# A small cylinderical piece does not bend but when a long piece of same material and dia. tend to bend. Which Principal apply on this phenomenon?

Similarily when height of a building is increased it tend to bend with little force of wind. Please explain?

It sounds like this is the lever principle. This happens when two forces are operating in different directions on the bar or the building. For the building, the wind pushes it in one direction at the top, and the ground has a force that pushes back in order to keep the ground in place.

I agree.  This is the principal of a lever.  The idea is simple enough.  Letâ€™s imagine you have both small and large cylindrical pieces fixed at one end like this:

|—————-    (Long Cylinder, 5 Meter)
|—-                     (Short cylinder, 1 Meter)

Now apply the same force to the right side of both cylinders. Using Statics you would find that the total amount of force in the longer cylinder is great than in the shorter cylinder, causing the larger cylinder to bend.  The equation to find how much each beam would deflect is:

y=(WX^2/6EI)*(3L-X)

W= Weight Applied
X=Location of the deflection
L = Length of the beam
E= Modulus of Elasticity
I = Moment of Inertia
y = Deflection