Wikipedia:Reference desk/Archives/Science/2020 January 6

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January 6

Gears in gym equipment

The most common weight lifting machines around me just consist of a set of metal plates (can be configured) attached to a cable that go through two pulleys. I know that alternative ways of creating resistance for exercising exist, like pneumatic, elastics, and so on. I suppose these are not as resilient and low on maintenance as a simple block of metal plates with a cable attached. I wonder, couldn't they just make a machine with one weight and a set of gears to add more resistance? I imagine you could also use a level to alter the resistance, but that seem more of clumsy solution. --31.4.128.9 (talk) 08:20, 6 January 2020 (UTC)

They could, but a possible downside is that people like to know how much weight they are lifting, and the usual arrangement makes that obvious. Alansplodge (talk) 13:59, 6 January 2020 (UTC)

• What's "resistance"? Are you after a large force on a small movement, or a lower force over a long distance? Weights have several advantages: they're simple, reliable and cheap. They also provide a high force, and they provide a constant force. Most spring devices will change force over their travel. It's also easy (with a pin in a stack) to change the force from the same machine, keeping the same overall distance. It's also easy to assemble a weight-based machine, whilst a spring machine might need an assembly jig and carry a risk of finger injury.

For high force devices, most use weights. Some use the operator's own weight - a common design for outdoor park exercise machines is based on lifting the user's own weight, which is usefully self-calibrating for a wide range of users, without needing to adjust anything. These tend to use levers as force adjusters, as the most long-term reliable machine. A few machines use springs or hydraulics, but these are more complicated and have maintenance issues. Elastic (i.e. polymer elastomers) is rarely used (small home-use devices), as it has lifetime issues and a sudden snap is a real problem to design for in a "safe" machine.

For higher forces, a pulley system is more common than gears. It stands up better to wear and exercise equipment is in a very litigious market, where reliability is important. There's sometimes a need to offer a longer movement (some horizontal pulls) and pulleys can do that too - otherwise it's easier to just make more weight go up higher.

For aerobic exercise rather than strength, the load was usually an air fan, a viscous drag or a magnetic eddy current in an aluminium disc. Nowadays though, machines are computer controlled, so active electrical devices (motors) are used, even though they're expensive, complicated and need a power source, just because they're more easily controlled. Andy Dingley (talk) 14:51, 6 January 2020 (UTC)

A convict in Victorian Britain might find exercise machines crank and treadmill provided for his benefit. DroneB (talk) 16:25, 6 January 2020 (UTC)

Neither of those were 'exercise' machines. They (deliberately) weren't particularly hard labour, they were mostly boring. Where convicts did actual hard labour (and Dartmoor was one of the few UK prisons where this was the classically hard-working stone-breaking work) they had to be fed better, or else they starved and died. In the typical prisons, the work was tedious but not so energetic, and so the food costs were lower. Andy Dingley (talk) 16:42, 6 January 2020 (UTC)

Torture and Democracy by Darius Rejali (p. 301) says: 'Nineteenth-century British prisons and reformatories preferred exhaustion exercises. Guards required prisoners to perform shifts on various devices, notably the treadmill (1817) and the crank (1840). As one ascended the wheel, the previous step slid away. It exhausted the strongest of men in fifteen minutes. Turning the crank required turning a handle twenty revolutions a minute, for a total often thousand revolutions in 8.5 hours. “No human being, whether adult or juvenile, could continue to perform such an amount of labour of this kind for several consecutive days, especially on a prison diet, without suffering much and wasting greatly".'

The quote is from Report of the Commissioners Appointed to Inquire Into the Condition and Treatment of the Prisoners Confined in Birmingham Borough Prison, and the Conduct, Management, and Discipline of the Said Prison (1854) p. vii. Alansplodge (talk) 18:02, 6 January 2020 (UTC)

"Victorian" goes on a long while after that though, and these were falling from use. I think the crank stayed for a long time, but it was specifically punitive (they still have one in Lincoln Castle). Andy Dingley (talk) 18:13, 6 January 2020 (UTC)

We have both a section of a treadmill, and a crank, in Ripon Prison and Police Museum. --ColinFine (talk) 19:10, 7 January 2020 (UTC)

Other sources of resistance are flywheels and generators. The latter have the advantage of offering a "reward" such as powering an electric fan. NonmalignedNations (talk) 06:22, 8 January 2020 (UTC)

Thought experiment: Star with mass in the Brown dwarf mass range with a lot of fissionable elements

Could a brown dwarf with enough Uranium or Potassium 40 shine above its weight class? Perhaps it could do normal hydrogen fusion, instead of deuterium fusion?144.35.45.50 (talk) 23:29, 6 January 2020 (UTC)

