Well, this is what happens when a sociology major attempts to describe physics ...
Carleton -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of James Frysinger Sent: Sunday, January 25, 2009 19:39 To: U.S. Metric Association Subject: [USMA:42574] Re: The real physics (was Small item seen on TV) As long as we are going to be fussy about technical correctness, let's do it right. The public's understanding of physics is woefully thin already. Worse, misconceptions abound, including the one I am responding to here. The astronauts orbiting the Earth are not weightless. Their gravitational weight is the centripetal force that pulls them around in orbit. Likewise, their orbiter's weight pulls it around in orbit. The physics of the situation is that the two orbits can be identical. The weight of an object at Shuttle Orbiter altitudes is roughly 92 % of its weight on the surface of the Earth. At ISS altitudes it's roughly 91 %. Since the astronauts are in orbit with their orbiter and since both experience the same centripetal acceleration, the astronauts feel "weightless" and float around inside the orbiter. A similar phenomenon can be experienced (briefly) in an elevator whose cable has been cut. The elevator accelerates toward the center of the Earth at the value of gravitational acceleration. (We ignore air drag here.) Likewise, any passenger in that elevator, now unsupported by the floor of the cab, will freefall and will accelerate towards Earth equally with the elevator. The passenger will feel "weightless" and will float around inside the elevator until this event reaches its natural conclusion. Do not try this experiment at home. The astronaut and the elevator passenger are willing to believe they are weightless because their visual frames of reference are non-inertial. That is, their frames of reference are accelerating. Also, their compartments are not exerting any upward force to balance their gravitational forces (weights). One can mimic this effect by using a paper cup into which two opposing holes have been made in the side of the cup, just above the bottom. Fill the cup with water while holding finger and thumb over the holes. Over a sink or wastebasket, use the other hand to hold the cup while uncovering the holes. The water will flow out, making nice parabolic arcs. Cover the holes and refill the cup. This time, just drop the cup, uncovering the holes in the process. The cup and water will fall together and no water will be emitted from the holes as they descend together. Any standard high school or college textbook on introductory physics will show the math, which is at the week 6 of Algebra I level. Jim Carleton MacDonald wrote: > You are right, and I've explained the distinction to many people (using as > an example the astronauts whose weight goes to zero but not their mass). > The problem of course is the doctors and nurses never ask you "What is your > mass?" and as a result it falls out of mind. > > Carleton > > -----Original Message----- > From: [email protected] [mailto:[email protected]] On Behalf > Of [email protected] > Sent: Sunday, January 25, 2009 18:28 > To: U.S. Metric Association > Subject: [USMA:42566] Re: Small item seen on TV > > > Carleton, > > In SI, the measurement language in this forum, the kilogram is the unit of > mass. The newton is the unit of force, which includes thrust, drag, lift, > and weight and some other quantities which are also forces. > > Newcomers may not be aware of this factual distinction in SI. > Let's try not to confuse mass and weight (a force). > > In recent medical literature I see the terms muscle mass, bone mass, body > mass, and body mass index ever more frequently, specified in grams or > kilograms. I also see the term percentage fat, but never yet the term fat > mass, which would complete the set of body mass measurements. > > I believe you already know all of the above, but might confuse others by > stating your weight in kilograms. > > Gene > > ---- Original message ---- >> Date: Sun, 25 Jan 2009 14:06:12 -0500 >> From: "Carleton MacDonald" <[email protected]> >> Subject: [USMA:42556] Re: Small item seen on TV >> To: "U.S. Metric Association" <[email protected]> >> >> When I give my weight in kg to my doctor or a member >> of her staff (because that's the only way I know it) >> they try to convert it back to pounds. >> >> >> >> Carleton >> >> >> >> From: [email protected] >> [mailto:[email