Chapter I : Mechanics

In the study of mechanics we study motions of bodies that range in size from enormous stars and planets through things of ordinary size. Automobiles move through highways or wind in and out of traffic, passenger planes fly high above jet planes, artificial satellites streak and across the sky. Stars perform their regular progression. Everything in this physical world from the tiniest particle to the giant stellar bodies is in motion. In fact the subject of motion was the first aspect to be studied by man. What makes anything move? Is there a single cause common to all motion? Is this cause necessary? 

Mechanics is fundamentally the study of motion of material objects. It is one of the oldest of physical sciences. Mechanics can be classified as statics and dynamics. Statics is the study of objects at rest. In statics we deal with forces in equilibrium. Dynamics deals with objects in motion. Whereas time has no importance in statics, dynamics involves time. 

Questions about the cause of motion arose in the minds of man more than twenty-five centuries ago but our present answers were not found until the time of Galileo (1564-1642) and Newton (1642-1727). What sort of thing do we associate with motion? The answer is muscular pulls and pushes. To move a piano across the room you have to do a lot of pushing. To move a sheet of paper off your desk takes very little push. These pulls and pushes are called forces. The notion of force as used in Physics certainly started this way. Later as understanding grew, the idea of forces was extended to include all causes of motion. The pull of a magnet on a nail is a force since it can change the motion of the nail in the same way as that of a muscular force can. 

More specifically what is the relation between force and motion? Everyday experience seems to indicate that it is necessary to exert a force constantly to maintain a steady motion such as motion in a straight line with constant speed. Aristotle (384-322 BC) had noted this fact. He concluded that a constant force was required to produce a constant velocity. It then follows that in the absence of force bodies would come to rest. All though it explained many practical observations it did not explain all the motions, which occur in nature. 

For example the Greeks were aware that bodies fell with increasing speed without the application of any evident outside force. They were also acquainted with the motions of the sun, moon and stars, which seem to occur without pushes or pulls. There seems to be three kinds of motion. We must not only explain the motion of things we push around on the surface of the earth but also the motion of bodies falling on earth and the unceasing motion of heavenly bodies. 

Today our understanding of the motion of falling bodies, heavenly bodies and of bodies, which we ourselves push and pull around on the surface of the earth, is now described in a single fundamental law of motion. Satellites are designed, built and fired according to the same law. 

For two thousand years after the time of Aristotle the apparent difference between celestial motion and motion on earth halted significant progress in dynamics. Then in the seventeenth century Galileo took the first big step towards creating a single explanation of both types of motion. He asserted that any velocity once imparted to a body would be rigidly maintained as long as there are no causes of acceleration or retardation a condition that is approached only on a horizontal plane where the force of friction has been minimized. Briefly it says when no force is exerted on a body it stays at rest or it moves in a straight line with constant speed. 

By studying motion of bodies on inclined planes Galileo was convinced that friction provided the forces, which stop bodies in horizontal motion, and in the absence of all forces the bodies would continue to move forever. He therefore stated his result for the idealized situation in which no forces act. Galileo’s principle of inertia was a great breakthrough, which enabled Newton to build up our present understanding of dynamics. Galileo put forward the idea that everything in nature should be subjected to an experimental test. 

He studied simple phenomena of motion and set up simple experiments to examine them. Thus Galileo contributed a lot to the growth of mechanics and clarified the idea of motion before the world was ready for Isaac Newton to investigate the cause of motion and consequences of motion. Newton ‘s works cannot be underestimated. His contribution laid the firm foundation for modern physical thought. Historically the concept of Mechanics has been used to build up other areas in Physics. Thus without proper understanding of mechanics one cannot evolve an understanding of Physics. 

Particle Kinematics is that branch of Mechanics, which deals with motion without considering the cause of motion. Some important concepts involved in understanding motion are rest, motion, velocity, speed, time and acceleration. The word ‘Kinematics ‘ is derived from the Greek word ‘kinema’ which means motion. 

Particle Dynamics is the study of motion and also the cause of motion. Dynamics comes from the Greek word ‘dynamis’ meaning ‘power’. The cause of motion as we all know is force. Therefore the study of dynamics is largely about force. Dynamics rests on one important pivot - namely Newton’s laws of motion, which is universally known to one and all. 

Isaac Newton was born in 1642, the year Galileo died. He brought together the discoveries of Copernicus, Kepler, Galileo and others in the fields of astronomy and Mechanics. To this he added his own findings and fused them into a magnificent structure that still stands today a testimony of the brilliance of this great scientist. 

An apple fell on Newton’s head, Moved downwards from the tree, And that was when Sir Isaac said: "The force is gravity"