I believe what the master is trying to say is density. A cloud contains a lot of mass, and weighs a lot, but it’s not very dense. A cloud falling on you isn’t going to hurt all that much.
There is a difference between weight and mass, but it can be confusing. Technically, mass never changes. If you have a mass of 100 kilograms on the Earth, you’ll have a mass of 100 kilograms on the moon too (and still be overweight). If you weigh 220 pounds on the Earth, you’ll only weight around 35 pounds on the Moon (and be the envy of your weight watchers class).
Weight is the mass times the acceleration. Your weight on the moon and Earth is caused by the acceleration of your mass due to gravity. However, since the Earth’s gravity has six times the acceleration, you weight six times more on the Earth than the moon.
The confusion stems from kilograms being a unit of mass and pounds being a unit of weight. Technically, weight in the metric system (SI Units) is measured in Newtons. Mass in the English (FPS System) is measured in slugs.
A person who weighs 220 pounds on Earth will have a mass of about 6.8 slugs on both the moon and on Earth. A person who has a mass of 100 Kilograms will have a weight of about 980 Newtons on Earth, but only about 160 Newtons on the moon.
However, to add to the confusion, the National Institute of Standards has officially defined the pound as 0.453_592_37 kilograms. Since pounds are defined against a unit of mass, the U.S. government considers pounds as a unit of mass instead of weight.
Note that in the metric system, the official designation of mass isn’t the gram, but the kilogram (which is why the pound is defined in kilograms and not grams). A gram is defined against the kilogram as 1/1000 of a kilogram, and not the other way around.
And the kilogram is defined by a specific hunk of metal sitting in France, the last of the fundamental units to be defined in such a way.
If you want to say that buoyant things are “weightless” , then you have to say that a hot air balloon, a blimp, the Titanic, and the iceberg that sank it are weightless too.
In actuality, a cloud is not even buoyant: it is more dense (both in aggregate and in parts) than the surrounding air. Rather, it is a colloidal suspension of small, heavier-than-air particles in a gas: an aerosol
Technically, the kilogram is the standard unit of mass in the SI system of measures, and in the old style MKS system (meter-kilogram-second). Those of us of exceptionally great age will remember the CGS system (centimeter-gram-second), which indeed uses the gram as the standard unit of mass. None of these are strictly “metric” in the sense that a second is 1/60th a minute, which is not a multiple of 10.
Wikipedia gives the weight of a 747-100B as 358,000 lb presumably at sea-level. This is the measure of the force of gravity that’s acting on the 747. This same force is still acting on the 747 even when flying at constant altitude. The difference is that while flying at constant altitude, the wings create an airfoil that provides 358,000 lb of lifting force, adding the two forces gives a total force of zero, thus level flight. So if we define “weight” strictly as a measure of the force of gravity, then the 747 still weighs 358,000 lbs. If we define it as the total force acting on the airplane, then we can say the 747 is “weightless”.
This article gives the weight of a typical cumulus cloud at around a million pounds. The derivation seems sound to me, and apparently seems sound to The Master as well. If I’m allowed to continue where the article leaves off, these falling raindrops impede the air’s uplift. The standard thunderstorm is a self-destructive phenomenon. In a severe thunderstorm, the column of uplifting air is separate from the rain column. Thus it can continue for quite some time and continue to grow. These are the dangerous storms that produce all the wicked weather we associate with severe weather (tornadoes, severe downdrafts, hail, exceptional wind gusts, foreign terrorism and very high rainfall rates).
The CGS system isn’t just for those of exceptionally great age. It’s still in common use in some disciplines. Astronomers, for instance, usually use CGS (when not using astronomy-specific units like parsecs and solar masses, at least), and I think chemists might, too.
It might not be much longer RadioLab has an interesting story about the attempts to define the kilogram via a magnetic field on a mass balance scale. The Podcast also talks about a rather embarrassing issue. It turns out that the Ur-Kilogram so well protected in France might be a little light. The seven weights that were based against it are all slightly heavier than the official kilogram.
Of course, since the kilogram is based upon that weight, that hunk of metal is not lighter than a kilogram. Instead, all of the other kilograms in the world are just a tad heavier than they should be. Officially, you now weight a fraction of a gram more than you use to.
Yeah, I’ve heard of trying to get of the “hunk of metal in France” standard, and am frankly surprised that we haven’t already.
The explanation for the weight loss that I’ve heard is that it’s probably losing weight from trace amount of osmium, which will react with oxygen and become volatile. The kilogram is made of a platinum-iridium alloy, and iridium is pretty chemically similar to osmium, making it hard to get a pure sample.
I haven’t listened to the podcast, so I’m not sure if that was covered or not.
It’s a tough standard. Officially, a gram was defined as “the absolute weight of a volume of pure water equal to the cube of the hundredth part of the metre, and at the temperature of melting ice.”. The problem is that water has different volume depending upon air pressure. Low pressure, and the kilogram will weigh less. Higher pressure, and it would weigh more.
You can’t define pressure without using mass, so it becomes a recursive definition. The other two measurements (time and length) were fairly easy to standardize. A second is so much decay from a particular atom. A meter is so many wave lengths of the emission of a particular kind of light. Mass is much harder to define. The current attempt is to use a watt balance scale.
Actually, currently the second is defined by a certain number of periods of a certain type of light (transition between hyperfine levels in a cesium atom), while a meter is defined as the distance that light travels in a certain fraction of a second. /pedant
obPedant: the critical plot element was not the gravity or mass of the titular neutron star, but the tidal forces caused by the neutron star’s steep gravitational gradient at close approach
I don’t see the connection to how much a cloud masses (or weighs). Maybe I lack imagination that way.
There is a second attempt to create an alternative to the current Platinum-Iridium ur-kilogram. The world’s roundest object(youtube video link) has some info on The Avogadro Project which is making a pure sphere of Silicon-28 which may, when precisely measured, allow us to mathematically calculate the mass of it, and define the kilogram based on it. Assuming the sample checks out as pure.
Helium is heavier than I thought: a balloon 3 meters across would weigh five pounds. That’s a lot of helium. But that’s not a trivial weight. It’s just that it displaces a sufficient mass of air so that it would float upward if given a chance. The concept is apparently buoyancy.
A cloud’s a little different though. A cloud is heavier (denser) than the surrounding air. If you boxed it up and put it on a scale, it would read a positive value, albeit much smaller than the true weight (it’s not that much denser than air). It’s just made up of particles small enough that they essentially don’t settle out on a reasonable timescale. It’s a colloid, specifically an aerosol.
[nitpick]… or ascending flight, or descending flight. The point being that when lift = weight, you have unaccelerated flight and thus constant vertical velocity - which can be positive, zero, or negative.[/nitpick]
Not sure this is correct … unaccelerated flight in the vertical would have to be in the absence of gravity. As long as the force of gravity is being applied, a 747 must create it’s lifting force. When lift > gravity, the plane goes up, when lift < gravity, the plane goes down. I understand that you’re applying of Newton’s second law of motion, but you have to include all the vertical forces involved. Gravity is constant, so the pilot can only adjust lift to adjust his altitude.
Sounds like the old joke: which is heavier, 100 lbs of lead or 100 lbs of feathers?
Answer:
100 lbs = 100 lbs. Duh.
If you take the cloud as a whole, then it doesn’t matter how dense it is, it still has the same mass.
The reason why a cloud falling on you doesn’t hurt all that much is that a cloud is not really a cohesive whole. You only get a tiny part of the cloud on you at any one time.