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        riddles >> microsoft >> How much does planet earth weigh?
        
(Message started by: hoferman on Aug 17^th, 2005, 7:41am)

Title: How much does planet earth weigh?
Post by hoferman on Aug 17^th, 2005, 7:41am

I am very curious what different methods this community can find to the question: How much does planet earth weigh?

Title: Re: How much does planet earth weigh?
Post by towr on Aug 17^th, 2005, 12:02pm

It doesn't weigh anything, because it's floating in space.
It also doesn't weigh anything because it can't operate scales, nor does it has any need to weigh things for that matter :P

Title: Re: How much does planet earth weigh?
Post by Icarus on Aug 17^th, 2005, 5:11pm

I once read an article by Isaac Asimov called "The Man Who Weighed The Earth", about Cavendish. In it, Asimov bemoaned the terminology, saying that it actually should be "The Man Who Massed The Earth", but that popular usage (including his own colloquial descriptions) required the inaccurate title.

How did Cavendish mass the earth? By measuring G, the constant of proportionality in Newton's Law F = GMmr^-2. If M is the mass of the Earth, and R is the radius of the Earth, then the acceleration due to gravity on the surface is GMR^-2. The acceleration is easily measured and found to be about 9.81 ms^-2. R had been determined to significant accuracy by French scientists some time in the 17th or 18th century. So, it had also been known for some time that GM = 3.97x10¹⁴ m³s^-2.

Cavendish hung a crossbar from a thin metallic thread of known elastic properties, so that he could measure how much torque was being applied to the bar by the angle through which the bar was deflected from its rest position. This allowed him to measure very small forces. On the ends of the bar he placed as massive weights as the supporting thread could handle without risk of deformation, and placed two other large masses on stands very close by, placed to produce maximum torque from the gravitational attraction. By measuring the resultant force, dividing by the product of the masses, and multiplying by the square of the distance between their centers, he was able to calculate G = 6.67x10^-11 kg^-1m³s^-2. (I'm not sure what accuracy he was able to obtain with this original measurement.)

Dividing 3.97x10¹⁴/6.67x10^-11 gives a mass for Earth of 5.95x10²⁴ kilograms. (A more accurate value is 5.975x10²⁴ kilograms.)

Title: Re: How much does planet earth weigh?
Post by Eros on Aug 25^th, 2005, 10:11pm

Weight is relative it depends on Gravitational pull that is acting on the object. So a person will have different weight on moon from what he has on earth. Now for us to weigh Earth it should be put under some standard gravitational force.[hideb] I think i am deviating from the problem. :-X[/hideb]