Talk:Drag (physics)

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Help with this template Comments: edit – history – watch – refresh The article does supply lots of interesting information but fails to indicate the full scope of the subject. While free falling objects through the atmosphere is indeed interesting it is only a small part of drag. Drag in water is largely ignored. Showing how a foot of fast flowing water can easily knock the feet out from under a grown man would illustrate the power of our second most abundant fluid when it moves. The large amount of time it would take a bowling ball to come to rest through sheer air drag could be contrasted with the huge wind loads that can form on buildings. Air drag is relatively weak but given enough speed and area to work with it has power. In short the article fails to show the importance of drag in our daily lives. RobertJDunn (talk) 09:28, 8 December 2007 (UTC)

Contents 1 Merge with Drag (physics) 2 b vs Cd 3 v^4 term 4 Clarity of v^2 term in drag equation 5 Discussion of "Power equals force times velocity" 6 Limit of low/high velocity 7 terminal velocity 8 Would this formula derivation for the velocity over time be useful? 9 Discussion for generalizing the page. 10 Units for the Equation

[edit] Merge with Drag (physics)

Merge?

Oppose. Two things:

It is a different subject. Fundamental and useful though the drag equation is, it is inapplicable to complete aircraft, ship hulls with wave drag, and just a minute part of what would be general drag article.

I believe that our efforts here are best directed to trying to make articles that are accessible, useful, and accurate. From the history and talk on drag_equation it can be seen that, even with such a narrowly bounded subject, a good deal of work is required to keep it comprehensible. Merging into a vast treatise on drag will not serve other users. Meggar 04:50, 5 March 2006 (UTC)

• Merge

The phrase "Drag Equation" is ambiguous. Are we talking about drag in a medium like the atmosphere, or a medium more like thick honey? Drag (physics) is hardly a "vast treatise" and would serve to give this equation context and help a reader quickly and easily find related topics - like terminal velocity and drag through a viscous medium.JabberWok 03:21, 7 March 2006 (UTC)

• Merge --MarSch 15:06, 13 April 2006 (UTC)

Oppose. Does it matter what type of medium we are talking about here? Density can be changed for the type of medium making it applicable to all mediums. It is an extensive topic in its own right. I think it is more fit as a separate article where it can be modified and added to on its own, as merging it would only bring more confusion to it.--Jadian Prime 19:06, 5 May 2006 (UTC)

I'm not quite sure what you're talking about? The issue here is whether Drag equation should just be a redirect into Drag (physics). The physics article is more general and already contains all the information found in the drag equation article. JabberWok 21:35, 5 May 2006 (UTC)

Merge - however the topic would need some scaling down. I think merging would allow a better discussion of why the equation is inaccurate in many cases - as meggar noted. Fresheneesz 06:24, 30 May 2006 (UTC)

Also, a large portion of this page was a simple duplicate of the information on drag equation, and so I removed it - replacing it with a very short suppliment. The information really belongs on one page (here). Fresheneesz 17:16, 30 May 2006 (UTC)

• Merge completed - I merged the pages, but moved the long derivation to its own page to make this page more concise. Fresheneesz 23:06, 31 May 2006 (UTC)

Unmerged. It works better here to talk and come to consensus. In a paragraph above Jabberwok expressed an opinion that is not correct. The term "drag equation" is not ambigious. A quick Google search [1] shows a lot of articles about this exact equation. The top one on the list is a page from NASA titled "The Drag Equation", of course about just this equation and none other [2]. This is not because the people who wrote it are lacking in knowledge or because it is oversimplified, but because that is unambigusly the long established usage of that phrase. As with wikipedia policy on neologisms, we don't use phrases for what we think they reasonably ought to mean, but as they are actually used. Therefore: The long discription and history about the drag equation belongs in an article titled "Drag equation". A suitably brief reference to it should be part of a general article on drag along with a lot of other approaches to the subject. Meggar 05:12, 15 June 2006 (UTC)

