 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Natural Science Forum / Physics / Relativity / December 2004'nother conundrum!
A (University) site somewhere said that when the sun is overhead, that a person is pulled toward the sun with a force of about an ounce (compared to sunset) due to sun's tidal pull on him. Unfortunately, they didn't say whether this effect is measurable on a bathorom scale, an experiment I am eager to do if it isn't a waste of time. So last night, I wasted all-too-much time trying to figure it out. All I came up with was either: YES, because every day at high noon, you're being "taken up", much like in the book of revelations, for the same reason that the oceans are sloshed around by the moon. NO, because the bathroom and the scale are being pulled toward the sun in just the same way. And maybe orbital centrifugal force has something to do with it, too. I tried to solve this by considering an extreme case of tidal pull: what if the moon broke loose and fell into the earth? As it got larger and larger directly above us in the sky, would we become noticably lighter in the last few seconds of life, when the moon filled the sky from horizon to horizon and the darkness was pierced only by the sudden, white glow of the lunar surface hitting the atmosphere at mach fifty? But like so many other things, I couldn't figure that out either. =[ d, to whom someone once said "you know david, you think too much about things that don't need to be paid attention to", to which I replied "That's because there's nothing else in this god-forsaken post-college business jungle interesting enough to pay any attention to at all". Dear DavidBowman: >A (University) site somewhere said that when the sun is overhead, that > a person is pulled toward the sun with a force of about an ounce [quoted text clipped - 4 lines] > time. So last night, I wasted all-too-much time trying to figure it > out. All I came up with was either: It is better with a very accurate setup, and a test mass that does not: - eat, - drink, - breathe out moisture at an unknown rate, - urinate, - defecate, and - shed dead skin cells. ... these things can easily hide an ounce (or more) in 6 hours. The Foucault pendulum shows a lot of what you describe. The problem is, that the Moon's effect is also a little larger than the Sun's. A pendulum's period depends on a local value of g, and if there is slightly less "net" mass (remember shells...) contributing to this g, then the period will be altered ever so slightly. David A. Smith Are you saying this would WORK? The difference is about .1%. So If I put a I Kg weight on a postal scale at 6am, it will weigh 999 grams (times the sine of my latitude) at noon?? =[ d Dear DavidBowman: > Are you saying this would WORK? > > The difference is about .1%. So If I put a I Kg weight on a postal > scale at 6am, it will weigh 999 grams (times the sine of my latitude) > at noon?? Try it at noon with a New Moon, so you can "double" the effect. I doubt that will vary by as much as you have calculated, but a strain gauge based sensor could likely detect a difference. Tides are as big as they are because where the effect is strongest, is where the effect of the Sun and Moon are at 90 deg to the direction of g. Nothing to retard the "gravitational force" of the other body on the water, except the momentum of the fluid itself. Foucault pendula are *very* sensitve. You'd be hard pressed to develop something that was more sensitive, without extensive shielding and temperature controls. I believe the Foulcault pendulum was responsible for locating a 3 meter "lump" in the crust of the Earth that follows the Moon around... David A. Smith > A (University) site somewhere said that when the sun is overhead, that > a person is pulled toward the sun with a force of about an ounce > (compared to sunset) due to sun's tidal pull on him. Approximately, depending on the person's actual weight. The sun's gravitational force is about 600 parts per million of the earth's. > Unfortunately, they didn't say whether this effect is measurable on a > bathorom scale, Not on mine, or any I have ever seen. But a laboratory scale with an accuracy of a few milligrams weighing a kilogram of brass should easily do it, as long as the scale is NOT a balance. > Are you saying this would WORK? As long as you use an accurate-enough spring scale, sure. but finding such a scale is non-trivial. Several Eotvos-type experiments used the sun's gravity, not the earth's (even though the strength is much less, they can reduce systematic errors even more so their sensitivity is higher). Tom Roberts tjroberts@lucent.com Man, that is AMAZING! I'm definitely going to do it. was I right about the effect being porportional to the sin(L), where L = lattitude expressed as a fraction of 90 degrees, or is it just proportional to L? -d Dear DavidBowman: > Man, that is AMAZING! > > I'm definitely going to do it. was I right about the effect being > porportional to the sin(L), where L = lattitude expressed as a fraction > of 90 degrees, or is it just proportional to L? Read what Tom Roberts supplied... 600 parts per million. If your spring (elastically) deflects 1 meter to support the 1 kg mass, the the change in deflection over 12 hours, at New Moon, will be on the order of 1 mm. You could easily get that much change just from thermal effects on the spring... And it is not an easily located spring... Your latitude will just pull it off kilter. You might do like Tom glancingly recommended, copy an Eotvos experiment. Place a curved mirror on your weight, and shoot a laser at the mirror at some steep angle. Where it ends up on the wall, some long distance away, should make a sine wave with the frequency of 24 hours... I'll be gone until Monday. I hope one and all have a Happy and Safe New Years... David A. Smith > was I right about the effect being > porportional to the sin(L), where L = lattitude expressed as a fraction > of 90 degrees, or is it just proportional to L? Neither. Assuming your scale only responds to vertical motions (required by most spring scale designs), the gravitational pull of the sun is proportional to cos(theta) where theta is the angle between vertical on your scale and the line from scale to sun. So at the northern-winter solstice the maximum upward pull is at solar noon at 23.5 S lattitude. Tom Roberts tjroberts@lucent.com The Georga State University Astrophysics site http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html (at the very bottom of the page) says that for a 160 lb. person, the solar tidal pull is .09 lb. (call it .1), which is more than an ounce. Can't a good doctor's office scale weigh a person to within an ounce? It also says that the force of the moon is .0005 lb, which is less. Did they make a mistake and switch the two? Assuming that one or the other is correct, that's a difference of one part in 1,800. So at 45 deg latitude, a scale would have to weigh a 1Kg mass to an accuracy of about .2 mg to see the effect. Is this reasonable for, say, a high-school lab electronic scale, or is this NIST-type accuracy? =[ d PS It also gives the tidal pull for other bodies. In the case of Jupiter, it;'s .0000056 lb, stronger than the much-closer mars! |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2009 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.