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Index of Subjects --Apple-Mail-84-649708184 Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes Content-Transfer-Encoding: 7bit I'm not sure I see the confusion arises over the kg being a unit of mass. I just covered this in my last astronomy class and why people use the words mass and weight interchangeably when they are two totally different things. Mass is a measure of how much matter an object contains. You measure mass with a scale. A 1 kg bag of sugar has the same mass here as well as on the Moon, as on a scale you would still need a 1kg mass to balance it. Weight is a force, not a measure of mass. The problem is not helped when people still use the English unit of "pound" for both weight and mass. The English system unit of mass is the slug, not the pound. Your weight is the force of gravity acting on your mass and depends on your mass, the mass of the Earth and the radius of the Earth. if you are standing on the Moon (different mass and radius) you will "weigh" less because the gravitational force is less. If you stood on a doctor's scale on the Moon you would find that your mass is the same as it is on Earth. In the metric system, the unit of force is the Newton (the symbol is N). 1 N is the force that is required to accelerate a 1 kg mass at the rate of 1 m/s/s. (i.e. Force = mass X acceleration). At the surface of the Earth, the gravitational force = G x m x M / (r x r), where G is the gravitational constant, m is the mass of a small object (like a person), M is the mass of the Earth and r is the radius of the Earth. If you drop that small object, this force will also produce an acceleration (because Force = mass X acceleration). So F = m x a. Since the force comes from gravity: m x a = G x m x M / (r x r) Note that since the small object is on both sides of the equation, you can cancel it, and thus you get: a = G x M / (r x r) This acceleration gets a special symbol, g, and has a value of 9.8 m/s/ s (which you can round of to 10 m/s/s). This is the rate at which all objects at the surface will fall (in a vacuum). If you go to the Moon, since M and r are different the surface gravity there is much lower, 1.7 m/s/s of about 1/5 that of the Earth. You can also use the surface gravity as a quick way to calculate your "weight" by multiplying your mass (in kg) by g. A 1 kg mass has a "weight" of 9.8 N and a person such as myself with a mass of about 100 kg would have a weight of 980 N. Thus a proper metric scale would NOT be graduated in kg, but in Newtons. I expect there are two reasons they don't. One is that most people would freak if their "weight" appeared to be 10 times what they thought it was, and secondly, since they assume you are using the scale at the Earth's surface, they have "converted" the force back into "mass". Just don't take it to the Moon and expect it to work properly! Pat PS: For those who are curious, a mass of 1 slug has a weight at the Earth's surface of 32 pounds. The kg a unit oef mass On Sep 22, 2012, at 9:58 AM, David & Alison Webster wrote: > Hi Fred & All, Sept 22, 2012 > Explanations of weight/mass usually resemble inconsistent > confessions obtained by torture but I guess that is ok now. > > It seems logical to me that weight should be treated as a special > case of F=ma where a in this case is g and F is measured by exerting > an equal and opposite force upward to keep the mass from falling. > > One could then determine the magnitude of mass as F/g and then > proceed to work out units of momentum, inertia etc from there. > > But unfortunately kg has been defined (or perhaps redefined) as a > unit of mass which leads to everything being a hopeless muddle. > > I would try to reform the system of units dealing with weight, > mass and distance but I am tied up this afternoon. > > Yt, Dave Webster, Kentville > ----- Original Message ----- From: "Fred Schueler" <bckcdb@istar.ca> > To: <naturens@chebucto.ns.ca> > Sent: Friday, September 21, 2012 9:19 PM > Subject: Re: [NatureNS] Correction; Fw: Velocity of light > > >> Quoting David & Alison Webster <dwebster@glinx.com>: >> >>> I just dug out my 1st yr Physics (Weber, White & Manning, 1952) >>> and they consistently refer to 'speed of light' as opposed to >>> 'velocity of light'. Four other sources (1941, 1948, ~1965 &1962) >>> have velocity. >> >> * indeed, it would be ineffective to purge society, or even >> literature, of all who misuse speed/velocity or weight/mass. Or, >> to bring it closer to natural history, all who refer to >> nonhemipterans as "bugs." >> >> fred. >> =============================================== >> >>> ----- Original Message ----- From: "David & Alison Webster" <dwebster@glinx.com >>> > >>> To: <NatureNS@chebucto.ns.ca> >>> Sent: Friday, September 21, 2012 7:44 PM >>> Subject: Velocity of light >>> >>> >>>> Dear All, Sept 21, 2012 >>>> The recent discussion about transmission of electricity, >>>> reading of HEAT and reading some of Energy... brings to the >>>> forefront a question that has nagged me for decades. Perhaps >>>> someone can clarify. >>>> >>>> In the typical elementary physics text we are told that >>>> velocity is a vector quantity, and to quote one text >>>> "velocity... may be defined as the rate of change of position in >>>> a given direction". But "In cases where the direction of motion >>>> does not require to be considered , the term speed is employed >>>> to express the rate of travelling." >>>> >>>> Based on the above I would think that the rate of movement of >>>> light should be called speed, i.e. speed of light but I think it >>>> is always called velocity. >>>> For example, if light from the sun is reflected from two 45o >>>> mirrors then a beam of light could be directed from the earth >>>> back to the sun and, the velocity would then be minus 3 x 10^10 >>>> cm/sec. Or if variously scattered or reflected then the >>>> velocity, relative to the initial sun to earth direction, would >>>> always be less than 3 X 10^10 and after several reflections >>>> might be zero. >>>> >>>> Is th