Weighing Long Things

[Chesterton]

Like the energizer bunny, Gravity keeps going and going.

No, the universe is expanding. And I am not the slightest bit attracted to you way over there in California.

 

[essentialsaltes]

And I am not the slightest bit attracted to you way over there in California.

I think you're protesting too much. Besides, I know better.

 

[Chesterton]

I think you're protesting too much. Besides, I know better.

I think this thread has gotten silly.

 

[Tanj]

I think this thread has gotten silly.

Scales put me back under 110, so everyone can start using them again.

Also, with regards to the flibberty Gibber going on in this thread. The issue is that you are not in the slightest bit interested in how much the board weighs, you care about it's mass, and that is a constant not affected by gravity (assuming no velocity differences). We also have ways of measuring mass that do not rely on gravity. Use one of those.

 

[Chesterton]

The issue is that you are not in the slightest bit interested in how much the board weighs, you care about it's mass, and that is a constant not affected by gravity (assuming no velocity differences).

Excuse me, but please refer to the thread title - Weighing Long Things, not Determining The Mass Of Long Things.

 

[Tanj]

Excuse me, but please refer to the thread title - Weighing Long Things, not Determining The Mass Of Long Things.

yes, yes, you have no idea what you really want. You are not alone. Turns out you wanted a guitar with less mass. If you just wanted one that weighs less, you would have gone into orbit with the one you have.

 

[sjastro]

If you are satisfied with your weight there is an upward proper acceleration.

If you are overweight the acceleration is definitely improper.

 

[iluvatar5150]

So if it extended outside Earth's atmosphere, only the section of the wood subject to Earth's gravity would be weighed accurately?

The atmosphere is not a magic barrier, but distance from a large mass like the Earth is. I wonder how a scale would measure a hypothetical beam which extended outside the atmosphere. Some of it would be normal, some of it lighter, and at the ends it would be weightless.

There is no such thing as true weightlessness - everything is impacted by gravity, though, sure, in some cases, it's so small as to be imperceptible.

The situation you're thinking of, however, where an object is in orbit isn't true weightlessness, either, but rather a sort of perceived weightlessness. What happens with, say, a space craft, in orbit is not that it's weightless and unaffected by gravity, but rather that it's moving at such a high speed that, as it falls towards the earth (which it is doing all the time), the path along which it falls matches the curvature of the earth. The space station is literally falling in a big circle (or ellipse, if you want to get fussy).

The ends of your 2x4 wouldn't be in orbit like that and would still "weight" something, though it would be somewhat less their weight were they at a lower altitude.

Stand the 4x2 on end and let go for a moment as you take the reading. It will take a few seconds before it starts to tip.

I have also weighed long objects by weighing each end.
If you put one end of the wood on a brick, and the other end on the scales, each end will bear 50% of the load, so just double it. You can check by swapping the scaled from one end to the other.

Its the same as using 2 or even 3 sets of scales on a large object. Add the scale weights together to get the total.

Yup, stand it on end, or suspend it from something and have the scale under or attached to the thing from which you are suspending it.

On a related note, I still work in the entertainment industry a little bit and keep an eye on some of the rigging groups just because I think it's interesting. I recently came across some discussions regarding the distribution of weight across multiple motors lifting a single straight truss and the numbers aren't what I would've guessed:

Rigging Loading Calculations for Multipoint straight truss | HireHop.com

Assuming the weight is evenly distributed across a truss, then putting a hoist at the location of each arrow in the diagram causes those hoists to bear unequal percentages of the total load:

 

[durangodawood]

All the mass of an object acts as if it resided entirely at its center of mass. If it is of uniform density the center of mass will be at the geometrical center. So if the 2X4 is balanced on the scale there is no problem. For very long objects two scales could be used.

But he wants the weight... not the mass.

Some portions of the flat object will be further away from the Earth than others and so record a lighter weight than they would at the scale. Of course, this is negligible over 20ft.

 

[JackRT]

But he wants the weight... not the mass.

Some portions of the flat object will be further away from the Earth than others and so record a lighter weight than they would at the scale. Of course, this is negligible over 20ft.

Weight is a measure of the force of gravity on a mass. My mass would be the same on Earth and Mars but my weight would be different on each.

 

[durangodawood]

Weight is a measure of the force of gravity on a mass. My mass would be the same on Earth and Mars but my weight would be different on each.

Exactly. And portions of a long flat object would be further away from the Earths gravity than at the scale. I guess that difference is effectively part of "the weight" of the object tho.

 

[FrumiousBandersnatch]

Exactly. And portions of a long flat object would be further away from the Earths gravity than at the scale. I guess that difference is effectively part of "the weight" of the object tho.

Which suggests to me that the weight would be greater when weighed lying horizontally on the scale than when standing vertically on the scale. In each case, the point resting on the scale is deepest in the gravity well, but when standing vertically, more of it is further away from that point than when it's lying horizontally