Merge The basic result of a merge would be to reorganize a chaotic mass. Technically, the term drag is only pertaining to fluid dynamics. Physics is the more broad subject, yet to include drag in physics might not be entirely correct. Sadly it is clear that the effort to merge drag with physics is a cover-up to hide the sorry state of our fluid dynamics entries. In time I suppose I will work to amend this. For the time being a merge with physics is most appropo I suppose. This will free up the base wiki for other things like a disambiguation page (slang, races, transvestites etc.) Eventually, once fluids is well developed, drag should join its fellows there.

[edit] b vs C_d

I would like to see some discussion about the relationship between b and C_d. Also, it would be very interesting and useful to have more discussion on when the approximations are accurate, and more importantly, when they are not accurate - and why - and what to do in those cases. Fresheneesz 06:51, 30 May 2006 (UTC)

[edit] v^4 term

there's a v^4 term and higher even powers of v involved. I would like to see something about this in the entry.

[edit] Clarity of v^2 term in drag equation

"Since v is a vector, it isn't entirely clear what v^2 means, since usual matrix multiplication doesn't work. Does the drag equation hold if you use component-wise multiplication to calculate v^2, or do you really mean speed, not velocity? It would be great if this could be clarified in the article."

Seriously, it is clear that v^2 means speed squared, in the direction of the velocity. Mechanical Rose 06:12, 12 December 2006 (UTC)

Also, I edited the page to say v is "the speed of" the object. Now it is unambiguous and also technically correct. But I really want v to be velocity - maybe we should have |v| in the equation? Mechanical Rose 05:21, 28 March 2007 (UTC)

The problem here is with the standard notion. I checked my textbooks and found that when they are talking about vectors they use indicate a dot product. Normally the vectors are indicated by small bold letters. If the context were matrix math they would use AB^T. In a vector context the transpose is assumed as nothing else really makes sense. The shortcut v² is often used for . Now the dot product results in a scalar equal to |v|² which is exactly what we want in our case . We are however being a little loose with the signs and if we want to retain v as a vector it could causes problems.

Consider our opening equation:[[Media: ]]We are talking about forces which are vector quanities. This formula assumes and as being positive in a down direction and in an up directions. This is fine as none of these forces can ever be negative using these directional assumptions. The problem occurs at low Renynolds numbers where Stokes's Drag Equations applies. v appears unsquared so is still a vector. This works well as it rightly reports a force being applied in the opposite sense of the velocity. Now if we substitue this into our loose equation above we get: which is bound to cause problems. I think this is a problem we must solve before we can fully describe how to use Reynolds numbers to determine what estimating formula to use. RobertJDunn 15:50, 6 December 2007 (UTC)

[edit] Discussion of "Power equals force times velocity"

Someone added a paragraph about how the Power equation (proportional to v^3) is wrong. This person is incorrect, and I have removed his/her comment, and I will explain why here.

The person claims that work = force * (delta velocity). This has the right units, but is incorrect. Work is actually equal to force * velocity. If you push a block through a fluid at 1 m/s by applying 1 N of force, you are doing work. You are doing 1 watt of work, and the fluid is heating up at a rate of one watt. Saying that you are not doing work because velocity is not changing is absurd.

Put another way, d(work) = force * d(x). Differentiating both sides with respect to time, we find that d/dt(work) = power = force * d/dt(x) = force * velocity.

