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. INTRODUCTION

When forces are applied to a body (solid, liquid or gas), internal forces are set up in the body and it

deforms and/or moves. The object of our study here is to understand how the applied load relates to

these internal forces and deformation/motion of the body. Why do we need to know these things? Let

us take an example. Let us say you are asked to design an antenna for a space application. You are

required to design the antenna to a specific shape so that a satellite can communicate with earth from

the outer edges of the solar system. So you take special care and produce (after spending millions of

dollars) a beautiful antenna which has on earth the exact shape that it should have in space. Then you

send it up into space and it does not work! The satellite's messages are being beamed to the Oort

cloud instead of to Houston. What have you done wrong? You have forgotten a very simple thing ---

gravity! A body on earth is always subject to the earth's gravitational attraction. The antenna was

perfectly shaped under earth's gravitational force, but in space (or on any other planet) when it is

under either zero-g or someother-g, it has quite a different shape! So what happens next? You are

fired, of course...

Anyway, there are other less esoteric reasons for us to understand the mechanics of deformable

bodies and I am sure you can think of hundreds of them. Figure 1 lists a few examples. So, granting

that we are embarked on an important mission of discovery and all that, how exactly are we going to

characterize the internal forces and deformation of a body?Continum Assumption: When external forces are applied to a solid body, the atoms or molecules in

the body may move apart a little bit from each other. The reason the atoms hopefully do not

completely come apart (if the load is sufficiently small) is because they resist the applied external

forces by developing internal forces until equilibrium is achieved. If the internal forces cannot resist

the external forces, the body breaks or fractures. Though internal forces are due to the atoms or

molecules inside a body, it is too complex to study mechanical deformation from the atomistic point

of view (even though some people make a living doing so). Therefore, we will now adopt the first of

our many approximations, namely the so-called continuum assumption. Under this assumption we

can forget about the details of the atomic structure of the solid, and instead treat the solid

(equivalently, a fluid) as if it were one continuous thing, whatever that means. In essence, our theorywill hold only for length scales which are much larger than atomic distances.

(nrml stres)

(mechanics) The stress component at a point in a structure which is perpendicular to the

reference plane.

Type your answer stress; the force applied perunit surface area of the body that produce or tends

to produce deformation in a body is called stress if a force F to a area A of a body then

stress=force/areaor o=f/a

where o (stigma) denotes stress.obviously, the SI unit of stree is newton pper square meter(Nm-

2).

strain; the fractonal deformation resulting from astree is called strain

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shiri stres)

(mechanics) A stress in which the material on one side of a surface pushes on the material on the

other side of the surface with a force which is parallel to the surface. Also known as shear stress;

tangential stress.

(beri preshr)

(mechanics) Load on a bearing surface divided by its area. Also known as bearing stress.

(d'fr-m'shn, df'r-)

n.

1.a. The act or process of deforming.b. The condition of being deformed.

2. An alteration of form for the worse.3. Physics.

a. An alteration of shape, as by pressure or stress.b. The shape that results from such an alteration.

(tr'shn)

n.

1. a. The act of twisting or turning.b. The condition of being twisted or turned.

2. The stress or deformation caused when one end of an object is twisted in one directionand the other end is held motionless or twisted in the opposite direction.

a tangential stress applies to a circular shape, such as a hoop, and is a tensile or

compressive stress, not shear stress

A body is refered to be in equilibrium when the forces acting on it cancel each other out, that is,

the body has no resultant force.

because F=ma , it can also be stated that a body is in equilibrium when there is no acceleration or

deceleration in any direction.

Therefore, if a body is moving at a steady speed, although it IS moving, it is still at equilibrium.

We normally do not think of a moving object as one in equilibrium but that is because in an

actual situation, air drag would cause an object to be in equilibrium with 0 velocity.

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