[Jennifer Stern]

Jennifer Stern  asked the Naked Scientists:
   
Hi
 
I was listening to Cape Talk/Radio 702 today, and something you said concerned me a bit but I didn't really pay it too much attention, as I was driving at the time. But now I have worked it out. (No prizes for guessing what made me think about it.)
 
 You said that
 
a)      When you go to the loo you lose weight. No problem with that.

But you also said:

b)      When you fart, you can gain weight because methane is lighter than air. Surely that will only be the case if you actually decrease your volume as well when you fart. And do you? Does the momentary release in bloating actually change your external volume? Because if you don't decrease your volume when you fart, you will have lost the actual weight of the methane, but you may not have changed your volume, so you will still be displacing the same amount of air. Let's not complicate matters by assuming that, when farting, you don't breathe.

Not a bad assumption, really!
 
Great programme, by the way.
 
Cheers
 
Jennifer Stern

What do you think?

[LeeE]

Shouldn't the question be 'Do you really gain weight when you fart?'

It doesn't matter whether your volume changes when you fart; 1 kg of feathers occupies much more volume than 1 kg of lead but still weighs the same.

There's also the issue of buoyancy to consider, for we generally measure our 'weight' in the Earth's atmosphere, so the reason we might gain weight by farting is because we become less buoyant in air after losing the methane.  If we were to measure our weight after farting in a vacuum then we would lose weight because the buoyancy issue doesn't apply.

As it happens, farts can, and will often be heavier than air - up to 90% of it can be nitrogen:

http://en.wikipedia.org/wiki/Flatus#Composition_of_flatus

[Geezer]

It doesn't matter whether your volume changes when you fart; 1 kg of feathers occupies much more volume than 1 kg of lead but still weighs the same.

There's also the issue of buoyancy to consider, for we generally measure our 'weight' in the Earth's atmosphere, so the reason we might gain weight by farting is because we become less buoyant in air after losing the methane.  If we were to measure our weight after farting in a vacuum then we would lose weight because the buoyancy issue doesn't apply.

At the risk of seeming overly pedantic (but why would I let that stop me  []) isn't that a slight contradiction? If your volume changes wouldn't that alter your displacement in a fluid and alter your weight.

Maybe you meant to say the mass of 1 kg is always 1 kg?

[LeeE]

If your volume changes wouldn't that alter your displacement in a fluid and alter your weight.

Yes Geezer, it would, which is why I said you should also consider buoyancy as, in general, we weigh ourselves in a fluid/atmosphere.  However, I was being pedantic too, and was trying to allow for the highly improbable case of weighing yourself in a vacuum, hence the two different answers.

[Geezer]

If your volume changes wouldn't that alter your displacement in a fluid and alter your weight.

Yes Geezer, it would, which is why I said you should also consider buoyancy as, in general, we weigh ourselves in a fluid/atmosphere.  However, I was being pedantic too, and was trying to allow for the highly improbable case of weighing yourself in a vacuum, hence the two different answers.

It was the first bit that I was whining about. I don't think a 1 kg mass of feathers will weigh the same as a 1 kg mass of lead. They have the same mass, but they will not weigh the same when measured in our atmosphere. (A kilogram is a measure of mass, not weight.)

[LeeE]

A kilogram is a measure of mass, not weight.

...and weight is a measure of force acting upon a mass...

[Geezer]

A kilogram is a measure of mass, not weight.

...and weight is a measure of force acting upon a mass...

A kilogram is a measure of mass, not weight.

...and weight is a measure of force acting upon a mass...

Right. So does 1kg of feathers weigh the same as 1 kg of lead? (when weighed at STP on Earth.)

Or, if you prefer, does 1 kg of hydrogen weigh the same as 1 kg of lead? (when weighed at STP on Earth.)

[LeeE]

Right. So does 1kg of feathers weigh the same as 1 kg of lead? (when weighed at STP on Earth.)

Well, it'll depend upon how you measure those 1kg of masses.  If you do so by weighing them then they'll weigh the same.  Really though, to find their actual masses, as opposed to their weights, you should measure their acceleration resulting from a known force.  However, if you try to do that in STP conditions the air resistance will result in incorrect results too  []

[Geezer]

Right. So does 1kg of feathers weigh the same as 1 kg of lead? (when weighed at STP on Earth.)

If you do so by weighing them then they'll weigh the same. 

Lee, Sorry, but I think you're a bit confused about that. They won't actually weigh the same.

[LeeE]

Ah - I think I may have misinterpreted what you were getting at.

If you take a known 1kg of feathers and a known 1kg of lead then yes, the two will have different weights in STP (assuming that you haven't compressed the feathers into the same volume as the lead, of course).

I was going on about establishing their masses via weighing, and not assuming that the masses were already known, which is why I then went off about finding their masses via acceleration.

I did raise the buoyancy issue in my first response though.

[Geezer]

Phew! I'm glad we were able to sort that out  []

I was being pedantic of course, but strictly speaking, despite what your greengrocer says, 1kg is a mass, not a weight. I suppose the customer is getting a slight advantage though, so we might not want to make a big deal out of it.

[Don_1]

OK, its a 'toilet' question and I have refrained from poking my nose in for obvious reasons.  But I can resist the temptation of toilet humor no longer.

Since most of the gases we evacuate from our nether region are heavier than air, I would have to assume that a good Trumpet Voluntary emanating from the exhaust pipe would result in a loss of weight. However, I rather doubt that it would be of any truly discernable proportion, so I don't think it would earn you any Brownie Points ( snigger) at a Weight Watchers meeting. On the contrary, I should think you would be far more likely to find yourself loosing points due to the discomfort brought upon other members.

But, your expulsion of gases surplus to requirements, could result in weight loss of those around you. A frantic waving of limbs, or indeed, a hasty retreat from the affected atmospheric zone could have beneficial effects on those who find your obnoxious byproducts too much to bare.

The extent of weight loss during the act of 'trumping' could be increased if the humidity of said expulsion were high. This is commonly known as a 'wet fart' and would be greatly increased in event of the syndrome known as 'follow through'.

It should be noted that both these eventualities can result in serious embarrassment and an urgent requirement for clean, fresh undergarments.