protected]] On Behalf Of >> Martin Vlietstra >> Sent: Sunday, January 25, 2009 12:38 >> To: U.S. Metric Association >> Subject: [USMA:42535] Re: Small item seen on TV >> >> >> >> Medical staff will record your weight in kilograms >> (that is a legal requirement), but will expect you >> to talk in stones and pounds (another weird UK unit >> of measure). They will be pleasantly surprised if >> you use kilograms for your weight - something which >> is becoming more common amongst health fanatics who >> go to the gym or soldiers (such as my son), both of >> whom like to know how much they are carrying (or >> pushing) as a fraction of body weight. >> >> >> >> You are right, it is a mess, hence the title of a >> publication by the United Kingdom Metrication >> Association entitled "A Very British Mess". (see >> www.ukma.org.uk). >> >> >> >> ---------------------------------------------------- >> >> From: [email protected] >> [mailto:[email protected]] On Behalf Of >> Jeremiah MacGregor >> Sent: 25 January 2009 17:16 >> To: U.S. Metric Association >> Subject: [USMA:42529] Re: Small item seen on TV >> >> >> >> When you say the UK is bi, do you mean they use both >> metric and English equally,? 50 % ? Or is there >> more of a leaning towards one or the other? How are >> both use equally without causing confusion? Say for >> instance in the medical field. Would a doctor speak >> metric and a nurse respond in English? It must make >> for some strange communications. >> >> >> >> Jerry >> >> >> >> ---------------------------------------------------- >> >> From: Stephen Humphreys <[email protected]> >> To: U.S. Metric Association <[email protected]> >> Sent: Sunday, January 25, 2009 11:40:08 AM >> Subject: [USMA:42515] Re: Small item seen on TV >> Because the UK is not metric (it's 'bi') and in the >> case of tyre pressures there are not laws forcing >> the use of metric. >> >> ---------------------------------------------------- >> >> Date: Sun, 25 Jan 2009 08:00:18 -0800 >> From: [email protected] >> Subject: Re: [USMA:42494] Re: Small item seen on TV >> To: [email protected]; [email protected] >> >> Stephen, >> >> >> >> I interpreted the statement to mean that bar and kPa >> were the most common. It doesn't mean the is no >> psi, it just means it isn't very common. If the UK >> is metric then why would psi dominate and not kPa? >> >> >> >> Jerry >> >> >> >> ---------------------------------------------------- >> >> From: Stephen Humphreys <[email protected]> >> To: U.S. Metric Association <[email protected]> >> Sent: Sunday, January 25, 2009 10:27:02 AM >> Subject: [USMA:42494] Re: Small item seen on TV >> Except in the UK (which is part of Europe) where >> PSI dominates. >> Maybe you meant "Mainland Europe" >> >> ---------------------------------------------------- >> >> From: [email protected] >> To: [email protected] >> Subject: [USMA:42439] Re: Small item seen on TV >> Date: Sat, 24 Jan 2009 15:22:34 +0000 >> >> The most common units of measure for tyre pressures >> in Europe are bars or kPa. (100 kPa = 1 bar). >> >> >> >> ---------------------------------------------------- >> >> From: [email protected] >> [mailto:[email protected]] On Behalf Of >> Jeremiah MacGregor >> Sent: 24 January 2009 14:59 >> To: U.S. Metric Association >> Subject: [USMA:42430] Re: Small item seen on TV >> >> >> >> Harry, >> >> >> >> Aren't they suppose to be in pascals or something >> along that line? >> >> >> >> Jerry >> >> >> >> ---------------------------------------------------- >> >> From: Harry Wyeth <[email protected]> >> To: U.S. Metric Association <[email protected]> >> Sent: Tuesday, January 20, 2009 11:39:58 PM >> Subject: [USMA:42388] Small item seen on TV >> A minor point of interest: on PBS's US broadcast of >> the BBC World News tonight, in a piece re the >> resumption of natural gas to Europe, there was >> "footage" showing close-ups of presssure gauges on >> pipeline fixtures out in the snowy fields. One >> showed pressure in kg/cm2, and the other in "bar". >> >> HARRY WYETH >> >> >> >> >> >> ---------------------------------------------------- >> >> Share your photos with Windows Live Photos - Free >> Find out more! >> >> >> >> ---------------------------------------------------- >> >> Windows Live Hotmail just got better. Find out more! >> >> > > > > -- James R. Frysinger 632 Stony Point Mountain Road Doyle, TN 38559-3030 (C) 931.212.0267 (H) 931.657.3107 (F) 931.657.3108
