Talk:Free body diagram

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I found this page rather muddled. The basic points about the free body diagram are missing:

• free body diagrams are useful for statics and dynamics, depending on whether or not the system is in equilibrium,
• a free body must never include its supports (otherwise it would not need a reaction force to maintain equilibrium),
• the force vectors on the diagram must show magnitude, direction and point of application.

Rather than getting bogged down with weight and inclined planes, it would be more useful to give clear instructions for static analysis (resolving forces and taking moments). That could lead to non-rotational dynamics, showing statics is a special case where acceleration is zero), and possibly also going on to rotational dynamics, introducing moment of inertia.

--Jack Hale 10:52, 1 May 2006 (UTC)

I am in absolute agreement with you. A free body diagram does not contain any objects other than the one which is being acted on by certain forces. The given examples are not, in my opinion, even free body diagrams. I am editing the first paragraph to define a FBD before it describes what it is used for.--Erik the guy 06:52, 5 November 2006 (UTC)

Tried to clean up. -AndrewDressel (talk) 02:54, 5 March 2008 (UTC)

[edit] Merge?

I think this article should be merged with section 1.2 of Classical mechanics. Jonathan48 18:28, 16 Feb 2005 (UTC)

I considered a merge also, but then decided that maybe some example figures of free-body diagrams could go here and was going to make one or two. As-is I'd say its worthless, but I think it'd be worth keeping seperate with a figure. Laura Scudder 00:35, 6 Mar 2005 (UTC)

I do not think a merge will be helpful. I, and several of my classmates, have found it handy to use this individual page. It would benifit from being expanded however.--Erik the guy 06:50, 5 November 2006 (UTC)

[edit] Types of Forces

I believe that this article should be editted to include

Contact forces

• Friction
• Tension
• Normal force
• Air resistance
• Applied Force

Non-Contact forces

• Gravity
• Magnetism

--Erik the guy 07:34, 5 November 2006 (UTC)

This approach is, in my opinion, a dangerous road to tread pedagogically. It makes the student think that there exists an exhaustive list of every type of vector they'll ever need, and if they memorize that list, everything will be ok. But a physics student is constantly exposed to additional forces over their tenure. Even the sticky issue that there's no such thing as a "contact force" is further complicated by the question of what a fictional force is - it doesn't seem to fit into either of those categories, but should certainly be listed on a FBD when appropriate. Rather, the student should be encouraged to think about the specific situation at hand and attempt to list all the relevant effects on the object. I've always felt it was a mistake to categorize forces at this level, the whole beauty of FBDs is that they make no distinction between forces, every force is just a vector.

What I'd like to do is change the vector section to a very general discussion of how to construct a FBD, and then move the existing material into a separate "Example" section. I'll try to do that and see how people feel.

--Hyandat 23:53, 5 November 2006 (UTC)

Yah i sort of agree with you. We cannott offer a definitive list of all possible forces. We should attempt to compose a list of some of the most common forces however, and those given above are some of the standard ones. Fictional forces can even be lister seperatly from the other two (since they are not technically forces) I just think the wiki needs a bit more info than weight and normal force.--Erik the guy 00:32, 6 November 2006 (UTC)

[edit] Internal Forces

Needs generalising to the use in eg beams (sometimes called a'cut-body diagram'??), to show internal forces, such as internal moment and shear.Linuxlad 11:35, 10 January 2007 (UTC)

[edit] Issues with additional diagram

A free-body diagram of a block resting on a rough inclined plane, with its weight (W), normal force (N) and friction (F) shown.

I've moved this additional diagram here to discuss the issues I believe it has:

1. It is redundent, showing the same situation as the existing diagram except for the ramp sloping down to the left instead of the to the right.
2. It includes the ramp, which is supposed to be replaced by forces in a free body diagram. The article already states "All external contacts and contraints are left out."
3. It does not have the force arrows at their point of action: the normal and friction forces act on the bottom of the block, not not at the center of mass.

If another example would be helpful, it should at least be of a different object under different forces, and it should comply with the rest of the article. -AndrewDressel (talk) 14:08, 29 March 2008 (UTC)

The picture Image:Free_body_diagram_mod.png has been changed since the above comments were made. The current version, as of June 9, still has the following issues:

1. It includes the ramp, which is supposed to be replaced by forces in a free body diagram. The article already states "All external contacts and contraints are left out."
2. The normal force has been repositioned to reflect some type of analysis which cannot be performed without additional information such as the coefficient of friction and the acceleration of the block, if any. Wait a sec. My mistake. I see now that the caption specified "resting" on a ramp. While less general, that would be sufficient to located the resulting normal force correctly.

