10 Jun 08
3 edits
Originally posted by FabianFnasIt is an incoherent question. Weight is a measure of the gravitational force acting on a body on the earths surface. The earth is not a body on the earths surface and thus to talk about its weight (without an expanded definition - which you did not specify) is incoherent.
How much does the Earth weigh?
Is this an interesting question?
Is the answer interesting?
Is the answer as obvious as one can imagine?
We could use an expanded definition of weight to mean a the gravitational force on a body on the surface of a given planet, but that too would not work. Maybe you could use the gravitational force of the sun, in which case the earths weight would be Zero at its present position (yes position matters in the definition of weight and the centrifucal force due to the motion of the object is included too.)
10 Jun 08
1 edit
Originally posted by twhiteheadYou're parsing. You know what the kid meant and you're trying to big-time him.
It is an incoherent question. Weight is a measure of the gravitational force acting on a body on the earths surface. The earth is not a body on the earths surface and thus to talk about its weight (without and expanded definition - which you did not specify) is incoherent.
We could use an expanded definition of weight to mean a the gravitational force on ...[text shortened]... definition of weight and the centrifucal force due to the motion of the object is included too.)
10 Jun 08
Originally posted by sasquatch672I think it was intended as a trick question and he certainly did not mean 'mass'.
You're parsing. You know what the kid meant and you're trying to big-time him.
It reminds me of a cartoon strip called Calvin and Hobbes where the teacher tells the student (calvin) that he got the answer wrong because it was a trick question and Calvin replies "well it was a trick answer".
10 Jun 08
Originally posted by PalynkaThe earth is about 200lbs - and it varies.
Zero?
I'm holding the whole earth up by my feet as I rotate around the sun. When I put a scale between me and the earth to measure the strain the earth is putting on my feet it shows around 200lbs - so the earth weighs around 200lbs.
11 Jun 08
Originally posted by FabianFnasWas it a trick question? Or was it a straight question?
How much does the Earth weigh?
Is this an interesting question?
Is the answer interesting?
Is the answer as obvious as one can imagine?
Normally, for non scientific people, weight and mass is the same thing. Weight watchers, for example, is for reducing weight, but they measure their weight in kilograms. Correct or not? Well, as long we live in a quite homogenous gravitational field it could consider being correct. But not scientifically. (This is the science forum, remember?)
But, however, they correctly use Body Mass Index, BMI, to measure obesity.
sasquatch672 gave an correct answer (I haven't checked) but of a another question. The question should then be "How much mass does the Earth have?"
Palynka gave the correct answer (zero) but as a physics teacher I would only give half a point for it.
The correct answer is - 0 Newton. Full point if you answered this. Weight is measured in Newton, a force. One kilogram at the surface of Earth is equivalent of about 9.8 Newton.
Zero Newton... Why is it so? Well, the planet Earth is in the state of free fall around the Sun. twhitehead gives a more elaborate explanation.
So was it a trick question? Well, the question was crystal clear. And I gave some clues to think it over some.
"Is it an interesting question?" Yes, I think so. Because it make you think of why the question is posed in the first place. Like "Is 2+3 really 5?" It makes you think of what circumstances 2+3 is not 5, and why it is like this. (Perhaps worth a thread of its own.)
"Is the answer interesting?" If the answer is zero Newton, yes. If the answer is som high number metric tonnes, no not particularly. It is too big number to relate to. (My opinion.)
"Is the answer as obvious as one can imagine?" If you give the answer as tonnes, it's not really obvious. If you give the answer in Newton, then it really is obvious. Can the Earth have any other value than zero Newton? If so, how come?
These kind of question are very interesting, in my humble opinion.
11 Jun 08
Originally posted by FabianFnasAnd in every day English, weight means what mass means in scientific language. So its not so much a case of 'weight watchers' being wrong but a case of science using a popular common word in a new or stricter sense.
Weight watchers, for example, is for reducing weight, but they measure their weight in kilograms. Correct or not?
(I am not saying you claimed otherwise, just thought I would comment on it)
I still think that your origional question is incoherent in that the standard scientific definition of 'weight' simply cannot be applied to the earth as a whole.
11 Jun 08
1 edit
Originally posted by twhiteheadWhere did the reference to the Earth come from? It's not in the definition:
Weight is a measure of the gravitational force acting on a body on the earths surface.
http://dictionary.reference.com/search?q=weight&x=0&y=0
http://en.wikipedia.org/wiki/Weight
"Weight" is simply a measurement of the gravitational forces acting on an object. So the "weight" of the Earth is really the vector sum of gravitational forces acting on it due to all other bodies in the universe, which (I would think) is unlikely to be zero.
11 Jun 08
2 edits
Originally posted by PBE6Don't forget that since it is a sum of vector forces then many cancel out.
Where did the reference to the Earth come from? It's not in the definition:
http://dictionary.reference.com/search?q=weight&x=0&y=0
http://en.wikipedia.org/wiki/Weight
"Weight" is simply a measurement of the gravitational forces acting on an object. So the "weight" of the Earth is really the vector sum of gravitational forces acting on it due to all other bodies in the universe, which (I would think) is unlikely to be zero.
With respect to the forces we can perceive, it seems to be zero because the Earth is in free fall w.r.t. the sun, which is the next logical reference.
Note that you need a reference to calculate that vector. Since we have no idea where the center point of the universe is, it makes sense to choose the sun as the next reference.