can anybody explain me the bending moments if a beam supported at both ends at 1. knife edges, 2. rigidly fixed at the ends.
the problems are, regarding equilibrium of the part of the beam say oc we donot consider the reaction forces at the end other than o.
and its stated there that if the beam is fixed at both ends then the reaction torques are considered at the ends otherwise not. why?

the beam on a knife edge works like a hinge, the ends of the beam can turn at an angle and therefor there is no reaction to their rotation. Fixed ends stop the rotation of the ends of the beam. The only way a rotation can be stopped is to apply a torque which will cause a bending moment at the ends of the beam.

if there be rotation then there must be torque for initially it was at rest.

swaha,

Remember that we are talking about reaction torques here which keep the body in equilibrium. The torque that you refer to would be transient will not be important in analysing the problem.

if there be no rotation why do we need to assume that there exists torque.

swaha,

Look at this diagram, it should explain things better than words.

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