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# A Massive Bag of Feathers?

19
Nov
2009

By Miranda

The IQ test question (or is it a common-sense test question), Which is heavier, a ton of lead or a ton of feathers?, will always catch a few people out.

But the difficulty of distinguishing the idea of density from the idea of mass is made more complicated by the physicists having taken the word mass, a common, previously-existing word with long-established meanings, and giving it a new, very precise, use which, while related to the older meanings, just isn't the same.

It was bad enough in science classes trying to understand the difference between mass and weight . Yes, the two are different. Hence weightlessness in space – you have the same mass but there's less gravity – no, sorry, gravity is as it ever was, there's less gravitational force acting on you.

But mass and weight can be the same, for all intents and purposes, if your items with mass are all being acted on by the same gravitational force. Understanding this is not necessarily helped by the unit for measuring mass, the kilogram, being the one that we use to identify the weight of things here on the Earth's surface where (and please don't tell me there are regional variations and/or anomalies) the gravitational force is always the same.

The word mass was originally associated with things that are big or with the concept of 'lots of it', whatever 'it' was. And mass entered a grey area in art-speak, becoming a 'large area with similar characteristics in a painting', thus taking on a feeling of weight when the balance of the work was explored.

So, as far as the feathers fly (and I expect them to after this blog), we can have a massive (big in size – non-physics meaning) bag of feathers but it won't have as much mass as an identically massive (big in size) bag of lead (which is dense).

Further, the bag itself can be large or heavy (or both) regardless of whether its contents are lead or feathers. And now I've just used heavy to describe 'mass with density'. Sigh...

And, since we're all on this planet together (global-warming sceptics, take note), is it OK to use weight as a clue for mass, and vice versa?

Comment on this blog

17 Responses to

## A Massive Bag of Feathers?

November 19, 2009 at 10:05 AM

I hate to put a wrench in the works, but mass, in the context of a puzzle, can also mean the Catholic religious service, or a stack of objects, regardless of their size or weight. But then, the physics meaning of mass (it is physics, isn't it?) can actually mean weight, i.e. weight for its size. Good one, Miranda.

said:
November 19, 2009 at 6:30 PM

As a physicist I can safely say that is the most painful thing I have ever read. You have taken a simple issue and made it needlessly complicated. Your mass is how much matter you have. Your weight is how much gravitational force is exerted on you (or vice versa), as a result of possessing mass. And yes, gravitational force does vary across the earth's surface. This is because the earth is not a perfectly smooth sphere of uniform density. And I haven't even touched on the Borehole Effect.

kragzy said:
November 19, 2009 at 10:42 PM

So Spica, when your doctor asks your weight, do you say so many kilograms or so many Newtons? My weight is 804 Newtons. Try finding someone to whom that makes sense! Anyway, I'm off to Mass now to say some weighty prayers about the mass of people for whom this matter holds no weight. I love your blogs Miranda - sounds like I weight for them every week :-)

November 22, 2009 at 8:13 AM

And if we take the leaad and the feathers to the moon - what then? A very different answer me thinks. And do we weigh ourselves or mass ourselves?

Lillypop said:
November 23, 2009 at 12:10 PM

Oh dear, no wonder I have trouble trying to help Miss 10 with her maths homework! (or is it physics?)

luddite said:
November 23, 2009 at 9:05 PM

In reality, Kragzy, we should say to the doc "My mass is 82.04 kg." or "My Earth weight is 804 newtons." All the confusion stems from the imperial system of using a weight standard for measuring - the pound - and then continuing to try to use the language of that mind set when we changed to metrics. This sort of thing happens when everyday language usage gets involved in technical topics.

November 23, 2009 at 9:41 PM

If I take my scales to the moon what will they read? The same or different? Would gravity change the nuber of kilograms I measured? It might seem simple to you Spica but to the layperson Miranda has expressed well the confusion which occurs from the language usage. Well done Miranda for another thought provoking blog on our rather interesting, ever changing language.

November 23, 2009 at 9:42 PM

"number" sorry

luddite said:
November 24, 2009 at 9:06 AM

To Stichpuzz : If you take domestic bathroom scales, which work by measuring the force applied by the mass placed on them, to the Moon they will read one sixth of their earth reading, the Moon's surface gravitational attraction being one sixth of the Earth's. If, however, you take a beam balance, which directly compares masses, you will get the same kilogram reading as on Earth. This is another part of the trouble, expressing what is actually measured ( force, unit newtons) in terms of a different quantity ( mass, unit kilograms ).

November 24, 2009 at 4:12 PM

Thankyou Luddite. A very straightforward answer to my question. i think I will measure myself with bathroom scales on the moon :-)

luddite said:
November 24, 2009 at 5:13 PM

No probs Sttchpuzz. The reading there may please, but won't change your mass. Oh, be careful jumping about while you're there.

Sierra said:
November 25, 2009 at 9:28 AM

"sCeptics"?? Is that an Australian spelling?? Just wondering?? A new area for you to explore?? *smile*

kragzy said:
November 25, 2009 at 2:07 PM

Sceptic and skeptic are both ok. The English word came from Latin 'scepticus' and the Latin word came from the Greek 'skeptikos'. Take your pick. Who knows where the Greeks got it from! Babylon?

spoggy said:
December 20, 2009 at 2:36 PM

i thought that Mass was identified through atomic density ?? hence, lead is more dense in atoms than feathers ? ergo, more feathers to make up the density of lead ??

spoggy said:
January 17, 2010 at 3:01 PM

a ton is a ton is a ton !! a ton of feathers, a ton of lead , a ton of hair, a ton of gold , all are a ton ?? some just take up more room than others ?

lamby said:
January 17, 2010 at 3:54 PM

I know a lot of people who would like to reduce their mass - both the weight and how much room they take up.

said:
January 20, 2010 at 10:55 AM

the answer to that's so easy, lead.....,