# Free body diagram involving normal force, gravitational force, and applied force.

Question:

Bob is applying a horizontal force to a box across a level floor. The box can slide across the floor without friction.

Identify the forces acting on the object of interest. From the list below, select the forces that act on the box.

A. gravitational force acting on the box (box's weight)
B. force of the floor on the box (normal force)
C. force of the box on the floor
D. force of Bob on the box
E. force of the box pushing on Bob

I know that A, B, and C are true, but I'm confused of D and E.
In free body diagram, do you also include Newton's 3rd law, which is D and E?

Thank you.

## Answers and Replies

Doc Al
Mentor
ysk1 said:
I know that A, B, and C are true, but I'm confused of D and E.
In free body diagram, do you also include Newton's 3rd law, which is D and E?
Careful! The key to a proper free body diagram for the box is to show all forces acting on the box and only those forces. The fact that the box, via Newton's 3rd law, also exerts forces on other objects is irrelevant. (They may be important for another part of the problem, of course.)

Do I include only the forces that act ON the object, but not the forces that the object exerts on others?
Considering that, I think the correct answers are A, B, and D?
Am I correct?

Doc Al
Mentor
ysk1 said:
Do I include only the forces that act ON the object, but not the forces that the object exerts on others?
Exactly.

Considering that, I think the correct answers are A, B, and D?
Am I correct?
Yes you are.

Now I have a clear grasp of drawing the free body diagram.
Thank you so much for your help.