Free body diagrams (FBDs) are really helpful tools that help us understand how things stay balanced and still in 2D situations.

The main job of an FBD is to take a single object and show all the outside forces that are acting on it. This makes it easier for us to break down complicated systems into simple pictures. These pictures help us analyze and solve problems more clearly.

When we talk about static equilibrium, we say an object is in equilibrium when all the forces acting on it add up to zero. We can express this with a couple of equations:

1. The total force in the horizontal direction ($x$) should be zero:
   $\sum F_x = 0$

2. The total force in the vertical direction ($y$) should also be zero:
   $\sum F_y = 0$

These equations mean that the forces pushing or pulling the object left, right, up, and down balance each other out. An FBD shows these forces, including how strong they are and which way they point. This is especially helpful when we have lots of forces, like on structures such as bridges or beams carrying weight.

Also, FBDs help us see moments, which are another kind of force that causes things to rotate. For something to be in rotational balance, the total of these moments must also be zero:

$\sum M = 0$

FBDs show where we measure these moments from, which helps us make sure all forces and distances are taken into account.

By clearly showing the forces and moments, FBDs help us spot any problems and make better decisions about how things are balanced. So, when we work on problems about static equilibrium in 2D, using free body diagrams not only makes things easier to understand but also gives us a clear plan to apply our knowledge to real-life situations.

In short, FBDs are very important tools for engineers. They help them design and check that structures are safe and stable.