Lines of thought leading to Newton's three laws of motion

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Lines of thought leading to Newton’s three laws of motion

Newton’s three laws of motion that underlie all of mechanics are not deep, hidden facts of nature that required someone of deep and profound intellect to discover. In fact, one is led to each of them by rather simple lines of thought and it is likely that others, by a little reflective thought, were led to suggest them before Newton.

First law of motion. A body remains in a state of rest or of uniform motion in a straight line unless acted upon by some unbalanced force.

Rationale. An arrow that has been set in motion by a bow follows a straight line with constant speed. The same is true for a stone that has been put in motion by a sling. A marble set in motion by a push on a smooth, horizontal surface follows a straight line with constant speed. All of these represent motion in which there are no external forces acting on the object. It is likely that Galileo and other people who experimented with rolling marbles down inclines considered the above cases of motion in which there were no external acting forces and pondered over them.

Before Galileo it was assumed that rest was the natural state of a body, that a body in motion would come to rest, and that motion, in general, was caused by some force. See Beliefs prior to Galileo. It is easy to understand how these assumptions would arise. In nature, bodies in motion do usually come to rest and motion is usually caused by some force. A loaded wagon doesn’t move unless some force is applied. To move a stone or any other object you need to apply force. Tree branches and leaves move from the force of a wind. However, Galileo and his contemporaries had to deal with the question of what force was keeping a flying arrow going. The solution to this riddle lies in bringing in the concept of frictional forces that resist motion. The reason that a heavily loaded wagon requires a force to keep it moving is that the motion is being resisted by a frictional force. Once one introduces the concept of frictional forces and how they operate everything falls together and makes sense. One can then state the First law of motion and apply it to all motion.

Second law of motion. An unbalanced force acting on a body produces an acceleration of the body. This acceleration is in the direction of the force and in proportion to the force and is inversely proportional to the mass.

It is natural to guess that an unbalanced force acting on a body will produce an acceleration of the body and this acceleration will be in the direction of the force and in proportion to the force and inversely proportional to the mass. This is a natural hypothesis and one that would need to be checked out by experiment.

Third law of motion. Whenever one body exerts a force on another, the second body exerts a force equal in magnitude and opposite in direction on the first.

This law is pretty much intuitively obvious.