A very basic doubt in Strength Of Materials

Hi Members,

                   This may seem a very basic or even stupid question to you all , but it still leaves me with a doubt regarding the concept behind the WHY and HOW of this. So however trivial this may seem Please bear wiith it and Help me Clearify my concept and getting the right idea.

 This is regarding the " Seperation Of the Beam at the Internal Hinge"  during the solution for S.F.D and B.M.D. in any general example involving a beam loaded with point loads and moments.

Suppose there is a fixed beam with one single point load acting vertically at 0.5m from the free end ( suppose it is fixed at the left end and has a point support at the free end at right) and the beam is of 4m length, and there is an Internal Hing at 1.5m from the right.

When the beam is seperated at the Internal Hinge and the right hand part is considered under equilibrium then the right hand part is considered seperately.

Here the right hand part of the total beam now has its own left end ( ie at the seperated spot of the Internal Hinge ) and the point support at the right end of the original beam.

Now in the Free Body diagram drawn the equilibrium is considered and is said that the point load( vertically downwards )would have a reaction at the point support at the right end and the Internal Hinge in the direction vertically upwards.  This is okay, and the reason I deciphered for this was that at the Internal Hinge there is a Pin passing horizontally through the holes in each of the two component beams ( the right part and the left part, - supposing that the each part has a hole at its end and the pin passses horizontally through these holes connecting them together)

So if we imagine the action created by the point load at the hinge then it will be trying to press down the upper surface of the pin by the upper surface of the inside of the right hand beam hole and this inturn creates a Reaction by the pin surface on the beam hole surface in the UPWARD DIERCTION due to the contact. This explains as why the reaction at the left hand side ( hinge seperated end) of the right hand part of the original beam.

But Now the Original Question arises, that is the REACTION AT THE INTERNAL HINGE ON THE OTHER PART ( LEFT PART ) OF THE ORIGINAL BEAM IS TAKEN IN THE OPPOSITE DIRECTION  IE  DOWNWARDS in the free body diagram for the Left Part of the Original Beam.

The Text Book only mentions that the direction of the reaction at the Internal Hinge for the other part of the beam ( here the left part ) will be in the Opposite direction to the reaction on the right part a the internal hinge. But there is no proper reason explained for this.  

And this is What I want to know, WHY IS THE REACTION AT THE INTERNAL HINGE TAKEN TO BE IN OPPOSITE DIRECTION ON THE OTHER PART OF THE BEAM

AND HOW IS THE FORCES FROM THE RIGHT PART TRANSFERRED TO THE LEFT PART IF THE BEAM IS CONSIDERED AS NON SEPERATED AT THE HINGE. How to visualize the tranefer of forces through the hinge ie say the shear force on any point on the left part of the beam would still be equal to the point load given, but then how os the force transmitted through the hinge.

 Reason Thought :

To this I thought it to be like this  -   The pin is pressed downwards by the  right hand beam ( due to the point load pressing downwards on the right end point support and trying to rest on the pin in the hinge as its oher support ) which causes the pin to lift upwards on the upper surface of the hole of the left hand beam ( just like a see-saw movement where one side goes up and presses down the opposite side). this lifting up on the upper surface inside the left hand beam hole would cause a reaction pointing downwards. So this decides the direction of the reaction on the left hand beam to be Downwards. 

Is this thinking correct or is this the actual reason for the direction of the reaction on one side of the hinge being opposite to the reaction on the other side.

Please help me get the actual reason or concept behind this as there is no Explaination given for this assumption of directions in any Text Book or by any professor in class.