5.1 Introduction

INTRODUCTION

Newton was the first person to study motion fundamentally. He studied and clarified some of Galileo’s ideas and proposed three laws of motion concerning relations between force and motion. Newton’s laws of motion are of central importance in classical physics and a large number of principles and results may be derived from Newton’s laws.

The first two laws relate to the type of motion of a system that results from a given set of forces.

Let us consider some common experiences we encountered in daily life. To move a football at rest, someone must kick it. To throw a stone upward, one has to give it an upward push. A breeze causes the branches of a tree to swing; a strong wind can even move heavy objects. A boat moves in a flowing river without anyone rowing it. Clearly, some external agency is needed to provide force to move a body from rest. Likewise, an external force is needed also to retard or stop motion. You can stop a ball rolling down an inclined plane by applying a force against the direction of its motion.

In these examples, the external agency of force (hands, wind, stream, etc) is in contact with the object. This is not always necessary. A stone released from the top of a building accelerates downward due to the gravitational pull of the earth. A bar magnet can attract an iron nail from a distance. This shows that external agencies (e.g. gravitational and magnetic forces ) can exert force on a body even from a distance.

In short, a force is required to put a stationary body in motion or stop a moving body, and some external agency is needed to provide this force. The external agency may or may not be in contact with the body.

So far so good. But what if a body is moving uniformly (e.g. a skater moving straight with constant speed on a horizontal ice slab)? Is an external force required to keep a body in uniform motion?