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# Motion

Motion may be of two kinds. If a body moves from one position to another in a straight line, it is said to have a motion of translation; if it moves round a fixed point, it is said to have a motion of rotation. However complex the motion of a body may appear to be, it can always be shown to be a combination of these forms. When a body moves through equal distances in equal times, its motion is said to be uniform; but if it gains or loses speed, its motion is variable, and the speed gained in unit time is called its acceleration. Thus, if a body is moving with a speed of 44 feet per second and during one second increases from 44 to 50 feet per second, it has an acceleration of six feet per second. We can only consider relative motion, for we know nothing about the absolute motions of bodies in space. In saying that a train moves at the rate of 30 miles an hour, we are merely comparing its speed with that of the places it passes on the earth, and we entirely neglect the earth's motion round her axis or round the sun, which motion belongs alike to the train and the places on its route. Newton enunciated three laws of motion: - (1) Every body remains in a state of rest or of uniform motion in a straight line, unless it is compelled by impressed forces to change that state. This is equivalent to saying that matter has inertia; it cannot, by itself, change its state of motion. (2) Change of motion is proportional to the impressed force, and takes place in the direction of the straight line in which the force acts. From this it at once follows that when several forces act on a body, each produces the same effect as if it acted alone, and this effect is produced whether the body be already in motion or at rest. (3) Action and reaction are equal and opposite, or the mutual actions between any two bodies are always equal in magnitude, but oppositely directed in the same straight line. Thus, the tension of a rope is the same throughout its length; it exerts, for instance, as great a pull on the towing-line as on the boat which is towed. The action and reaction can be expressed as the work done by a force; hence, we arrive at the principle of "natural work" and its application to machines. The third law may further be regarded as the first enunciation of the principle of the indestructibility or conservation of energy.
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