## Quite Interesting.

A forum set up for physics questions from students in the courses PHYS1121, 1131, 1221 and 1331 at the University of New South Wales. It is intended for questions that cannot readily be answered in class,
either because they fall outside the main syllabus and therefore would be distraction (however interesting) or for other reasons.

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tagitables
Posts: 10
Joined: Fri Jul 01, 2011 11:22 pm

### Quite Interesting.

When I was a little kid, someone had asked me a question.
What's heavier a ton of gold or a ton of feathers?
At first I answered gold but later I've been told that they were actually in balance. A ton of something is weighted a ton of something.

Nothing could be further than the truth.

One of them IS heavier because gold is measured in troy weight, rather than avoirdupois. They're different. So a ton of gold is different to a ton of feathers.

Reference:
http://en.wikipedia.org/wiki/Troy_weight
http://en.wikipedia.org/wiki/Avoirdupois
Signed.
angle.alpha
Posts: 21
Joined: Tue Mar 08, 2011 8:28 pm

### Re: Quite Interesting.

That is definitely not true if you are using SI units, the METRIC tonne which is NOT based on imperial units (pounds) which is what your two systems are referring to.
tagitables
Posts: 10
Joined: Fri Jul 01, 2011 11:22 pm

### Re: Quite Interesting.

angle.alpha wrote:That is definitely not true if you are using SI units....
Please note that when someone measuring gold, or any precious metals, especially silver and gemstones, troy weight is used.
Signed.
angle.alpha
Posts: 21
Joined: Tue Mar 08, 2011 8:28 pm

### Re: Quite Interesting.

Unless that someone is a physicist... but even so, feathers would still be measured in SI units unless you happen to live in north America. Regardless, once you reduce that question down to a matter of linguistics of units, it loses any scientific meaning... it becomes a question of perhaps historic and cultural significance. the original question i think was explaining to young children and laypeople the conceptual difference between mass and density.
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: Quite Interesting.

There is an interesting subtlety about the weight of a tonne of gold vs a tonne of feathers.

Normally, one would weigh them in air. A tonne of gold displaces less than 0.1 m^3 of air, and so the buoyancy force on it would be less than 1 Newton.
So, if its weight were 9800 N, then a balance in air would yield about 9799 N.

I don't know how big a tonne of feathers would be, but (not counting the air between them) I reckon that the feathers would displace about 1 m^3 and so have a buoyancy force of about 12 N, so the balance in air would yield about 9788 N.

Of course, one rarely finds a balance that reads 10 kN with that precision, and the problem of getting the tonne of feathers on the balance would be prodigious, and certain to produce new artifacts.

Joe

I doubt that there is a ton in troy measures. Fort Knox doesn't seem to use one. However, Scrooge McDuck (brother of Donald Duck) was American and rich enough to need one.
Hops
Posts: 1
Joined: Sat Oct 22, 2011 10:42 am

### Re: Quite Interesting.

tagitables wrote:When I was a little kid, someone had asked me a question.
What's heavier a ton of gold or a ton of feathers?
At first I answered gold but later I've been told that they were actually in balance. A ton of something is weighted a ton of something.

Nothing could be further than the truth.

One of them IS heavier because gold is measured in troy weight, rather than avoirdupois. They're different. So a ton of gold is different to a ton of feathers.
That's really interesting!) Not that I get it at once, but at least I was not totally wrong when I couldn't believe that a ton of gold and a ton of feathers have no difference
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Last edited by Hops on Mon Oct 31, 2011 12:00 pm, edited 1 time in total.
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: Quite Interesting.

Not that I get it at once
Don't bother trying to get it: the reference to Troy and to Avoirdupois is misleading. Gold is usually measured in either grams or in Troy ounces. The Troy system doesn't have a unit of a ton. A tonne of gold means 1000 kg of gold. A tonne of feathers means a 1000 kg of feathers. (What is meant by a ton depends on the country: in the US it could mean 2000 pounds and in the UK 2240 pounds, but very few countries use these units any more.)

So a ton of gold and a ton of feathers have have the same weight. As do a tonne of gold and a tonne of feathers. (Measuring their weight very precisely is complicated by the tiny buoyancy effect mentioned in this thread, but this doesn't affect the statement.)

Joe