the string. It was a really basic physics and engineering demonstration.

14. ### steveb Senior Member

Jul 3, 2008
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In this case, the friction between the bead and the string is added to the tension on one side of the string, and subtracted from the tension on the other side of the string. In particular, the side of the string behind the bead, as it moves forward, will have more tension and will be the side that is much more likely to break. Or, at least that is one possible setup you could have.

15. ### davebee Well-Known Member

Oct 22, 2008
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The answer to this question is complicated, but in a well-known way; it is wave propogation down a string.

When the string is cut, the layer of string molecules just below the cut suddenly experiences unbalanced force, so it instantaneously begins to accelerate.

As the first layer of string molecules starts to accelerate downwards, it no longer applies an upward force to the layer beneath it, so the layer beneath it also begins to accelerate downwards.

Solving the differential equations of each mass and tension comes up with an equation describing a wave having velocity equal to the square root of the tension on the string divided by the linear density of the string. v = sqrt(T/u).

So the object starts to fall after the time it takes for the wave to propogate from the cut to the object, based on the density of the string and the tension on it.

16. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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Even though the string is cut or not, nothing can float at any given time in a gravitational field.
The gravitational force exerted on the object is equal to the force exerted on the string. The string being attached to the crane being part of the load bearing force exerts an opposite force on the string which prevents the object from falling. But is the is no gravitational force the object will have no force acting on it and thus there wouldn't be any need to cut the string.

Umm! What was question again?

17. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
lol I figured this would get alot of responses which is good.

My friend and I were just think though about what would happen if the object was no longer a string and instead was a solid object. So here is the set-up.

REMEMBER this is just for thought.

Say the object is about 1000 light years across and is solid (it can not bend, break, or stretch for the sake of argument). The end of the object is near a black-hole, but not past the event horizon. The crane (ultimate Crane), lol, at the other end is holding the object back from being sucked in to the black-hole. Now what happens if the crane let's go? Does it really take 1000 years before the object even starts to fall. It is crazy, I know but I believe yes even as crazy as it sounds, lol, besides the unbreakable 1000 light year object, lol.

I did think that maybe it would start to fall before the information of cutting got to the end but that's impossible too. The reason is simply because of light speed, (information speed limit ), lol.

And the thing about floating in a gravitational field, that was just an exaggeration (sorry), I know that is not really what is happening but it would defiantly look like that if you knew the exact time the string was supposed to be cut, so there would be short period where the supporting force had been cut and the wave propagation just hadn't got to you yet. So it's just wierd way to think about it, lol. Same thing with the solid object idea too.

18. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
and think about that, 1000 years, pretty much all of human civilization as far as technology we know of today rose in that amount of time, it boggles the mind, lol

19. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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Has it been a 1000 years?

20. ### MrChips Moderator

Oct 2, 2009
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What time did you say it was? Or I am too old to understand!
Where is loosewire when you need him?