Shear and moment diagrams

Imagine you have a big piece of paper, like a piece of cardboard or poster board. We're going to pretend that this piece of paper is a beam that runs left to right.

Now, if you push down on the left side of the beam, what do you think would happen? The beam might bend or tilt! This bending creates something called "shear."

Shear is when a beam bends or tilts because of a force acting on it. For example, if you're holding a pencil in the middle and you push down on one end, the pencil might bend or tilt because of the force of your hand on the other end.

Now, let's talk about "moment." Moment is a fancy word that means "the turning effect of a force." So, if you're pushing down on the left side of our pretend beam, you're creating a moment because you're causing the beam to bend or tilt.

But what do shear and moment diagrams have to do with all of this? Well, imagine that you have a little trained tracking dog that can follow the bending and tilting of the beam caused by the applied forces. As the beam bends, the dog draws lines on the paper to show where the shear and moment are occurring.

The "shear diagram" will show you how the shear force changes along the length of the beam. For example, if the force is the greatest at one end of the beam and decreases towards the middle, the shear diagram will show that change.

The "moment diagram" will show you how the moment changes along the length of the beam. If the bending is greater on the left side of the beam than the right, the moment diagram will show you that, too!

So, in summary, shear and moment diagrams show us how forces applied to a beam cause it to bend or tilt. Shear diagrams show us how the shear force changes along the length of the beam, while moment diagrams show us how the moment changes.