What Is Shear Force And Bending Moment ? No question is silly, these are basic questions but are very very important as all of Structural Design depends upon primarily on Shear Force which in turn generates Shear Stress and Bending Moment, okay let me help you.
Consider a very little flexible stick which is kept rested on supports at its both ends, it is similar to a beam, and suppose you have imposed a little force by your finger or palm as necessary on that stick anywhere on it in its span, that is anywhere in between its two ends. Then What will notice!!!!!
You will notice that the stick is bend like and bow without string that is with concavity downwards.
Now, what is Moment??? Moment is a effect on a body due to the application of force which effects into rotation of that body, and it is a combined effect of force acting and the distance between the line of action of force and the point at which the body is trying to rotate about.
Practical example is any heavy gates like that of at the entrance of a school etc., You will notice that If you push the gate by holding at a nearer distance from the end hinge it takes lot more efforts than pushing the same from a far more distance, here as you increase the distance from the Hinge the moment will increase hence it takes less efforts to push the gate.
Now, in case of a structure the matter is almost same basically, loading on beam/stick or any other structures causes a rotating effect on it, as due to the effect of this moment beam/stick or structures bends this is why it is known as bending moment. Most of the people will be amazed and ask rotating and bending is not same, right????? But actually bending in structures is also a form of rotation, yes it is, I can show that
Now, Let us talk about Shear force. It is nothing but the NET Forces acting upon the structure at a point which tries to shear off that structure from that point. I said NET FORCE because suppose you are putting a 3kg load a point on that stick from the top, rather say flat wooden plank than stick 😀 , and now somehow from the bottom of that point on that wooden plank a 2 kg upward force is given, then actually this 2 kg upward force will neutralize the 2kg out of that 3 kg downward force, hence NET FORCE which will try to shear off the structure (Wooden Plank) about that point is 3 kg 2 kg = 1kg. So the Shear Force at that point is 1kg Downward Force.
Definitions are not at all important in Engineering, what matters is the real concept from the Grass Root Level. But as you have asked, I will define it as follows simply :

Shear Force :
It is the unbalanced vertical force about a section on a structure.

Bending Moment :
It is the unbalanced Moment at a section of a structure.
See the below the complete Video Lecture on this Topic with much more In Depth Detailing and Understandg witn Ease, Even a Child can Learn This, Take a Look
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