You have a bucket.
This bucket has no lid.
This bucket is standing upright.
I pour helium into said bucket.
Does the bucket, at least in theory, become temporarily lighter before the helium gas escapes into the air?
I say yes because the friction caused along the walls of the bucket as the helium escapes makes it lighter.
Now granted, there’s probably no measuring device sensitive enough to measure this but in theory I think it makes it lighter.
If the helium is in a gaseous state, I don’t think you can pour it into a bucket.
If it’s in a liquid state, of course it makes the bucket heavier.
I believe you cannot pour Helium into that lid-less bucket because Helium rises faster than you think.
When I was interpreting for a car company that wanted to make a Hydrogen-powered car there were concerns with the safety of the fuel tank. They showed that when you pour a glass full of liquid Hydrogen on a table (don’t try this at home, they showed a video) the Hydrogen evaporates even before it reaches the table and rises up into the atmosphere, if it does not combine chemically with other elements it escapes the atmosphere in a matter of minutes. Helium, of course, is a noble gas and does not combine chemically. Your Helium would be wasted.
Ok perhaps pour was a poor (heh) choice of words. How about blow from a helium tank instead?
If you blow hard enough, Helium will fill the bucket dynamically, and the pressure thus generated will make the bucket heavier. And as soon as you stop blowing, the Helium escapes and you have wasted even more Helium.
The bucket will always remain the same weight. What you are asking is if the helium escaping the bucket somehow pulls the bucket upward, not necessarily enough to levitate, but enough to be detected, if such a detection device existed. I would say no. The surface of the inside of the bucket wall probably does not have enough area to make a difference, accounting for the total volume of helium. Besides, the bucket is wider at the top than the bottom, so there would be less friction than you may think with the gas escaping out of the top.
Maybe it’ll help to alter the scenario slightly?
I think these scenarios are functionally the same as what you describe, except that they avoid the problem of pouring the helium.
I don’t think you are still asking the correct question. The bucket’s mass and weight on earth cannot change. I think what you are wanting to ask is do the weight of bucket and its contents change.
Goodness, why do we have to let semantics ruin a perfectly good question?
@snowthx understood what I was trying to ask perfectly.
Is it really that hard to get the gist?
If the “bucket” is open top and bottom with helium rising up through it, the bucket should weigh slightly less as the gases moving upward will exert a frictional force on the sides of the bucket in the upward direction.
If the bucket has a bottom, the volume of gas (He) moving up is exactly equal to the volume of gas (air) moving down to replace it.
If the bucket has a bottom and a top, it will weigh less, the same way as a balloon. Poke a hole in the top, and it will slowly begin to weigh more as the He is replaced with air.
In the OP’s case, I think it’s #2, in which the modest upward frictional force from the rising He is offset by the similarly modest downward force of the air replacing it.
True but once you stop blowing helium into it, it would become lighter ever so briefly…
When you weigh it, you are effectively weighing the bucket plus the air it contains.
If you replace the air with a lighter gas (Helium or Hydrogen), it will weigh less because the bucket contents weigh less.
Of course, the problem is to get the gas to stay inside the bucket – since it’s lighter than air, it will immediately float up, unless you have a gas-tight cover on the bucket.
Theoretically, I suppose there would be some fractional upward friction on the sides of the bucket from the gas floating up. But also downward friction from the air falling down to replace that gas. I’d expect those forces to roughly cancel each other out – and to be so tiny as to require really sensitive instruments to measure them.
Clearly.
I’m genuinely trying to understand the question. Would a similar question be whether a bucket would appear to weigh less if there were someone blowing up its side from the outside?
Wait–I’m not even sure this is right. You’re positing that the helium’s upward motion would, via friction, make it appear to weigh less, correct?
As the helium leaves the bucket, an equal volume of atmosphere is replacing the helium. That equal volume would apply downward friction, wouldn’t it? Possibly, due to its greater density, a greater friction?
If that’s correct, then the bucket might actually weigh less when you’re piping the helium in: it’s forcing the regular atmosphere out and up as it enters the bucket.
That’s a good point. I did not consider the air replacing the helium.
Your hypothetical presumes that the helium in the bucket would rise as a cohesive mass, sliding up the sides of the bucket, exerting friction. I don’t think that is accurate. The center of the helium would rise first, like a bubble, then the rest of the helium would pull away from the sides of the bucket, toward the bottom of the rising helium bubble without exerting any measurable friction.
An alternate version (attempt) of changing the problem to make it easier to figure:
You’ve got a “bucket” that’s approximately 1 inch in diameter and 50 feet deep (basically, a pipe with a capped bottom). Temporarily, you cap the top with an infinitely thin-yet-completely-unsurpassable zero weight film, pump the tube full of helium and then, through magic, you’re able to make the film disappear.
My personal sense would align with @Left_Hand_of_Dorkness. As the helium tries to escape, it creates a vacuum which, in turn, causes regular air to be pulled in. This will create a variety of turbulence and effects that largely counterbalance one another. However, like @Tim_T-Bonham.net , I’d still expect a transition from the initial state - in which you’ve effectively got a bucket with lighter gaseous materials pushing down on it from above than usual - to a bucket that is full of normal air and topped with normal air, and thus heavier overall.
If you could remove the turbulence and chaos, I’d expect a transition from lighter to heavier as it replaces the helium with nitrogen and oxygen.
That makes sense to me, and therefore I would say that if you are somehow adding helium to the bucket, then ignoring turbulence, the measured weight of the system will be less than just when the bucket is sitting there full of ordinary air (which state it will quickly return to when the added helium escapes out of the top)
Imagine you have a bucket with a helium balloon tied to the bottom, reducing its weight. When you cut the string, how long does it take for the bucket to to back to it’s normal weight?
I don’t think this is the case. Consider a steel bucket with a mass of 1kg, which can hold 5 litres of water. If you put that bucket on a set of scales underwater the scales do not say it weighs 5 kg as it is now full of water. The scales would actually record about 880gm because the steal is about 120cm3 of water.
The same is true for being in air the steal is displacing 120cm3 of air which would reduce its weight by about 1gm when compared to being in a vacuum
Copying Left hand of Dorkness idea, and going back to the underwater bucket. Say is has a screen bottom and I release a flow of air through is at a rate such that the bucket is mostly full of air. I would not expect the bucket to float as the mass is reduced by the difference in weight of the water and the air. The friction of the air going up against the sides might be enough to reduce it weight a little but not by 5 kg.
Sorry for all the metric measurements but it does make hte math easier especailyl if you have no idea of the density of water in lb/ft3