\r\n A free body diagram is a simplified graphical illustration that shows all the external forces acting on a body or a system of bodies.\r\n A free body diagram is also known as 'FBD' — the commonly used shorthand notation for it.
\r\n To obtain the free body diagram of a body, we isolate that body from its surrounding objects and replace those objects by the forces they exert on the body.
\r\n Note that while drawing the free body diagram of a system of bodies,\r\n we consider only the external forces on the system — that is, the forces or moments exerted by bodies outside of the system.\r\n
\r\n Below are the two steps that we need to follow in order to draw the free body diagram of an object.\r\n
\r\n In mechanics, a free body diagram is commonly used in an exercise known as balance of forces and moments.\r\n Balance of forces or moments is used for calculating unknown internal or reactionary forces.\r\n This exercise requires accounting for all the forces and moments acting on an object.\r\n Drawing a free body diagram is the first step in this exercise and is used to visualize all the external forces and moments acting on the object.\r\n
\r\n\n In this example, we will consider a massless wooden plank that is resting on a wall.\r\n The plank is in contact with two external objects: the floor and the wall.\r\n To construct the free body diagram of the plank, we will remove these surrounding objects and replace them by the forces exerted by them on the plank.\r\n
\r\n\r\n First, we remove the wall and replace it by the force exerted by it on the plank.\r\n Note that the contact between the plank and the wall is assumed frictionless.\r\n So, the wall only exerts a normal reaction force and no frictional force on the plank.
\r\n The resultant configuration is shown in Figure 1(b).\n
\r\n Next, we remove the floor and replace it by the forces exerted by it on the plank. Note that the contact between the plank and the floor\r\n is a frictional contact. So, the floor exerts both a normal reaction and a tangential (frictional) force on the plank. This completes the FBD for the plank.
\r\n The resultant configuration is shown in Figure 1(c).\r\n
\r\n Monkey Mo is being chased by a angry bull Bo. To save himself from the bull, Monkey Mo climbs up a wooden plank that is inclined against a brick wall.\r\n Bull Bo can't climb up the plank so, in an effort to bring monkey Mo down, he charges at the plank. Your goal is to draw the free body diagram of the plank accounting for all the external forces acting on it.\r\n You may assume that —\r\n