Drawing a free-body diagram
We already know what a free-body diagram is. Let us look at some examples to understand how to draw these diagrams.
● A box on a table
Draw a horizontal line that represents a tabletop, and draw a 2-dimensional box on it. We can now go ahead and mark all the forces acting on this box, along with their directions., using arrows.
First, there is a force of gravity acting on it in the downward direction. Second, there is a normal force acting in an upward direction. As these forces are equal, we draw arrows of equal length. Since they act opposite to each other, they cancel each other out and the next force on the block is zero.
Free body diagram of a box on the table
An accelerating box due to the force acting on it
Just like the previous free body-diagram, we draw a box and a table. We then mark the arrows of equal length for the force of gravity and the normal force that balance out each other.
We will now represent the force acting on the box, in a sideward direction. Let us assume that this force acts towards the right. We also know that a body in motion experiences a frictional force when in contact with a surface (In our case, the tabletop). We will draw the frictional force in an opposite direction to that of the force applied.
As the box is accelerating, we know that the forces acting on the box are unbalanced. The force applied is greater than the frictional force. Hence the rightward force is represented with a longer arrow than the frictional force.
Now, all we need to do is mention the direction of motion of the box (in our case, the rightward direction).
Free body diagram of a box with rightward force
A skydiver descending with constant velocity
In the free body diagram of a skydiver, there is a downward gravitational force acting on him/her, but there is no normal force to balance it out. This is because the skydiver is not in contact with the ground.
Now as the skydiver falls, there is a force acting in the opposite direction to that of gravity and forces him/her upwards. This force is the air resistance that is equal and opposite to the gravitational force. Hence the net force on the skydiver is zero and he/she descends with a constant velocity.
In this case, as the downward and upward forces are equal in magnitude, we draw the arrows of equal length.
