THE MASS OF THE EARTH

Although with Newton's pioneering discoveries, gravity was the first basic force of nature to be described and studied quantitatively, it is the weakest of all known basic forces of nature. The other basic forces are the forces of electricity and magnetism and the forces of ‘strong’ and ‘weak’ interaction which act on subatomic particles. It is a measure of the success achieved to date that physicists are able to explain all observed natural and laboratory phenomena in terms of these four basic forces. As we shall see in later chapters, many physicists hope that one day they will be able to bring all the basic forces under the umbrella of one unified force.

Although atomic physicists consider gravity to be the weakest of the four known basic forces of nature, for astronomers gravity is the most dominant force in the celestial environment. How do we assess the strength of gravity in any given situation? We will try to answer this question with a few examples in this chapter.

All of us on the Earth are conscious of gravity. The feeling of weight that we have results from the gravitational pull the Earth exerts on us. Newton's inverse-square law of gravitation described in Chapter 2 tells us how strong this force is on any given body on the Earth's surface. Let m be the mass of the body and M the mass of the Earth. The distance between the body and the Earth is denoted by d.