Talk:Force/Archive 2

message: This article doesn't talk about internal and external forces and the article's definition makes it seem like all forces are external. Below is a definition from here.
# Internal force: Forces acting between body parts
# External force: Forces acting between the body and environment. It can be distant forces (gravity) or contact forces.
Here is a page with diagrams. -- Kjkolb 11:43, 26 November 2005 (UTC)
thread_title:  Internal and external forces 

message: This article's definition of Total Net Force is unclear.
F = m(dp/dt)
More information can be found here.
: Actually it's dp/dt...the m is wrong in your source.Rpf 14:32, 18 April 2006 (UTC)
thread_title:  Total Net Force 

message: What is the significance of the phrase 'when acting alone'? THis seems to be causing some controversy.--Light current 21:39, 25 April 2006 (UTC)
replies: text: Well a number of forces may be operating on an object in such a way that they cancel each other out and the object doesn't move. I can stand on a bathroom scale and clearly see the force of gravity as a reading on the scale, yet I do not move. It is only a NET force that results in a change in momentum, so it is not sufficient to use it as a definition. If gravity was "acting alone" i.e.: there was no ground, then that WOULD produce a certain rate of change of momentum as I fell. Any physics text, including Haliday, Resnick & Walker discusses the distinction of force and net force (HRW extended edition pg 83 "Force"). Also note that HRW defines a force in the same cause and affect manner, not by simply using an equal sign. Rpf 01:21, 26 April 2006 (UTC)
thread_title:  Acting alone 

message: Enormousdude says that force is not used in higher-level physics because it is redundant to momentum conservation.  If that were true, why would it be used in classical mechanics?  At least in the case of QM, I think it's not used because it's easier to deal with potential.  Force is definitely mentioned in solid-state physics (exchange forces, for example).  Pfalstad 18:02, 30 April 2006 (UTC)
replies: text: Take three fundamental constants (c, G, h) and combine them to get Plank force. What do you get? Surprise - Plank constant h is not there. You see? - force simply does not exist on quantum level. It is just our macroscopic concept to help understand mutual momentum exchange in interactions.
Enormousdude 01:30, 2 May 2006 (UTC)
thread_title:  Why is force not used in higher-level physics? 

message: I was asked why I used a pitcher of water as the object of my examples. I wanted something that weighed a couple of kilos, so that it would be obvious that it would NOT move if you poked it lightly with a finger while sitting on a table, but WOULD move at a standard slow frictional rate if you shoved it harder. I used water because it's a common object with a uniform density, and a couple of kilos is a bigger container than a glass of it. If you have a better object to use as an example for these resistive and variable frictional forces which screw up the immediate idea of defining force in terms of motion, feel free to suggest them. My examples are aimed at simply trying to elucidate why Newton was a genius, and why his second law, in a world with no zero-g demos or air-hockey games, was genius.Sbharris 23:56, 15 May 2006 (UTC)
replies: text: We're talking force. Not why Newton was a genius.8-|--Light current 00:21, 16 May 2006 (UTC)

text: No. On the article page we're talking about force. Other people invesigated force as well as Newton. People still are investigating it, it seems. We should concentrate on 'force' on this page and leave out the Historical controversies if possible. Perhaps some of yout input could go on the Isaac Newton page.8-)--Light current 02:44, 16 May 2006 (UTC)
replies: text: It could go on the newton's laws of motion page.  Pfalstad 12:53, 16 May 2006 (UTC)

text: Yes--Light current 13:42, 16 May 2006 (UTC)
thread_title: Examples

message: The effect of a knock on the head, may be to affect one's judgement.Sbharris 23:50, 14 June 2006 (UTC)
replies: text: What about to 'effect' a change/ Is that a verb or a noun 8-)--Light current 00:04, 15 June 2006 (UTC)
replies: text: Okay, effect can also be used as a verb, but it has to be used in the fashion you do. You can effect an effect, though it sounds stilted to say so. You can also affect an affect (eg. by smiling at a depressed person). But it's usually best to use the common verb and noun forms. And some stuff is just wrong. You can say the effect of gravity is to affect weight. But saying the affect of gravity is to effect weight, is just wrong.Sbharris 17:56, 15 June 2006 (UTC)

text: Then of course you can have the adjective : effective. Yes if the grammar is wrong, just fix it no need for a fuss!--Light current 22:04, 15 June 2006 (UTC)
thread_title: Effect is a noun, affect is (usually) a verb

message: I've semi-reverted this several times, trying to make the examples illustrate the problems our scientific forebears faced. Please remember that Newton's second law arrived so late because friction is so tricky. Static friction PERFECTLY adjusts to balance forces, until it breaks. THEN, as if Nature hadn't been tricky enough there, SLIDING friction shows the same behavior, more or less, allowing velocity to increase quickly until THIS force "adjusts" upward to also perfectly oppose motive force. So now velocity is constant (terminal velocity) and acceleration is zero.  Both of these totally incorrect for a free object with no friction. But the usual and normal behavior for objects of ordinary experience: 1) Either it doesn't move at all when you push it, or 2) it takes off and moves at a constant terminal rate. No wonder it took so long to figure out the correct relationship between force and motion. SBHarris 05:27, 12 August 2006 (UTC)
replies: text: Dear SBHarris, what you are saying is correct for velocity-dependent forces like air resistance, but not for kinetic friction. The kinetic friction force is given by f_k = \mu_k N\, where N\, is the normal force and \mu_k\, is the coefficient of kinetic friction. For a horizontal table, N = mg\,, and \mu_k\, is a constant that depends on the properties of the surfaces of the object and the table, but not on the velocity. Therefore the force of kinetic friction is fixed. To create a uniform motion, the force of the finger is adjusted to be equal to the kinetic friction. Yevgeny Kats 06:30, 12 August 2006 (UTC)
thread_title:  Those tricky frictional forces