When a body is immersed wholly or partially in a liquid, it is lifted up by a force equal to the weight of liquid displaced by the body. This statement is known as Archimedes principle.

The tendency of a liquid to uplift an immersed body, because of the upward thrust of the liquid, is known as buoyancy. The force tending to lift up the body is called the force of buoyancy or buoyant force and it is equal to the weight of the liquid displaced. The point through which the buoyant force is supposed to act, is known as centre of buoyancy. It may be noted that

1) If the force of buoyancy is more than the weight of the liquid displaced, then the body will float.
2) If the force of buoyancy is less than the weight of the liquid displaced, then the body will sink down.

The important dimensionless numbers in fluid mechanics are
1. Reynold’s number
2. Froude’s number
3. Weber’s number
4. Euler’s number
5. Mach’s number or Cauchy’s number

Reynold’s number: Reynold’s number is the ratio of inertia force to the viscous force.

Froude’s number: Froude’s number is the ratio of inertia force to the gravity force.

Weber’s number: Weber’s number is the ratio of inertia force to the surface tension force.

Euler’s number: Euler’s number is the ratio of inertia force to the pressure force.

Mach’s number or Cauchy’s number: Mach’s number or Cauchy’s number is the ratio of inertia force to the elastic force.