# Gravitational field and gravitational field strength

When you push a book on the table surface until the book moves, your hand touches the book. Likewise when you tie an object with a piece of rope then pull it until it moves, your hand touches the rope, the rope touches the object. In this case, the push force, pull force, tension force of the rope, and forces like this are called touch forces or contact forces. Earth’s gravitational force that pulls the fruit falling toward the surface of the earth or the gravitational force of the earth that pulls the moon to the orbit of the earth occurs without touch between the earth and the fruit and moon.

Therefore gravitational forces or forces like this are called non-touch forces. How could fruit fall and the moon “fall” towards the earth without touching between the earth with fruit and moon? Scientists, including Newton, find it difficult to imagine the concept of non-touch force. In order to more easily imagine and understand the concept of non-touch force, the concept of field is raised.

**Gravitational field **

When an object with mass is in space, the object produces a gravitational field. The gravitational field of an object with mass m is illustrated in the form of field lines as shown in the figure below.

The farther from the surface of the object, the distance between the lines of the gravitational field is farther away. Instead, the closer to the surface of the object, the distance between the field lines is getting closer. The closer, the greater the gravitational field strength. The farther, the gravitational field becomes smaller. Likewise, the greater the mass of objects, the more and closer the gravitational field lines. If a test particle is placed near an object that produces a gravitational field, the test particle will experience a gravitational force. The direction of the gravitational force toward the center of the object produces a gravitational field.

The farther from the surface of the object, the distance between the lines of the gravitational field is farther away. Instead, the closer to the surface of the object, the distance between the field lines is getting closer. The closer, the greater the gravitational field strength. The farther, the gravitational field becomes smaller. Likewise, the greater the mass of an object, the more and closer the gravitational field lines.

If a test particle is placed near an object that produces a gravitational field, the test particle will experience a gravitational force. The direction of the gravitational force toward the center of the object produces a gravitational field.

**The gravitational field strength**

The strength of the earth’s gravitational field experienced by an object above the earth’s surface:

g = gravitational field strength

F_{g }= force of gravity

m_{B} = mass of earth

R_{B }= radius of earth = 6.37 x 10^{6 }m

h = height of objects above the earth’s surface

If we want to calculate the gravitational field strength of a planet, the mass and radius of the earth (m_{B} and R_{B}) are replaced by the mass and radius of the moon or the mass and radius of a particular planet.

If we look at the free-fall motion of an object above the surface of the earth, then it is seen as the gravitational acceleration of the earth which is experienced by the object which is 9.8 m/s^{2}. If the object is still above the earth’s surface (objects do not fall freely), then g is seen as the earth’s gravitational field strength experienced by the object which is 9.8 N/kg.