# Explain free body diagram and draw the free body diagram of a ball of weight W placed on a horizontal surface

A body in equilibrium is subjected to a number of external forces and reactive forces, provided by the supports. The equilibrium of such bodies may be considered by removing the supports and replacing them by reactions, which they exert on the body. The body so isolated from the system is called as a free body.

When this isolated body is drawn, with all the forces and reactive forces acting on it, such a diagram is called a free body diagram.
Free body diagrams are found to be very useful when two or more bodies forming a system are subjected to more than three forces. In such cases equilibrium equations for each body can be formed by drawing its free body diagram.

Consider a ball of weight W placed on a horizontal surface as shown in fig.(a).
The support will exert a normal reaction (RA)
to the body. The point of application of Ra will be
the point of contact, i.e. A. Now according to the
principle of equilibrium of forces reaction RA must be equal and opposite to weight W. The free body diagram of the ball which can be obtained after removing the support is shown in fig.(b).

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