Newton's Law of Inertia

Newton, like Galileo, claimed that force free motion is motion with constant speed. However, Newton had what we now think is a better understanding of forces and realized that circular motion necessarily involved forces. According to Newton, force free motion is always in a straight line.

Short segments of the earth's surface, even segments as long as 1 km, are nearly straight lines. Thus, for distances of this order of magnitude or less, there is very little difference between Newton's and Galileo's concepts of inertia. Nevertheless, there is a difference. According to Newton, a body that starts out horizontally and that is moving inertially, after having travelled 1 km, would have left the surface of the earth and risen a small distance above it because the earth would have curved away underneath it.

Newton's concept of inertial motion is expressed in his First Law of Motion, which is his law of inertia,:

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

(Translated from the original Latin by I.E. Cohen and A. Whitman.)

This law of inertia is assumed not only in Newtonian mechanics, but also in Einstein's theory of special relativity which has improved on Newtonian mechanics.

Please note that uniform straight-line motion of a body can also occur when the body is interacting with other bodies, as long as these interactions give rise to forces that taken together add up to a zero net force.