How to draw Free body diagram

Free body diagram is a graphical way of depicting various forces acting on a body in a certain scenario. A free body diagram shows how much is the force acting on the body under consideration i.e. magnitude of forces acting on that object and also the direction of those forces.

Suppose you are trying to drag a desk through your classroom. Then how many forces acting on a body in a static state can be depicted easily by a free body diagram.

How to draw free body diagram

To draw free body diagram, as the name reveals, we do not need to figure a whole body under the action of forces. We can just draw a square or a box in place of the body and then draw arrows from the center of the square keeping in view the direction and magnitude of forces acting on a body under consideration. The size of the arrows represents the magnitude of the forces where the direction of arrows represents the direction of forces acting on a body. Suppose you are asked to draw a free body diagram of the situation (dragging desk) described above, then all you need is to draw a square or a box and then draw vectors through from the center of the box outward depending on the number of forces acting on the body. We know that there are four forces acting on our body we are trying to drag; weight, normal force, applied force and friction force. All these forces can be depicted by free body diagram as shown below

Here I’m not going to discuss how these forces act on our body when we try to drag a body down the floor. There is no restriction over how many forces you need to draw in free body diagram, it all depends on the situation under discussion. For example, when you are standing on the ground, there are only two forces acting upon you; one is the force of gravity which tries to pull you down and the normal reaction forces which exactly equals to your weight due to gravity (that’s why you are stable on the ground). In this situation, only two forces need to be shown in our free body diagram.

Now imagine a situation when a body accelerates rightward under the action of applied force. In this scenario, the frictional force can no more balance the applied force. And we can draw the free body diagram as given below

It can easily be understood from the free body diagram that the body under consideration is accelerating rightward while the weight of the body has been canceled by the normal force. The purpose of describing the physics behind the figure is to show the size of arrows, rightward and leftward arrows are not equal in size.

Similarly, we can come across a situation where any body gets under the action of one force only. A book is free falling from your hand and accelerating towards the ground is under the action of one force only and its free diagram is

In this way, you can draw free body diagram of any body under consideration.The most important thing is to disclose all the forces acting on a body on free body diagram and for that purpose, it is essential to have knowledge of different types of forces.