I am a mechanical engineering student at MIT. Trust me, or buy a mechanics textbook. Mechanical Rose 06:12, 12 December 2006 (UTC)

I agree with User:Mechanical Rose and it looks like her comments have been incorporated into the article; however, I still believe that the text is is incorrect when it talks about the power quadrupling for a doubling of velocity. In fact the power goes up by a factor of 8 (as stated later on) not by 4. I am not sure what the protocol is for correcting this so I thought the discussion page was the best place to mention this. TRamsey 03:06, 14 January 2007 (UTC)

The text actually talks about the Force quadrupling for a doubling of velocity (which it indeed does, being proportional to v^2). Power gets multiplied by 8 (since it is 4 times the force in half the time). Also, I am a guy. Mechanical Rose 05:24, 28 March 2007 (UTC)

I think it might be good if the example hp/speeds for this section are adjusted. If you take the 80hp for 100mph figure, then 200mph would require 8 times the power, or 640hp, which sounds a little high, unless you are driving a brick. I've looked around, and haven't found any definitive sources, but I know I've heard that it takes around 400 hp to get to 200mph, and I found a comment on a car thread indicating 450hp. In any case, many of the people that read this article are reading to understand vehicle performance better, so a real-world example might be in order. --216.84.45.198 14:28, 24 July 2007 (UTC)

Adjust the numbers if you like, but it is the concept, the abstraction, that we are trying to state here. If you have data specific to cars it would go well in the automotive aerodynamics page. Meggar 06:01, 26 July 2007 (UTC)

[edit] Limit of low/high velocity

Shouldn't one mention the concept of a Reynold's number here? --Benjamin.friedrich 15:28, 1 November 2006 (UTC)

Agreed, strongly. -- David W. Hogg 05:21, 9 January 2007 (UTC)

[edit] terminal velocity

The equation for the approach to terminal velocity is not particularly instructive, and only applies in the low-speed limit (viscous drag dominates). This should be noted or the equation (and section) should be dropped, leaving only a reference to the terminal velocity article. -- David W. Hogg 05:21, 9 January 2007 (UTC)

[edit] Would this formula derivation for the velocity over time be useful?

I am new at this so I am wondering if this work I did as I was studying this topic would be a useful addition and if so how to add it. You will need a mathml enabled browser to view it. Probably a links from where the formuls is cited on this page would be best.

Formula derivation page[3]

RobertJDunn (talk) 02:13, 5 December 2007 (UTC)

[edit] Discussion for generalizing the page.

I have been thinking that the scope of this page is too narrow for the topic of drag. This is in part due to the example given of an object free falling through atmosphere. Though simple it does not tress the very important concept of drag force always being created in an direction directly opposite to the velocity.

I have been thinking of various examples that might be better that might be used in concertthe the given example of an object falling.

1. A cart on perfect wheels (no friction) is taken up to the speed of 100 kilometers per hour and then let glide. Show how drag slows it down. This is a no external force example.

2. A plane on level flight at a constant velocity. Two external forces, one provide by engine and the other by gravitiy. Shows how lift generated by wing shape adds a drag component to the forward velocity of the plane. This is basically an equilibrium example with the two external forces, one vertical and the other horizontal balanced out by drag.

3. A glider flying at a contstant velocity and falling at a constant rate(this is around 50 meters forward for every meter dropped). One external force but the shape of the wing causes the glider to be squeezed forward between the graivitation force and the vertical component of the drag force. The total drag force generated is of course opposite to the slightly downward velocity. Nice example of one external force generating motion in an oblique direction.

4. A sailing ship tacked slightly off from the wind being directly behind it. With a good hull design they can move faster than the wind. Considering good design the effect of the wind on superstructure should be minimum so we have the drag of the sail through the air and the drag of the hull through the water.

5. Water flowing downhill through a pipe. The dag force would tend to push the pipe the opposite direction to the flow of the water.

Just suggestions here. Feel free to correct or add or subtract. I feel that this general approach would add greatly to people being able to get a better grasp on the subject faster.

RobertJDunn (talk) 18:38, 7 December 2007 (UTC)

[edit] Units for the Equation

Shouldn't the equation have units of measurement listed below it, where the legend is? Just to clarity things.

Also, for density in SI, is it measured in kg/m^3 or g/cm^3? 203.122.106.197 (talk) 18:02, 3 May 2008 (UTC)