Earth’s Gravitational Field
People have always known that free objects drop to the ground, but it was not until the early seventeenth century that philosophers started to formulate theories as to why this happened.
Galileo was intrigued by gravity. People of his day thought that heavier objects fell more quickly than lighter objects, and that the rate at which objects fell was a measure of their weight – ‘Air does not fall, so it has no weight. Galileo demonstrated very simply and effectively that objects of different mass dropped from the same height fell to the ground in very nearly the same time.
Before the time of Galileo, most people assumed that heavier objects fall faster.
Galileo showed that all objects fall at the same rate, regardless of mass: 9.81 m s–2.
This means that two objects falling from the same height land at the same time.
The Earth is surrounded by a gravitational field, in much the same way that it is surrounded by a magnetic field. There are, however, major differences between the Earth’s gravitational field and its magnetic field. In the case of the gravitational field, the Earth behaves like an electrostatic negative pole, and all ‘lines of gravitational flux’ (as it were) are directed towards the Earth. In the case of the magnetic field, the Earth is bipolar-there is an N-pole and an S-pole, and the ‘lines of flux’ originate and end on the Earth.
The word field is used in the gravitational and magnetic contexts to mean a region in which a body, pole or charge (as appropriate) can experience gravitational or magnetic forces. It refers to a field of influence. It shows the directions of the forces experienced by masses when placed in the Earth’s gravitational field; the sizes of the forces will depend on how close the masses are to the Earth’s centre. Because the Earth’s gravitational field is always directed towards the centre of the Earth, the gravitational force acting on a mass in the vicinity of the Earth is always directed towards the centre of the Earth too. Someone standing near the mass sees this as ‘vertically downwards’.