**Gravity:** The gravitational force due to earth is called gravity.

**Newton's law of gravitation:** In this universe, all things attract each other. The force of attraction acting between two bodes of masses m_{1} and m_{2} separated by a distance r is directly proportional to product of their masses and inversely proportional to square of the separation between them.

F = Gm_{1}m_{2} / r^{2}

Here, G is called Gravitation constant. Gravitational forces are very weak forces.

G = 6.67 × 10-11 Nm^{2} × kg^{-2}

**Relation between g (Acceleration due to gravity) and G (gravitation constant)**

F = GMm/r^{2} = mg

g = GM/r^{2}

The value of g near the surface of the earth is taken as 9.8 ms^{-2}. The value of acceleration due to gravity (g) is maximum at poles and minimum at equator.

**Motion of object under gravity:**

When a body is falling down, then a is replaced by g

- v = u + gt
- s = ut + ½gt
^{2 } - v
^{2}= u^{2}+ 2gs

When a body is thrown up, then a is replaced by (-g).

**Buoyancy:** Whenever a body is immersed in a fluid, an upward force is exerted by the fluid on the body, called force of buoyancy or buoyant force. This is also known as upthrust. The magnitude of the buoyant force acting on a body at a given place depends on the density of the liquid and volume of the body immersed in the fluid.

**Archimedes Principle:** When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to weight of the fluid displaced by it.