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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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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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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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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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