the only forces acting on it are

the forces at its end from the surrounding.

When we draw the free body around here,

there is no force between C

and D. So there is only one force,

but here is no couple applied

because we assume that this end is pinned,

and similarly here, there is

some only one force acting at this end.

We have said that

such a member is called a two force member.

That means in reality,

we assume that we have four unknown reactions here.

But these four reactions,

these basically two forces,

if we consider the equations of equilibrium of this body,

then the three equations of equilibrium,

then we can conclude that the only way this body

can be in equilibrium is

under one of these two scenarios.

One scenario is that we have two forces acting at its end

along the direction of the member and

opposite each other pulling on the member.

Let's call this force FCD,

or we have again

two forces opposite to each

other which are in this case compressing the member.

So this is tension,

the force is pull on the member and this is what we call

compression, and so on.

So each of these member is

either a tensile or compressive member

or possibly it could be what we

call zero force member if it happens that

the force within the member is zero.

Now, rather than using

the adjective tensile or compressive force,

we are going to make a convention,

accept the convention that we will call

all tensile force as

positive and all compressive forces negative.

That way, if we give you the force

of member FCE to be plus AD,

it will mean it is a tensile force of 80.

If we state that FBD is minus 150,

it means it is a compressive force of 150.

Now, when we consider the equilibrium of a body

like this which is comprised of seven rigid members,

we have said that it is not

sufficient to talk about the overall equilibrium.

That means writing the three equations of

over-roll equilibrium do not

satisfy equilibrium of the structure.

To satisfy equilibrium, we must make sure

that each sub member,

each rigid body comprising

the structure is under equilibrium.

All right. Now, let's try to write

all the necessary and sufficient conditions

of equilibrium for this structure.

So as we said, for each member,

we can draw a free body diagram like this.

For example, for member CE,

we are going to draw

and we will assume for now that everything is tensile.

So in our drawings,

we will assume that all forces are tensile.

If the member force turns out to be negative,

automatically we will be able to

conclude that the force is compressive.

So for each of these members,

for each of the seven members,

we can draw a free body diagram with

a tensile force FCE acting on it.

That already will have guaranteed

equilibrium of each member from one end to another,

from one end to another,