Let us quickly study two very simple cases of free-body diagrams to make the principles a little clearer.
A ball is at rest on a table, as shown below below. Two forces act on the ball, gravity (red arrow) and the supporting normal force from the table (blue arrow). They have the same length, but opposite direction and thus add up to zero. The two forces are balanced. The free-body diagram on the right shows this.
The same ball is on the same table, but now a string is attached to it. The string runs over a pulley and is connected to a weight that can drop. This situation is more complicated than the one above, but we can also draw the corresponding free-body diagram, see the right drawing below. Now the free-body diagram shows that the forces are unbalanced and there is a resulting force acting on the ball which will make it move.
You can, of course, also draw a free-body diagram of the weight that hangs down from the string. This diagram will have two arrows, one pointing straight down and one pointing straight up. The up arrow would be again the string force, and the down arrow that of gravity. Together with Newton's second law (which we will discuss soon), these two free-body diagrams would enable us to solve the entire problem.
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