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An object at rest "tends" to stay at rest?


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#1 Baby Astronaut

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Posted 9 January 2009 - 09:49 PM

Isn't that a bit oddly written? Seems like a popularization.

Newton's First Law (also known as the Law of Inertia) states that an object at rest tends to stay at rest and that an object in uniform motion tends to stay in uniform motion unless acted upon by a net external force.


If it is just a popularization, I'm unsure why it had to say "tend" when it's more accurate to say "object at rest stays at rest and that an object in uniform motion stays in uniform motion unless acted upon by..."

Otherwise, it implies that an object doesn't always stay at rest or in motion even if it weren't acted upon by an external force.

Plus it says nothing about an internal force. I can imagine a ball with a propulsion device inside firing a small object at its inside, causing it to begin rolling.

Just something I wondered about.
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#2 Mokele

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Posted 9 January 2009 - 10:16 PM

Could it just be translation, since the original was written in Latin?
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#3 insane_alien

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Posted 9 January 2009 - 10:21 PM

Lex I: Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.

Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.


what newton actually wrote and its translation.

the tends slipped in there when someone tried to put it into modern english and failed.

the one i always seen was 'an object remains at rest or in uniform motion unless acted upon by an unbalanced external force'
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#4 swansont

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Posted 10 January 2009 - 12:38 AM

I can imagine a ball with a propulsion device inside firing a small object at its inside, causing it to begin rolling.



How would it do so?
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#5 Baby Astronaut

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Posted 10 January 2009 - 03:14 AM

How would it do so?

Like so.

Posted Image
I can't draw :D

However, this next image perfectly illustrates what I mean by an object being acted upon by an internal force.

Posted Image

Say hello to the cute hamster, the newest physics volunteer experimenting for science.
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#6 swansont

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Posted 10 January 2009 - 10:46 AM

You're looking at rotation and composite systems. Newton's laws are meant to be applied to linear motion of rigid bodies, though there is an analogous set of laws for rotation. Even so, if the body is the shell of the ball, the cannon/cannonball or mouse are external forces.

The danger in looking at composite systems and is that you may leave a component out, like the earth, which is also exerting a force on the system. If you look at that whole system, the motion is unchanged.
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#7 Mokele

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Posted 10 January 2009 - 01:53 PM

Also, technically a hamster ball only works with an external force: friction. If there's no friction to prevent the ball slipping against the substrate, the hamster will only succeed in spinning the ball around, generating no forward movement.

As for the gun, remember that guns have recoil. Equal and opposite.
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#8 swansont

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Posted 10 January 2009 - 02:07 PM

Also, technically a hamster ball only works with an external force: friction. If there's no friction to prevent the ball slipping against the substrate, the hamster will only succeed in spinning the ball around, generating no forward movement.

As for the gun, remember that guns have recoil. Equal and opposite.


And the friction is supplied by the earth (or whatever surface is attached to it), so what Mokele has said is another (more specific) view of what I did.
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#9 Baby Astronaut

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Posted 10 January 2009 - 04:15 PM

Also, technically a hamster ball only works with an external force: friction. If there's no friction to prevent the ball slipping against the substrate, the hamster will only succeed in spinning the ball around, generating no forward movement.

Good point, but now it attracts a new observation.

swansont mentioned how an analogous set of laws exists for rotation. Is it possible these laws are further collapsible into one with Newton's laws of motion?

I mean, if a spinning ball on a frictionless surface would continue its state forever, then I'd imagine it's in a kind of static motion, but motion nonetheless. Thus all you'd have to do is remove the "straight forward" criteria from insane_alien's quoted translation of Newton's First Law, perhaps, to merge them into one law. I could be wrong since I'm less familiar with the rotation laws.

As for the gun, remember that guns have recoil. Equal and opposite.

Well, the gun isn't attached to the base. It would fall soon as the ball rolled forward enough. However, it would be enough to get the ball rolling. And if it were next to top edge of a steep hill...
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#10 Sisyphus

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Posted 10 January 2009 - 05:07 PM

swansont mentioned how an analogous set of laws exists for rotation. Is it possible these laws are further collapsible into one with Newton's laws of motion?


Yes. It involves calculus. You treat objects as collections of infinitesimal connected point-objects exerting forces on one another, each point is in continually accelerated by the others, bla bla bla. It's far easier to use rotational equivalents.

Well, the gun isn't attached to the base. It would fall soon as the ball rolled forward enough. However, it would be enough to get the ball rolling. And if it were next to top edge of a steep hill...


Whether or not the gun is attached, it would be impossible to change the ball-gun system's center of mass by any internal action. If it's not attached, then the gun flies backwards with as much force as the bullet flies forwards.
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I think you'll find it's a bit more complicated than that.

#11 PlayStationX

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Posted 13 January 2009 - 06:22 AM

"object at rest stays at rest and that an object in uniform motion stays in uniform motion unless acted upon by..."


you are right. its badly worded, too many words.

it should go like this:
-an object in uniform motion stays in uniform motion unless acted upon by force


1.) not "external force", any force

2.) mentioning object at rest is not important, it is still "uniform motion", only with zero velocity



maybe better yet, it should go like this:
- only force can accelerate objects


...may the force accelerate you!

Edited by PlayStationX, 13 January 2009 - 06:27 AM.


#12 Baby Astronaut

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Posted 13 January 2009 - 07:37 AM

you are right. its badly worded, too many words.

it should go like this:
-an object in uniform motion stays in uniform motion unless acted upon by force

I like where you're heading. :-) Maybe it can be broken down even further.

