How it works : Newton's Laws

NEWTON'S LAWS Newton's work on gravitation was inspired, he maintained, by seeing the fall of an apple. From this he reasoned that the force which attracted the apple towards the Earth's centre was the same one which held the Moon in its orbit around Earth. This 'giant leap' in thought culminated in the Apollo Moon missions, 30 years later. In the course of his pioneering experiments in many branches of science, Sir Isaac Newton discovered several of the fundamental laws of PHYSICS. A scientific 'law' is a general statement which can explain the results of a number of different experiments; this generalization can then be used to predict the outcome of other, similar, experiments. The basic principles of DYNAMICS (the study of how forces act on objects) are summed up in Newton's three Laws of Motion. Laws of Motion Newton's First Law says that any moving body will continue to move in a straight line and at a constant speed unless it is acted upon by an outside force. This is not immediately obvious, since on Earth we are used to moving objects eventually stopping. But this is because outside forces, such as friction and air resistance, act on the body to slow it down. What happens when, as in most practical cases, an outside force does act on a moving body is covered by the Second Law: a force is applied to a body, its momentum will change in such a way that the rate of change of momentum is equal to the magnitude of the force. The momentum is the mass of the body multiplied by its velocity (speed), and so another way of stating the second Law is that the force on a body is equal to the mass of the body multiplied by the acceleration produced by the force: Force = mass ´ acceleration. A consequence of this law is that if the same force is applied to two objects with different masses, the less massive body will accelerate more than the more massive one. Below: the inverse square law of gravitation elongates the waters of the Earth, but the Earth cannot distort in the same way; the result is two 'heaps' of water- the tides. Lower picture: the way a body cools, when in an airflow, is shown by an exponential curve. The Third Law states that for every action there is an equal and opposite reaction. The action and reaction refer to the forces on two different bodies; for example, the weight of a chair standing on a floor must be balanced by an upward 'reaction' force of the floor on the chair, or else (according to the second Law) the chair would be accelerated towards the centre of the Earth. Anyone who tries to jump off a stationary toboggan notices an effect of the Third Law: the toboggan begins to move in the opposite direction even before the person's feet touch the ground. The action of a rocket demonstrates all the laws of motion. A rocket at rest on the ground, or coasting through space with the engine switched off, is obeying the First Law. When the engine is on, the force with which the propellant is ejected from the rocket must he balanced by a reaction force of the propellant on the rocket, and it is this reaction force which drives the rocket forward. These two forces must be equal, and act in opposite directions (Third Law). Since the mass of the rocket is much greater than that of the ejected propellant, the rocker is accelerated to a very much slower velocity than that of the propellant (Second Law). Above: Newton's laws of motion can be applied wherever moving bodies are concerned such as in this game of bowls. Gravitation Newton's Law of Gravitation describes how the gravitational force between any two objects varies with their masses and the distance between them. Each body experiences a force equal to the product of the masses of the two bodies multiplied by the universal Constant of Gravitation, G, and divided by the square of their separation. Newton was unable to explain the origin of gravitation, and Einstein's General Theory of RELATIVITY (1915) proposed that the geometry of space near massive bodies is altered, so that the quickest distance (the geodesic) between two points is not a straight line. By substituting the word 'geodesic' for 'straight line' in Newton's First Law, Einstein was able to incorporate gravity in the First Law of Motion. In spite of relativity, however, the Second and Third Laws are still, three hundred years after their formulation, fundamental to modern science. Cooling Newton's investigations of the cooling of a hot body led to his law of cooling: the rate of cooling is proportional to the difference in temperature between the object and its surroundings. The rate of cooling is measured by the rate at which the temperature of a body falls; so a body at a high temperature relative to its surroundings will initially cool fast, and its temperature falls rapidly. As its temperature decreases, however, the cooling becomes slower, and so the temperature falls less rapidly. This type of behaviour is known as an exponential decay (explained in LOGARITHM) and strictly speaking, although the temperature of the body becomes closer and closer to that of the surroundings, it will never become exactly the same.

Reproduced from HOW IT WORKS p1586