Gravitational Field

Gravitational Field

We have weight. Weight is a force from earth that pulls us down. This force is also called gravitational force. It is a mutual attraction between my own mass, and the mass of the earth. This is also true for any two bodies that have mass. Between a ball and a stone, there is gravitational force. It is just so weak that we do not notice it.

About 300 years ago, Henry Cavendish in England had measured the force between two metal spheres extremely carefully, and confirmed that there was indeed a force.

The region of space around any mass can potentially attract another mass, if one is nearby. This region is called gravitational field. It is a region of spacewhere a gravitational force can act. The a gravitational field around a stone pulls the earth up (by a tiny bit, so we don't notice it). The field around the earth pulls us down. The field around the sun pulls on the planets so they don't drift into the outer space.

The earth's gravitational field is the most familiar one. A bigger person feels more weight, while a smaller person feels less. It is really the same field from the same earth. A "fair" measure of the strength of this field is to look at the force acting on the same mass, e.g. 1 kg. We know that this is 9.81 N. So we say that the earth's gravitational field strength is 9.81 N/kg. This does not depend on the person or the object, so we can use this to refer to the earth's field. So if we go to the moon (not that we do that very often) and find that the field there is about 1.6 N/kg, we can say that the moon's field is weaker.

We can define the gravitational field strength g as the force per unit mass, or g = F/m, where F is the force on a body, and m is the mass. Note that the same symbol g as the acceleration due to gravity, is used. It is important to check that we are talking about the earth's surface, before using g = 9.81 N/kg. If we are far from the earth's surface, like on a satellite, then g would be smaller.

The Physics Notes