Newton’s first law

1. For centuries the problem of motion and its caus­es was a central theme of natural philosophy. Before Galileo's time most philosophers thought that some in­fluence or "force” was needed to keep a body in the state of motion. They thought that a body was in its “natural state” when it was at rest. For a body to move¹ in a straight line at constant speed, for example, they beleved that some external agent had to propel it continually: otherwise it would “naturally” stop.

2. If we wanted to test these ideas experimentally, we would have to find a way to free a body from all influ­ences of its environment or from all forces. This is hard to do, but in certain cases we can make the forces very small. If we study the motions as we make the forces smaller and smaller, we shall have some idea of what the motion would be like if the external forces were tru­ly zero.

3. Let us place our test body, say a block, on a rigid horizontal plane. If we let the block slide along this pla­ne, we notice that it gradually slows down and stops. This observation was used in fact to support the idea that motion stopped when the external force, in this case the hand which was initially pushing the block, was re­moved. Galileo argued against this idea. He was reason­ing as follows: Let us repeat our experiment. Now we shall be using a smoother block and a smoother plane. We notice that the velocity decreases more slowly than before. Let us use still smoother blocks and surfaces. We find that the block decreases in velocity at a slower and slower rate and travels farther each tijrpe before it comes to rest. We can now extrapolate and say that if all friction could be eliminated, the body would continue inde­finitely in a straight line with constant speed. This was Galileo’s conclusion. Galileo asserted that some exter­nal force was necessary in order to change the velocity of a body but that no external force was necessary in order to maintain the velocity of a body.

4. This principle of Galileo was adopted by Newton as the first of his three laws of motion. Newton stated his first law in these words: “Every body persists in its state of rest or of uniform motion in a straight line un­less it is compeled to change that state by forces impress­ed on it”.

5. Newton’s first law is really a statement about re­ference frames. For, in general, the acceleration of a body depends on the reference frame relative to which it is measured. The first law tells us that, if there are no ne­arby objects then it is possible to find a family of refe­rence frames in which a particle has no acceleration. Newton’s first law is often called the law of inertia and the reference frames to which it applies arc therefore called inertial frames. Such frames are either fixed with respect to the distant stars or arc moving at uniform ve­locity with respect to them.

6. Notice that there is no distinction in the first law between a body at rest and a body which is moving with constant velocity. Both motions are “natural" in the ab­sence of forces. That this is so becomes clear when a body at rest in one inertial frame is viewed from a second Inertial frame, that is, a frame which is moving with con­stant velocity with respect to the first. An observer in the first frame finds that the body is at rest; an observer in the second frame finds that the same body is moving with uniform velocity. Both observers find that the body has no acceleration, that is no change in velocity, and both may conclude from the first law that no force acts on the body.