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Newton's first law of motion says

"A body will remain in a state of rest or in uniform motion in straight line, unless acted by an external force."

So if a body is accelerated or moved uniformly in a circle, the state also changes its state by an external force.

Why does Sir Issac Newton use the state of rest or uniform motion in a straight line?

Qmechanic
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Syed Uzair Ali Shah
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  • I'm voting to close this question as off-topic because it shows lack of research-efforts. –  Nov 06 '15 at 14:23
  • What you mean ? This is a valid question – Syed Uzair Ali Shah Nov 06 '15 at 14:25
  • Don't you answer your question the line before asking it? – Kyle Kanos Nov 06 '15 at 14:45
  • The first law of Newton (discovered by Galileo Galilei) wants to say that a body can move (with constant speed and in a straight line) even if no force acts on it. Aristotel believed that a body can move only if pushed or pulled by a force. – Energizer777 Nov 06 '15 at 14:45
  • Related: http://physics.stackexchange.com/q/13557/2451 and links therein. – Qmechanic Nov 06 '15 at 14:55
  • Newton's 1st Law is making a statement about the very nature of straight-line constant speed motion: no outside force needed. That includes zero speed, too. Do you think it means something else? – Bill N Nov 07 '15 at 03:53
  • When there's no external force on a body it could be at rest and stay at rest. It could be moving in a straight line at a uniform speed and keep moving in a straight line at a uniform speed. And then after the law he starts to go into other examples, such as a uniform spinning body spinning uniformly subject to no external forces (and not weird internal forces) and it continues to spin uniformly. You could even have other cases too, but to try to make other cases your object starts to be non uniform and so you really have subparts and then you'd want to use all three laws on the subparts. – Timaeus Nov 08 '15 at 03:51

3 Answers3

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That "unless" in Newton's law is key. Maybe a more useful way of understanding Newton's statement is as follows:

If we see an object that has changed from being at rest to moving (or the other way around), or from moving in a given straight line to another, different straight line (think here of the tangents to a circle at two different points), then we'll say that an external force has acted on it.

And viceversa: if an external force acts on an object, then its effect will be to change its state in one of the ways stated above, i.e., from moving to rest (or viceversa) or from one straight line to another.

I deliberately simplified things here. This however captures the essence. For instance, in a circular movement, there is a constant force acting on the object and this forces is thus constantly (and continuously) changing the tangent (its velocity) along which the objects moves at any given instant.

However, the precise way to think of Newton's first law is that

  • There is a force $\mathbf{\vec{F}}$ acting on a body whenever $\frac{d}{dt}\mathbf{\vec{V}}\neq0$.
  • and viceversa, if there is a force $\mathbf{\vec{F}}$ acting on a body, then it will be $\frac{d}{dt}\mathbf{\vec{V}}\neq 0$

That is as much as Newton's laws go into actually defining what a force is. Strictly speaking they only tell us how to identify that a force is acting on a body. Or, to put it in yet another words, when we shall talk about force.

EDIT: Note: After reading the last comments I get your problem is in understanding how it can be enough to just "talk" about straight lines or at rest in this law. Isn't Newton missing other states? doesn't he need more generality? The answer is no. In order to convince yourself, try thinking about any kind of movement as a succession of straight-line moves, but very, very short, actually, infinitesimally short. Mathematics can help us in making sense of this. It involves a differential equation for the rate of change of the velocity of an object.

MASL
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  • Minor quibble on notation: \mathbf makes \vec superfluous (and vice versa) – Kyle Kanos Nov 06 '15 at 15:28
  • Sir I want to ask if you say that Sir Newton not missing any state but he state only two why ?? Where the other states ?? – Syed Uzair Ali Shah Nov 06 '15 at 15:28
  • @KyleKanos That's right. Have used both here just to highlight it more for the OP. – MASL Nov 06 '15 at 15:35
  • @SyedUzairAliShah He doesn't need to discuss all possible states. He realized that we can understand any movement as a succession of straight-line movements. That's what I pointed to in my very last paragraph. – MASL Nov 06 '15 at 15:37
  • @SyedUzairAliShah Consider the dodecagon of this picture https://en.wikipedia.org/wiki/Dodecagon It follows quite closely the circle. You can imagine that, if instead of 12 sides we would use a poligon with say 100 sides, the approximation to a circle would be way better. It is in this sense that Newton saw he didn't need to worry about more complicated states of movement. – MASL Nov 06 '15 at 15:43
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Newton's first law is not about forces changing the way things move. That's the content of the second law.

The first law states what happens when no force acts. It defines an object on which no net forces act as an object which travels in a straight line or is at rest.

ACuriousMind
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  • Sir uniform circular motion also contain no net force. Why Sir Issac Newton use the statof rest or uniform in straight line ?? – Syed Uzair Ali Shah Nov 06 '15 at 14:45
  • @SyedUzairAliShah: It does. Centripetal force is required to sustain circular motion. – ACuriousMind Nov 06 '15 at 14:46
  • @SyedUzairAliShah: Hint: for circular motion $a=v^2/r\neq0$ – Kyle Kanos Nov 06 '15 at 14:47
  • @SyedUzairAliShah: (you can include the @ symbol to respond to a particular user) I suppose you could say that $r=\infty$ for linear motion so that $a=v^2/\infty\approx0$. Outside of that, I'm not sure what you mean. – Kyle Kanos Nov 06 '15 at 14:54
  • ACuriousMind centripetal forc cancel by centrifuge fore. Uniform circular motion haven't any net force. Why sir Issac Newton use the state of rest or uniform motion in straight line – Syed Uzair Ali Shah Nov 06 '15 at 14:55
  • @SyedUzairAliShah: That's just not true, they don't "cancel". Fictitious forces don't cancel anything. Also, you don't need to repeat your question at the end of every comment. – ACuriousMind Nov 06 '15 at 15:00
  • Kyle Kanos I want to ask, to stop or move any thing we need a force .. It's doesn't matter the object in rest ,accalarate ,uniform in straight line and unform circular motion but in first law of motion Sir Newton say's " if a object remain at rest or moves uinformaly in straight line it always remain at there position unless acted by an external force". Why here use only two states – Syed Uzair Ali Shah Nov 06 '15 at 15:06
  • "centripetal force cancel by centrifuge force". This is a common misunderstanding. Many people believe that if $F=ma$ is true then the two forces acting on the body, F and ma (which is measured in N but is not a force in reality), cancel each other and no net force pushes or pulls the body. – Energizer777 Nov 06 '15 at 15:08
  • @SyedUzairAliShah because the universe just happened to be like this, if he included circular motion his laws would not work. –  Nov 06 '15 at 15:11
  • Energizer777 you mean to say uniform circular motion have completely net force less ?? – Syed Uzair Ali Shah Nov 06 '15 at 15:18
  • bruce smitherson if he including circular motion why her law would not work?? – Syed Uzair Ali Shah Nov 06 '15 at 15:19
  • I told you, because our universe is not that way. It might be valid in a different universe with other laws. Physicists are just trying to figure it out how this particular universe works –  Nov 06 '15 at 15:27
  • ACuriousMind your concept help me thank you very much . – Syed Uzair Ali Shah Nov 06 '15 at 15:37
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Moved uniformly in a circle is not correct.Circular motion is always accelerated due to change of direction.Straight line is here important as Newton's first law talks about two cases-what happens to a body in rest/motion when force acts on it or when it does not.To keep a body moving in a circle a constant external force is needed to change it's direction at every point.(You can imagine a circle as a polygon with infinite number of small points.)

Soham
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