**Gravitational field **

It is a region of space where a mass experiences a force.

**Newton’s law of gravitation**

It states that the two point mases attract each other with a force that is proportional to the product of their mases and inversely proportional to the square of their separation:

F∝m_{1}m_{2}/ r^{2}

F = – Gm_{1}m_{2}/r^{2 }where G is Gravitational Constant; G= 6.67 × 10 ^{-11}Nm^{2}kg^{2}.

When one of the masses is of planetary size M,

F = – GMm/r^{2}

The minus sign means that the force is attractive.

**Gravitational field Strength, g**

A gravitational field is the area around a mass where any other mass will experience a force.

We can think of gravitational field strength as the concentration of field lines at that point. We can see from the figure that the field strength is constant in a uniform field but drops quickly as we move further out in a radial field.

Gravitational field strength is the gravitational force exerted per unit mass on a small object placed at that point.

So, it can be written as.

g=F/M

If we use this equation for gravitational force at a distance r then,

g=- GMm/(r×r×m)

g= – GM/r^2

**Gravitational Potential**Gravitational potential at a point in gravitational field is defined as the work done per unit mass in bringing a unit mass from infinity to the point.

Ø = -GM/r

It is negative due to the attractive gravitational force. Therefore, the work done by mass is decreasing its potential difference.

**Orbit Period**The orbit period of a satellite is the time taken for one orbit.

The **orbit period** can be calculated by equating the gravitational force GMm/r^{2} to centripetal force mv^{2}/r.

Geostationary satellites have an orbital period of 24 hours and are used in telecommunications and television broadcasting.