Newton's first law states that in an inertial frame, a body at rest continues to be at rest, and a body in constant rectilinear motion continues its motion, unless an external force is applied upon the bodies. My question is how far have these assertions been tested experimentally? More precisely, is there any need to test this statement, or is it taken as a "definition" of sorts?

I assume the body at rest is a trivial case. On the other hand the case of the body in constant motion would be considerably harder to test, since no external force on a body is an idealization. But I am still curious whether there have been any attempts to test the case of the moving body.

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    $\begingroup$ There is no point in testing the validity. This is what defines a force. $\endgroup$
    – Mathusalem
    Commented May 27, 2013 at 12:27
  • $\begingroup$ @Mathusalem this is what defines inertia. Newton's Second Law is what defines a force $\endgroup$
    – Jim
    Commented May 27, 2013 at 13:33
  • $\begingroup$ Well this is the eternal problem of the circularity of the two first postulates, isn't it ? Correct me if I am wrong, but I think it is rather hard to distinguish which of the first two defines a force, since both are mutually needed to define the other $\endgroup$
    – Mathusalem
    Commented May 27, 2013 at 13:49

3 Answers 3


Newton's laws are testable. An example of a modern, high-precision solar system test of the first law is Battat 2007. The Eot-Wash group has done various high-precision laboratory tests, such as searches for coupling of spin to a preferred frame. The third law has been tested to high precision in Kreuzer 1968 and Bartlett 1986. For modern tests of the first law, the keyword you want to search for is "Lorentz violation." For the third law, try "local position invariance."

It's particularly absurd to claim that Newton's laws are only definitions and not testable, since they have already been falsified by experiment. Special relativity contradicts Newtonian mechanics, so all the experiments that verified SR demonstrate that Newton's laws constitute a real, falsifiable theory. Similarly, quantum mechanics falsifies Newton's laws.

Some textbooks may, e.g., present the second law as a definition of force or as a definition of mass, but this has no effect on the validity of experiments that test Newton's laws. This is because the concepts of force and mass carry other baggage, such as conservation of mass and vector addition of forces. Someone who prefers to think of the second law as a definition of force will consider such an experiment to be a valid test of some other claim or set of claims of Newtonian mechanics, such as vector addition of forces.

With the first law, you have to be careful because Newton formulated it in one way, but later people have formulated it differently: History of interpretation of Newton's first law . In a modern context, we think of it as having to do with Lorentz invariance and the lack of a preferred frame.

Bartlett and van Buren, Phys. Rev. Lett. 57 (1986) 21, summarized in Will, http://relativity.livingreviews.org/Articles/lrr-2006-3/

Battat 2007, http://arxiv.org/abs/0710.0702

Kreuzer, "Experimental measurement of the equivalence of active and passive gravitational mass," Phys. Rev. 169 (1968) 1007, http://bit.ly/13Z6XAm


In science there are several ways of testing if something is really justified. But most of the times something is valid until the contrary is proven.

In the case of Newton's first law there haven't been observations which are in contradiction with Newton's first law (if we don't consider relativistic or quantum mechanic observations). Everything that is seen in everyday life is according to Newton's first law. Our head that moves forward when stopping the car is sometimes said to be a very good proof by the way.

Even in present scientific research, the same way of accepting theories is used. The researchers in CERN know with more than 99,9% certainty that they found the Higgs particle, but still there is a chance that they are wrong. But one has to accept it at some point.

Newton's first law has been used for over 300 years now, and still it is widely accepted. Actually, this could be seen as the fact that Newton's first law is accepted with a certainty of 99,99... %, so we could consider the first law to be valid.

  • $\begingroup$ actually, we are 100% sure of newton's first law. It has been tested and confirmed to the point of being axiomatic. $\endgroup$
    – Jim
    Commented May 27, 2013 at 13:08
  • $\begingroup$ I meant 99,99...% instead of 99,9%. Which can be seen as 100%. $\endgroup$ Commented May 27, 2013 at 13:16
  • $\begingroup$ I see. that's true but it seems to me that saying 100% would get the point across better. Also, if the future, if you want comment at someone, use @(person's name) otherwise they won't be told about the comment. $\endgroup$
    – Jim
    Commented May 27, 2013 at 13:24

Newton's first law is not an empirical law. It's what philosophers of science call "vaccum laws".

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    $\begingroup$ . . . which are? $\endgroup$
    – HDE 226868
    Commented Jan 3, 2015 at 0:59

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