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Force Diagrams or Free Body Diagrams

We know that forces are applied on an object from different directions. We use force diagrams to easily visualize the directions of these forces. A force diagram, which is also called a free body diagram, shows how an object is affected by forces.


Balanced

Let's say we have a soccer ball and it is kicked to the right. To show this in a force diagram, we can follow the steps below. Our finished product will be a balanced free body diagram depicting a force being applied to an object in the rightward direction at a constant velocity.

force diagram of a force being applied to an object in the rightward direction at a constant velocity

  1. First, we draw a square for the object, which represents the soccer ball in this example.

  2. Next, we can address the applied force, Fa, which is the force making the object move. Since the applied force is coming from the right, the arrow representing Fa is drawn towards the right.

  3. When objects move, there is friction (we will cover friction more in the next lesson). To show the friction force, represented by Ff, draw another arrow in the opposite direction of the applied force arrow.

  4. Notice that when an object has constant velocity, both vector arrows (the Fa and Ff) are the same length. This means the forces are balanced.

  5. When an object sits on top of another object, the top object exerts a force on the bottom object and vice versa. Since the soccer ball sits on the ground, it exerts a force downward toward the ground; we call this the gravitational force, which is represented by Fg. This is also known as force of weight, or Fw.

  6. The ground exerts an equal force upward toward the soccer ball; we call this the normal force, or FN. When an object is not moving up or down, the normal force (FN), and gravitational force, Fg, are balanced.

NOTE: The force of gravity is always exerted toward the ground even when the object is in the air. When objects are in the air, there is not a normal force, FN, to oppose the gravitational force.


Unbalanced

Below is an example of an unbalanced free body diagram depicting unbalanced forces. Notice the similarities and differences with the previous diagram.

  • The normal force and gravitational force are the same.
  • The difference is that there is a greater applied force that indicates the object is accelerating; therefore, the friction force is less.

unbalanced force diagram