Metacentre
When the body floating in liquid is slightly tilt from its equilibrium position, the centre of gravity (CG) of the body and centre of buoyancy (CB) of the displaced liquid do not lie on the same vertical line as in equilibrium position.
The point of intersection of the vertical line passing through CB of the tilted position & the original vertical line in the equilibrium position is called metacentre (MC) of the floating body. The Floating body can regain its original position or fall into the liquid when it is tilted which depends upon the position of CG of and MC of the floating body.

Fluid dynamics
The branch of physics which deals with the study of nature of fluid when it is in motion is called fluid dynamics.

Fluid
Fluid can be either liquid or gases.

Relation between Surface Tension and Surface Energy

Let us consider, a rectangular frame of wire ABCD in which the wire BC is movable as shown in figure. If we dip the frame in a soap solution, a thin film is formed which pulls the wire BC towards left due to surface tension.

Let, T = Surface tension of film

[latex]l[/latex] = BC = length of the wire

Then, the force F on BC due to T is

[latex]F = T\times 2l[/latex]

Since, the film has both upper and lower surface, so total length of contact = [latex]2l[/latex]

Suppose, the wire BC moves to B’C’ through a distance x at constant temperature, work done in increasing the surface area against force F is given by,

[latex]W = F\times x = T\times 2l\times x[/latex]