See further discussion below. -AndrewDressel (talk) 20:37, 9 June 2008 (UTC)

[edit] Image:Free Body Diagram.png

It needs to be modified, since it shows the frictional force going through its true line of action but the normal reaction going through the block's centroid, which is not, which is somewhat misleading. Thanks, BigBlueFish (talk) 16:08, 9 June 2008 (UTC)

To where do you propose to move the normal force arrow? Its exact location is not known without additional information and further analysis. -AndrewDressel (talk) 16:26, 9 June 2008 (UTC)

I may stand to be corrected, but since the weight acts through the surface of contact, the normal reaction must act through the point on that surface vertically below the centre of mass, or there would be a net rotational acceleration. BigBlueFish (talk) 16:39, 9 June 2008 (UTC)

That would eliminate any moment due to the gravitational and the normal forces if they were in opposite directions, which they are not, and would not address the moment generated by the friction force. All of this only applies to the static case, which is not given. In fact, if the coefficient of friction is zero, then the normal force is correct as drawn. Otherwise, the acceleration must be known in order to correctly place the single, resultant normal force arrow. -AndrewDressel (talk) 16:50, 9 June 2008 (UTC)

If their lines of action intersect then there is no moment regardless of direction. BigBlueFish (talk) 17:06, 9 June 2008 (UTC)

This contradicts your statement above. Since the normal force is not parallel to the gravitational force, their lines of action will always intersect somewhere, no mater where the normal force is applied. That would mean that the normal force and the gravitational force could never create a moment, which is not true. -AndrewDressel (talk) 19:44, 9 June 2008 (UTC)

The normal and gravitational forces only produce a moment if the weight acts outside the surface of contact. Within this boundary the forces must cross on the surface or the frictional force exerts a moment with the other two. BigBlueFish (talk) 20:57, 9 June 2008 (UTC)

Yes, for a three-force member in static equilibrium. However, there is nothing about the diagram or the article that states that the block is not moving. -AndrewDressel (talk) 01:02, 10 June 2008 (UTC)

Even in a dynamic case the block has no angular acceleration, and all acceleration occurs in a plane perpendicular to the normal reaction, so it is not affected. BigBlueFish (talk) 17:06, 9 June 2008 (UTC)

No angular acceleration is necessary for angular momentum to be a factor. A braking car will experience "weight shift" to the front wheels even if the suspension and the wheels are rigid. No rotation is necessary for this to occur. -AndrewDressel (talk) 19:44, 9 June 2008 (UTC)

Forgive me if I find this a little hand-wavey. I'm not aware of any case in which the vertical force on the wheels of a braking car is imbalanced, or indeed there would be a net moment on the car. A truly rigid car has no angular momentum because its path has an infinite radius of curvature. BigBlueFish (talk) 20:57, 9 June 2008 (UTC)

No hand-waving necessary. It follows directly from the definition of angular momentum about a point, H_p = r_/p x mv, and the correct application of angular momentum balance. You may read about it in the Bicycle and motorcycle dynamics article. The citation includes a link to a PDF copy of a statics and dynamics textbook published by Oxford University Press. -AndrewDressel (talk) 01:02, 10 June 2008 (UTC)

Besides whatever effect or phenomenon you're trying to describe, the simple case that the diagram is trying to describe either shows a block in mechanical equilibrium on a plane or a block sliding down it, both of which are cases in which the free body illustrated experiences no net moment. BigBlueFish (talk) 20:57, 9 June 2008 (UTC)

There is no net moment only in the static (or constant velocity) case. If there is acceleration, then the net moment about any point must equal the cross product of the position vector from that point to the center of mass with the product of mass and acceleration vector of the center of mass:M_p = r_cm/p x ma_cm. Using a point that coincides with the center of mass provides no information, of course, because the position vector has zero length: the net moment about the center of mass is zero.

As for what I am showing in that diagram, it happens to coincide with option c of the three "sensible" ways to represent contact force distributions shown on page 95 of the same textbook mentioned above. One more option I've seen in a different text that is slightly more complicated is to indicate an unknown distance "d" from one edge to the point of application. -AndrewDressel (talk) 01:02, 10 June 2008 (UTC)