What is a free body diagram ?

Hint:Here, you are asked what is a free body diagram. Whenever you deal with forces, you need to work with Newton's laws of motions. Precisely, Newton’s first and second laws of motion. While working with Newton’s laws, we follow a systematic algorithm in order to write the equations of motion.

Complete answer:
Newton’s laws deal with particles and give relation between the forces acting on the particle, the acceleration of the particle and the mass of the particle. While writing the equations, we first consider the particle whose equations are to be written and in order to write those equations we follow a systematic algorithm. The first step is to decide the system on which the laws of motion are to be applied. The second step is to identify the forces acting on the decided system or particle of interest.

The third step is to draw the free body diagram. We will mainly focus on the third step. What we do after following the above mentioned two steps, we represent the system by a point in a separate diagram and then draw the vectors representing the forces acting on the system, by drawing we indicate the direction of the force. The collective magnitude and direction of all the forces acting on the system is drawn on a point using a separate diagram and this diagram is known as the free body diagram.

So, a free body diagram is basically a diagram which represents the forces acting on a system.

Note:We have discussed in brief about the systematic algorithm to write the equations of motion and free body diagram is part of that algorithm so you need to remember the systematic way of writing the equations. Keep in mind that we also choose the axes accordingly to write equations properly. The choice of axes is the fourth step.