Everyday Experience of Motion and Energy

Motion and Forces

There is clearly a great deal of very varied motion in the world around us. Even those entities which appear to be stationary for example the items of furniture in our rooms and the objects in and on them are moving at high speed as the earth rotates and moves around the sun. Further, at the other extreme, the atoms and molecules from which they are constituted are, as we shall see, in incessant motion. It is therefore essential to understand at an early stage the nature of motion and how it can be changed. First, to state the virtually obvious, the motion of a body is changed when a force is exerted on it where a force is characterized by two features-its magnitude and its direction. (Here it should be noted that entities specified by these two characteristics are known as vectors). By ‘changed’ is meant that the body speeds up (accelerates), slows down (decelerates) and/or changes the direction in which it is travelling. To start a supermarket trolley moving (i.e. to change its motion from rest to moving) it has to be pushed; the pusher exerts a force on it. Similarly a force has to be applied to turn it round a corner. Of  course, to keep the trolley moving at a steady speed in a straight line a continual push is still required and yet the motion is not changing. Here it must be realized that there is another force influencing the motion, namely friction, and in steady motion the ‘push’ and ‘frictional’ forces just balance. In other words the net force on the trolley is, in fact, zero and hence its motion does not change. If there were no friction then no push would be required to keep the trolley moving steadily. This state of affairs is, for example, nearly reached when an object such as an ice puck, experiencing very little friction, slides over ice. The statement that a body’s motion only changes when a force is exerted on it was formally enunciated by Isaac Newton in the 17th century and is incorporated in his First Law of Motion.

Newton’s First Law of Motion. A body continues in its state of rest, or uniform motion in a straight line, unless acted upon by an external force.

In a moment we will consider in a little more detail how the change in motion brought about by a force is related to its strength and direction, but before doing that we should consider the nature of force. Everyone is familiar with the force exerted on an object when it is pushed or pulled. Such a force is generally transmitted by direct contact between the pusher or puller and the object experiencing the force. But the force may also be transmitted through an intermediate agency-pushing a stone with a stick, hitting a ball with a racket, controlling a kite with a cord etc. Familiar to most will also be forces which are transmitted without any material contact, for example, the force exerted by a magnet on a piece of iron. Place a magnet near some iron filings and they will jump and attach themselves to it; wave a magnet near a compass needle and the needle will move. Here it will be recognized that magnetic forces can be repulsive as well as attractive; put two compass needles close to each other and the two north-seeking poles will move apart from each other. The iron filings and the compass needle change their state of motion under the influence of the magnet and a force, known as a magnetic force, is being exerted on them. Similarly, if a balloon is rubbed against a piece of material it can pick up pieces of tissue paper. In this case an electric force is coming in to play. It is the same type of force which raises the hairs on a hand or arm when placed close to a television screen. Finally, in this context, there is the force of gravity which pulls a ball down to the ground when thrown into the air and which keeps objects including ourselves-firmly on the face of the earth and keeps planets orbiting about the sun. Gravity, unlike magnetic and electric forces, is a force which is always attractive; it attracts the moon to the earth, the earth to the sun and is, in fact, experienced between all material objects. It is very weak, however, and is only noticeable when at least one of the objects is very massive (e.g. the earth). Magnetic, electric and gravitational forces, which are fundamental to understanding the behaviour and properties of matter at all levels of scale, are effective between bodies without there being any obvious direct physical contact between them. With such forces, the closer the two bodies experiencing them are, the stronger the force; you will not be able to detect the influence of a magnet on a compass needle placed on the other side of a room. The magnetic force dies away slowly as the distance from the magnet increases; similarly with electric and gravitational forces. The objects exerting such forces are surrounded by a ‘field of influence’ producing what might be called a ‘stress’ in space which becomes weaker the further you are away from the objects. It is conventional to refer to them as magnetic, electric and gravitational fields. In due course it will be explained how such ‘action at a distance’ forces and fields are propagated but, for the moment, just accept that they exist