# Why is free body diagram named as such?

## Main Question or Discussion Point

Let us consider a lift which is moving with a constant acceleration with a man inside. Now we know that the man exerts a force which is different from his weight depending on the direction of the movement. Now, when we consider the free body diagram of the lift do we take into consideration the force applied by the man on the lift (actually lift floor) and the force applied by the lift floor on the man? What are the guidelines for resolution of this kind of questions while writing the free body diagram.

Why is free body diagram named as such? A body in general is acted upon by a few forces and thus is not free.

## Answers and Replies

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jtbell
Mentor
Now, when we consider the free body diagram of the lift
(I added the emphasis here)

do we take into consideration the force applied by the man on the lift (actually lift floor)
Yes.

and the force applied by the lift floor on the man?
No.

In a free-body diagram you consider one object at a time, and you include only those forces that are exerted on that object. If you are doing a free-body diagram of the lift, you include the forces that are exerted on the lift, including the force exerted by the man on the lift. If you are doing a free-body diagram of the man, you include the forces that are exerted on the man, including the force exerted by the lift on the man.

The force exerted by the man on the lift and the force exerted by the lift on the man are equal in magnitude and opposite in direction, according to Newton's Third Law.

Thanks for the clarification.

Can you draw free diagram of the floor of the lift? The man is applying his apparent weight on the floor and the floor of the lift is moving (say up) with a constant acceleration. So there must be net force up on the floor of the lift. This upward force on the floor lift should be equal to the force applied by the man on the lift floor plus the mass of the lfit floor X acceleration.

Is it coorect to apply free body diagram for a part of the body (such as floor the lift)

Doc Al
Mentor
You can choose anything--or part of anything--as your "body" for the purposes of drawing a free-body diagram. But for any given problem, there is usually one obvious choice that makes the most sense. (Although it might not be so obvious at first. )

If you have the choice, for this situation I would pick the man as your "body" and do a free body diagram showing all the forces on him. (If you pick the elevator, you'll need to consider all forces acting on the elevator. Are you given information about those forces?)

It all depends on what you are asked to find.

Dale
Mentor
As mentioned by others, it is up to you to choose where to draw the lines that define your free body. You can draw that line anywhere you wish, and you only consider forces that cross the line. Usually, you want to look at the question and draw a body that has the force you are interested in, and as few other forces as possible.

To reconfirm, in this situation of lift floor, what will be the equation from free body diagram? Is the below eqn. correct, or did I miss something?

Net force on lift floor = lift floor mass X acceleration = Apparent weight of man + lift floor weight.

Thanks for the help.

No, I think the equation can not be correct. Is the below one correct?

Net force on lift floor = lift floor mass X acceleration = Upward pull force - (Apparent weight of man + lift floor weight).

My specific point in this equation is, do we take the weight of man or the apparent weight of man in the above equation? I think apparent one we should consider since that is what the man applies on the floor.

Dale
Mentor
You are correct, it is the apparent weight of the man. The weight of the man is the force of gravity and it acts on the man. Since that force acts on the man it is acting on a different body than the lift floor. The apparent weight of the man is the force that he exerts on the floor.