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Author Topic: How Do You Measure The Weight and Mass of A Star /Planet/ Thing ?  (Read 5526 times)

Offline neilep

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Dearest All,

I have some well posh weighing scales but I don't think even they are calibrated to weigh some heavenly bodies !! (well... ;))

So, how does one calculate the weight/mass of such a thing ?....how heavy would the moon be if it landed on my weighing scales ?


 

another_someone

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Depends on what it is you are measuring.

The easiest way is to work out how much the mass is effected by another nearby mass, or how a nearby mass effects your lump of mass.

If two bodies orbit each other, you can measure their masses by looking at the motion of the two bodies around each other.

For large bodies (e.g. stars), I would imagine that measuring the effect that the star has in bending light that passes close by is another way.

Where you know (or believe you know) the process by which a lump of mass (e.g. a star) is burning energy, you can often look at the rate of energy output as a measure of the amount of fuel that the body has to burn, as well as possibly the amount of compression caused by the gravity of the object upon that fuel.
 

Offline Soul Surfer

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To determine the mass (weigh) two bodies that are orbiting each other you need to understand the laws of gravity and also measure the gravitational constant.  This can be done using a torsion balance and this was first done in 1798  by Cavendish and he is sometimes called "the man who weighed the earth".  There had been some earlier attempts to estimate the gravitational constant before that date by looking at the displacement of a plumbline from vertical due the the  attraction of a large mountain nearby but they weren't very accurate.

You can use your posh sales to do the job if you want what you need is a largish lump of a nice dense material gold or platinum is best but lead will do at a pinch and some equal weights a heavy as your scales will take.  Set up the scales to be balanced with your weights on the pans and bring your largish lump of dense material close to each pan in turn and measure the deflection of the scale and correct it by using a tiny metal rider on the crossbar.  To get a really accurate answer you need to take account of the attraction of the weight between the far pan as well as the near one and also the attraction on the arm of the balance
« Last Edit: 15/03/2007 21:02:42 by Soul Surfer »
 

Offline neilep

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Thank you again GEORGE & IAN,

Gosh..it sounds complicated but I assume it's straight forward when you know how.

What about if I wanted to measure the weight/mass of Mount Everest ?...this must use a completely different technique yes ?
 

another_someone

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What about if I wanted to measure the weight/mass of Mount Everest ?...this must use a completely different technique yes ?

What is 'Mount Everest'?

What I mean is how would you define which patch of soil or rock belonged to Mount Everest, and which belong to something alongside or under Mount Everest.

In broad terms, the easiest direct measure would be to have an aircraft fly over  with a gravimeter (or you could just take the gravimeter up the slope, and measure the gravity as you go along), but it would only be a crude measure, but then you could only have a crude definition of what should or should not be part of Mount Everest anyway.
 

Offline neilep

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What about if I wanted to measure the weight/mass of Mount Everest ?...this must use a completely different technique yes ?

What is 'Mount Everest'?

What I mean is how would you define which patch of soil or rock belonged to Mount Everest, and which belong to something alongside or under Mount Everest.

In broad terms, the easiest direct measure would be to have an aircraft fly over  with a gravimeter (or you could just take the gravimeter up the slope, and measure the gravity as you go along), but it would only be a crude measure, but then you could only have a crude definition of what should or should not be part of Mount Everest anyway.


George !!

I said Mount Everest as an example....The real question is how do we calculate the weight of such a thing !!..not how do we determine the specs of it in the first place........I am using assumptions ......but..LOL..me still luffs ewe !!.......Ok...what about the Great Pyramid of Giza.....geezer ?...LOL
 

Offline Bored chemist

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For the great pyramid  I think measuring it's size to get a value for the volume and then calculate the mass based on the density of the rock it's made from would be the best bet.

If I wanted to weigh a star I think I'd hope there was something in orbit round it. From the period of the orbit and it's radius I think you can calculate a mass.
 

another_someone

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For the great pyramid  I think measuring it's size to get a value for the volume and then calculate the mass based on the density of the rock it's made from would be the best bet.

You could not work simply from the density of the rock - you would need the average density of the whole structure, bearing in mind that it is made from several different types of rock and plaster, and has large spaces within it.

But I do agree that the only really practical way you can do it is by taking the mass/density of its parts and adding it all up.

The only other possible way I could think of is by looking at the pressure it places upon the terrain upon which it is built.  The problem is judging how rigid the structure is - if the pyramids are not that rigid, they might deform themselves under their own weight, and thus distort the way they apply pressure to the ground beneath.
 

Offline Mjhavok

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Ask that Atlas guy how heavy he thinks it is.
 

Offline science_guy

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I would think that to measure the density of a star, you would need to look at the energy and mollecular signature of the light from said star, then you should find the size that should be a little simpler, multiply the density by the volume to find the mass, and then use the gravity modifyers to convert it to the weight of whatever planet you want to measure it on.
 

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