Posted by James R. Frysinger on January 06, 2000 at 00:32:14:
In Reply to: Weight of Hubble posted by Darrel on January 03, 2000 at 19:14:42:
> > > The Hubble weighs about 24000 lbs on Earth. That would be about 11000 kg.
> > > It is about the size of a big tractor-trailer truck (I don't know if there
> > > is a metric equivalent for this unit :>).
> > -
> > -The figure 11 000 kg is metric. Due to the ease of using prefixes in the international metric system (the SI), you could (if you wished) eliminate those zeros by moving the decimal place left three places and writing this as 11 Mg. All prefixes scale by powers of ten and so moving the decmal place makes scaling easy. You can't do that, of course, with ounces and pounds.
> > -
> > -There is a metric unit that is not part of the International System of Units (SI) but is acceptable for use with the SI. That is what we in the U.S. call the metric ton and most others call the tonne (its symbol is "t"). It is equal to 1000 kg, so you could say the HST weighs 11 metric tons. There is no ambiguity with that as there is between short tons and long tons.
> > -
> > -So 11 Mg or 11 000 kg are perfectly good and 11 t (or 11 metric tons) is acceptable as well.
> 24 000 lb is a WEIGHT and 11 Mg is a MASS. The relation you stated, is only true on earth, at sea level, where "g" is 9.806 65 m/s^2. On earth, Hubble would weigh 11 000 kg x 9.806 65 m/s^2 = 108 kN. In orbit, where Hubble is, at about 600 km altitude, Hubble would still have 11 Mg of mass, but its weight would be reduced to about 84 % of its earth value, or about 91 kN.
> Your use of pounds is confusing. You are equating it to kilograms instead of the proper unit of newtons. Forget pounds! Space craft should alwyas be quoted by their mass in kilograms. This way, one can deternmine there weight by knowing the location. If you measure Hubbles WEIGHT in orbit and convert it to KILOGRAMS, instead of newtons, using the common 2.204 6 factor, you will get the wrong result.
> Use of English units promotes errors and confusion. Only use English units if you want to make a mistake and confuse yourself and others. If you want to be correct, use metric!
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-Darryl, you are right on concerning weight and mass! I agree wholeheartedly. And in physics or engineering calculations where it really counts to keep those distinct, I do. Unfortunately, "weight" is commonly used to mean "mass" in everyday conversation and also in commerce; there is even documentation at the level of standards organizations that states this and allows it in those contexts. But that has nothing to do with the question I answered.
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-I addressed the issue of whether there was a "unit" for 11 000 kg. In doing so, I carried along the terminology used by Ed Cheng in his message, to which I replied. I felt that Ed was really thinking in terms of mass when he said "weight", especially when he converted pounds to kilograms. But then, he was probably refering to pounds avoirdupois as opposed to pounds (force).
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-And, AMEN!, again to the advantage of using the international metric system as compared to the American Hodgepodge of units, which are neither colonial, English, nor imperial, by the way!
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