I was watching some Mythbuster episodes yesterday and a couple of questions arose.
The first was about putting helium in a football. Adam was squeezing all the air out of some footballs and then weighing them when they were collapsed and empty. He then filled them up with air to standard pressure and weighed them again. Not surprisingly, the air increaded their weight by a small amount. He then emptied them out again, refilled them with helium to standard pressure, and weighed them one more time. But this time he said they weighed less - not just less than the air-filled footballs but also less than the empty footballs.
Adam seemed to take this in stride but I was very surprised. How can a football filled with helium weigh less than an empty football? The empty footballs were empty - they were collapsed and didn’t have any air in them. So how does a football weigh more than a football with some helium added? Helium is very light but it has some weight. Is there something here I’m missing?
The second question is simpler but probably more difficult to answer. There is a wall full of bins in the workshop with labels saying what’s inside the bins. Most have prosaic names like “electronic tools” or “hard hats” or “drill bits”. But I saw a bin labeled “raw meat”. I’m pretty sure they weren’t storing actual raw meat in a room tempature bin. Even if they were doing some experiment with letting raw meat spoil they wouldn’t have done it in a storage bin. So was the raw meat label a joke (because we all know how whimsical Jamie is about his workplace) or was it some kind of prop like a plastic replica of raw meat? Or something else?
I have no idea on your second, but the first is answered by the concept of buoyancy. Since helium is lighter than air, the ball is buoyed up. The same way that air has weight, but a football filled with air weighs less when floating in water.
For the first question, I would guess that buoyancy made the football lighter, the same way a helium balloon seems to weigh less than nothing. Either that or Adam just let his expectations influence his measurements.
For the second, yes, probably a joke … I’d guess by Adam, and merely tolerated by Jamie.
Imagine repeating the football experiment underwater. Weigh the empty football. Then fill it with air. Weigh it again, if it’s not floating. It’s the same process that makes the football slightly lighter when filled with helium while in the air.
The difference between the experiment in the air and under the water is the density of the fluid medium the football is in. Air is much less dense than water, so the buoyancy effect is much smaller.
An deflated football is not pressurized enough for to bounce or fly very well, but it’s not empty- it’s got some air in it. There is not a vacuum in there. So a bunch of helium is lighter than a little air. I think it makes sense.
I’ve wondered about those weird labels on the crates, too. It’s a special effects workshop, so maybe the ones with weird labels contain whatever they use as a substitute for raw meat when they are testing something?
Yes, the bouyancy of the helium made that football weigh less, though it had greater mass than an empty football. If you did all 3 measurements in a vacuum, you’d see (air filled ball) > (helium filled ball) > (empty ball) .
Before Mythbusters, M5 was a working special effects studio (they may still do some of that kind of work). Perhaps “raw meat” is fake meat used in effects work. There may be another bin somewhere with fake cooked meat.
No, the other posters have it right. With the air let out, the ball collapses and takes up less space. Fill it with helium and it fills out to full size, and that displaces some air.
Think of it in terms of density. The empty football has a certain weight and a certain volume, and from that you can figure out the density. Pump some helium in that sucker and the weight goes up a little and the volume goes up a lot. That gives it a lower density. It will have a very slight tendency to want to float on the air; not enough to actually lift off the ground, but enough to make it weigh just a little less when you put it on a scale.
No, Marley’s idea is at least reasonable. I’m not sure how inflating the ball displaces the air though. Wouldn’t the inflated ball still have the little bit of air plus the added helium?
Collapse a pool raft completely and will probably sink. Fill it with a gas less dense than water, and it will float upward, thus “weighing” less in the watery environment, as the heavier water pushes it up. Remember that weight is not mass. The scale registered a lower push downwards by the ball as it tried to float up because its filling gas was helium.
Depends on what you mean by 'deflated". If it was deflated in the sense of remaining at ambient pressure then yes, it would indeed still have the bit of air. However if it was deflated in the sense use din the OP, ie all the air had been squeezed out by collapsing the ball, then no, it won’t contain a little bit of air, because all the air has been… squeezed out by collapsing the ball.
The problem with Marley’s idea is that it requires the first meaning of collapsed for the air filled ball, but the second for the helium filled ball. So it doesn’t really work.
However from the description given in the OP the trial, I have discovered another flaw in the trial.
The reference weight should have been made with the balls at ambient pressure, not collapsed. By collapsing the balls to determine their reference weight the Mythbusters would have created a partial vacuum within the ball. That vacuum comes from the ball trying to return to its normal shape. As a result of that vacuum a collapsed ball would actually weigh slightly less than the materials used to make the ball. IOW the weight the team measured would have been lower than reality.
Once again, this comes from the bouyancy effect of a low pressure vessel. Because the pressure inside a collapsed ball is lower than ambient, the ball will actually have a slight tendency to float, just as when it is filled with helium.
Note that this doesn’t change the overall outcome of the trial. It just means that, in reality, the weight loss due to the helium would have been even greater than measured.
I didn’t watch it, so I can’t effectively comment without knowing what physical method and system he used to weigh it… was it a scale? digital, hanging, spring? I think this is more a confusion about weight and mass.
Hell, my answer is that it was lighter because of anti-grav.
It’s already been explained, but I’ll add a part that makes it a bit clearer in my opinion. Remember that weight, as measured by a scale, is merely a measure of how much downward force. So anything that adds an upward force would make the object weigh less. The same thing happens if you weight a heavy object, and try to lift it. For a brief amount of time, you’ll see the scale say it is lighter (before it jumps down to 0).
One last way to look at it - if the ball were made of titanium so that it didn’t collapse when you drew the air out but remained exactly the same shape, then yes, the helium-filled one would weigh less than the air-filled, and the one with a vacuum inside would weight the least of all. Because the displacement of air by the ball is then identical in all three cases.
Or think of this thought experiment - put CO2 cylinder with a spring-loaded firing pin inside a collapsed football. Seal the whole thing up. Weigh it. Now discharge the CO2 - ball inflates - and weighs less than it did before, even though no gas went in or out. The difference is that the displacement of the ball changed, and its mass is now being supported by more air, so it appears to weigh less.
But as you press the balloon down on the surface of the xcale (usually metallic) is there a moment of equilibrium or cohesion- when the rubber meets the road so to speak- and that balloon registers other than zero… Are those measured as weights? That miniscule amount of molecular attraction, surface tension, and adhesion? What does that weigh?
But what if there is mollecular adhesion- then repulsion by the metallic surface, here’s the thing they don’t tell ya… all metal is magnetized. So, you might not get accurate weights, weighing a metallic object on a metallic stage.
No, most of it is not only not magnetized, but cannot be magnetized. Ferromagnetism is a pretty rare property. All materials are either paramagnetic or diamagnetic, but those won’t leave an impression of any sort on the material.
And even if the weighed object does stick to the scale, what difference would that make? You could superglue something onto a scale, and it’d still weigh the same (aside from the weight of the glue itself, of course).