"Uniform motion is perpetual unless acted upon by a force"
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#13 PlayStationX

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Posted 13 January 2009 - 08:08 AM

"Uniform motion is perpetual unless acted upon by a force"


i agree with that.


but lets analyze it,
what is that sentence about, some motion and force?


well, i believe it is about 'CHANGE in MOTION' aka ACCELERATION.



so what can you possibly say about acceleration and force?

1.) force can cause acceleration (force accelerates)
2.) force always causes acceleration (force always accelerates)
3.) only force can cause acceleration (only force accelerates)

i think its #3, thought it sounds more trivial and even questionable

Edited by PlayStationX, 13 January 2009 - 08:46 AM.


#14 swansont

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Posted 13 January 2009 - 11:16 AM

you are right. its badly worded, too many words.

it should go like this:
-an object in uniform motion stays in uniform motion unless acted upon by force


Isn't circular motion at constant speed "uniform" ?
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#15 PlayStationX

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Posted 13 January 2009 - 11:42 AM

Isn't circular motion at constant speed "uniform" ?


i agree that question is valid. it goes on to show how original formulation is vague.


this is the essence of it cut with Occam's razor:
- only force accelerates


still, circular motion with constant velocity leaves a question - is there really some acceleration in there, or is it just a curvature in space-time?

#16 npts2020

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Posted 13 January 2009 - 01:30 PM

I think what swansont is trying to point out, is that direction is uniform as well as speed and you have to account for it. e.g. planets do not follow the curvature of space but have uniform (relatively) motion.
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#17 swansont

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Posted 13 January 2009 - 05:51 PM

i agree that question is valid. it goes on to show how original formulation is vague.


this is the essence of it cut with Occam's razor:
- only force accelerates


still, circular motion with constant velocity leaves a question - is there really some acceleration in there, or is it just a curvature in space-time?


Actually my objection is to your shortening of the statement. And there is no circular motion at constant velocity, by definition — velocity is a vector.

——

From wikipedia, (and quoted by insane_alien above) the first law is:

Lex I: Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.

Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.


i.e. "uniform motion" is defined, and not ambiguous

http://en.wikipedia...._law_of_inertia

———


In regard to the OP, I suspect the word "tend" may have some ties to the notion that Newton displaced, which was that an object's natural tendency was to come to rest, i.e. that was a dynamic thing. Moving objects tend to come to rest. The translation of Newton is saying that no, it's only objects already at rest that tend to remain that way, and they do not start to move on their own, i.e. there is no tendency for spontaneous motion: motion from a state of rest requires a force.
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#18 PlayStationX

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Posted 13 January 2009 - 06:51 PM

Actually my objection is to your shortening of the statement.


ok, i want to claim that "only force accelerates", contains all the information of the original statement, plus some more information, which makes it better definition. i want to claim that this short statement can substitute the original statement, completely and in every case.



And there is no circular motion at constant velocity, by definition — velocity is a vector.


i agree definition is such, i was referring that to lead these two statements in contradiction:

1.) when light bends due to gravity, it does not accelerate - it follows curvature of space-time

2.) magnetic field can not accelerate charged particles, but still it can change their direction



"uniform motion" is defined, and not ambiguous


i agree. it is explained, that if you read original text you may notice that he means constant velocity as well as constant acceleration. but when making new, modern formulation why not make it crystal clear. why not call "change in motion" - acceleration, we have a name for it, why not use it? why make it confusing? whats the point of words "tend", "external", "unbalanced"?

the sentence does not have "focus", its hard to see what is it about and why is it important. it kind of tries to be about "frames of reference" while it can not be about anything else but the force, since the "force" is a 'primary' word needing description.


In regard to the OP, I suspect the word "tend" may have some ties to the notion that Newton displaced, which was that an object's natural tendency was to come to rest, i.e. that was a dynamic thing. Moving objects tend to come to rest. The translation of Newton is saying that no, it's only objects already at rest that tend to remain that way, and they do not start to move on their own, i.e. there is no tendency for spontaneous motion: motion from a state of rest requires a force.


word "tend" has no place in there, it gives no explanation for anything , it only makes statement more undefined. it makes you feel as if there is some threshold to which object resist to change motion.

it is as if some more inert objects will not react at all, it makes you think that some minimal force will produce no effect what so ever, or that is a matter of chance.

definition should use words like: all, none, never, always, only...


...Galileo realized that force acting on a body determines acceleration, not velocity. This insight leads to Newton's First Law—no force means no acceleration, and hence the body will maintain its velocity.
http://en.wikipedia....ewton_third_law


this sums up my line of thought

Edited by PlayStationX, 13 January 2009 - 07:31 PM.


#19 swansont

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Posted 13 January 2009 - 09:39 PM

The objection I have to that is that it ignores the historical view, and appears to assume that the galilean/Newtonian view is obvious. But if it were so obvious, why did the Aristotalean view persist for 2000 years? And the OP did not ask if "tend" should be there, rather it asked why it was there.

All the rest is, I think, semantics and opinion. Is there a quantifiable way to show that one statement is better than another?

Edited by swansont, 13 January 2009 - 10:45 PM.
fix typo

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#20 Klaynos

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Posted 13 January 2009 - 09:46 PM

I'd like to take this opertunity to remind one and all of rulse 2.10 and 2.5 that requires that personal ideas/speculations are posted in the correct forum and in not in science threads, and also that new discussion threads are put into new threads, this will lead to a better experience for all and a better discussion of both ideas.

This is just a reminder as I can see it wandering in that direction.

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