Q.A spider of mass $50\text{ g}$ is hanging at rest on a single vertical thread of a cobweb, as shown in the figure. What is the tension in the thread?

A free body diagram (FBD) isolates one chosen object and shows only the forces acting on it — never the forces it exerts on other bodies. The systematic procedure: (1) identify every force acting on the object; (2) represent the object as a single point; (3) draw each force as a vector arrow from that point, in its correct direction and (where relevant) relative magnitude; (4) apply Newton's second law along each convenient axis.

The single most important — and most often violated — rule is that forces the object exerts on other bodies must never appear in its own FBD. For example, a book resting on a table has exactly two forces in its FBD (weight $mg$ down, normal force $N$ up); the (Newton's-third-law) force the book exerts back on the table, and the force the book exerts (upward) on the Earth via gravity, belong in other objects' FBDs, never the book's own.

For a system of several connected or stacked objects, the standard technique is to isolate one object at a time, giving each its own FBD, then solve the resulting set of simultaneous equations together — e.g. a stack of books, where each book's FBD includes its own weight, the downward push from anything stacked above it, and the upward normal force (from the book, or table, below) that must support the weight of everything from that point upward. A correctly drawn FBD is the essential, error-preventing first step before applying Newton's second law to any mechanics problem.