If it performed hydrogen fusion, it would by (current) definition not be a brown dwarf, but a low-mass red dwarf. The two catagories are not strictly segregated by mass (or any other single criterion), and may overlap in characteristics depending which one(s) one chooses to apply. {The poster formerly known as 87.81.230.195} 90.204.182.54 (talk) 23:55, 6 January 2020 (UTC)

Right, but beyond the question of classification, could such a star exist?Thanks. Rich (talk) 00:43, 7 January 2020 (UTC)

A natural nuclear reactor could easily exist if the concentration of uranium was high. Of course this is unlikely in a uniformly mixed gaseous body. Perhaps a star formed from binary neutron star merger hypernova output could be highly radioactive and hot, also containing plutonium and thorium. A lot of potassium would also make the body hot. Graeme Bartlett (talk) 03:06, 7 January 2020 (UTC)

Do you think it would require a fission chain reaction, rather than just many isolated fissions? The fissions here on earth produce a lot of heat.Rich (talk) 04:40, 7 January 2020 (UTC)

To explain what I meant, suppose a brown dwarf had a mass of 50 Jupiters, and for some bizarre reason, maybe advanced alien tech, that one of those 50 jupiter masses was entirely Pottasium 40. Then isolated disentegrations should provide much much more than just the heat of 32 Earths, because although jupiter is a mass of around 32 earths, fissionable atoms are a small fraction of theearths mass.Rich (talk) 04:51, 7 January 2020 (UTC)

This could actually make the star a more massive brown dwarf, possibly over the "typical" mass limit for a brown dwarf. A main sequence star is a ball of stuff in hydrostatic equilibrium; its gravity constantly tries to collapse it more, while the fusion in the core generates heat and tries to expand the star, and these two forces maintain a constant tug-of-war as long as there's sufficient fuel in the core. If there's another source of heat, this expands the star as well, opposing the gravitational collapse and therefore reducing the temperature and pressure in the core, possibly below that necessary for the p–p chain. --47.146.63.87 (talk) 07:47, 7 January 2020 (UTC)

Interesting!Rich (talk) 20:53, 7 January 2020 (UTC)

To stretch your noodle some more, there are various hypothetical stars supported against collapse by something other than heat produced by fusion. A couple are already known to exist: white dwarfs and neutron stars are supported by electron and neutron degeneracy pressure, respectively. A Thorne–Żytkow object would be a red giant with a neutron star as its core. Quasi-stars may have been some of the first stars in the universe, with black holes as their cores, supported against collapse by the radiation and heat generated as the black holes accreted matter. --47.146.63.87 (talk) 06:34, 10 January 2020 (UTC)

Fascinatinging!Rich (talk) 01:50, 11 January 2020 (UTC)

(edit conflict) Weight is not really a thing when talking about stars; scientists talk about them in terms of mass, which doesn't vary based on gravitational effects. If a star has enough mass to start the proton–proton chain, it's a red dwarf by definition, as noted above. A star with high metallicity will be denser than a low-metallicity star of the same mass, because the "metals" are denser and thus pack more mass into a smaller volume. This is an example of human intuition short-circuiting. We intuitively think of weight and mass as the same thing, because we think about things in our everyday existence under the influence of Earth's gravity. But stars are ginormous things that gravitate under their own gravitational fields. The primary thing that determines a star's character is its mass, because its mass is what produces its gravitational field. --47.146.63.87 (talk) 04:59, 7 January 2020 (UTC)

I don’t think any of the posters, including myself, have in any way confused mass with weight. You’ve misunderstood what’s been said.Rich (talk) 07:07, 7 January 2020 (UTC)

The OP, who 47 replied to did in fact use "weight class" in the question and "mass range" in the heading. I don't know if the OP is actually confused about the difference between weight and mass, but although the term weight is often used to refer to mass in a colloquial sense, I'd agree with 47 that this is one case where it important to appreciate the difference. Nil Einne (talk) 14:38, 7 January 2020 (UTC)

This is my bad, im the OP. I forgot i used that idiom/sports cliche. “Punching(or shining or performing) out of one’s weight class often does not, now, refer to weight and boxing anymore, for example if a usually poor student outstudies and outscores the class brain on an exam, or if a slow sprinter gets a fast time on the 400 meters due to high anaerobic endurance, it’s said they are performing out of their weight class.Rich (talk) 20:51, 7 January 2020 (UTC)

To clarify the question/premise: ⁴⁰K isn't a fissionable nuclide - instead it can decay by ordinary beta decay as well as electron capture and positron emission. Natural uranium and thorium isotopes show [[spontaneous fission], but at a very low rate - much more heat will be generated by the alpha decay of these elements, combined with decays of the daughter nuclides.

Anyway, you might be interested in a curious object which seems to contain short-lived actinides: Przybylski's Star

Icek~enwiki (talk) 22:30, 12 January 2020 (UTC)