Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: KrakenKing on 24/08/2020 02:15:51

Title: Frictionless Cogs
Post by: KrakenKing on 24/08/2020 02:15:51

I've had this question floating around in my head ever since one of my favorite YouTubers said this weird thing: "If you fell off a 30 story building and landed on a solid concrete FRICTIONLESS floor, could you ever actually touch it? if you can't touch it, would you actually die? Just ignore the fact the dynamic forces exist hahaha" I think it was a joke? whatever it was it made me think, everything in our life that has to do with mechanics uses friction to operate; including cogs in a clock.

Now here's the question: if you pinned two 100% frictionless cogs parallel to each other and on the same plane, tight enough so the cant move in any of the 6 axes, and close enough so that the cogs intertwine, would one move the other if you somehow made it spin? If one would move the other, WHY???

Title: Re: Frictionless Cogs
Post by: Kryptid on 24/08/2020 04:52:41

You would definitely die after jumping off a building onto a frictionless surface because what kills you is extreme deceleration.

One cog could indeed move the other because there is no room for one cog to slip past the other. If one cannot move past the other, then Newton's third law guarantees that the force applied by the teeth of one cog would result in an equal and opposite force from the other cog.

Title: Re: Frictionless Cogs
Post by: Bored chemist on 24/08/2020 09:06:38

The people who make cogs try to keep friction to a minimum.
Unlike two wheels in contact, the transmission of movement between two gogs does not rely on friction.
It's more like the rack and pinion rail system used for mountain trains.
That system has the advantage that it can run on steeper gradients because it doesn't depend on friction.

Title: Re: Frictionless Cogs
Post by: evan_au on 24/08/2020 10:35:24

The ultimate in long-life cogs has to be the 10,000 year clock being built by the Long Now Foundation.

They are trying to get people to think past tomorrow, past next week, and past the next election...
See: https://longnow.org/clock/

Title: Re: Frictionless Cogs
Post by: Colin2B on 24/08/2020 14:36:21

Quote from: Bored chemist on 24/08/2020 09:06:38

movement between two gogs does not rely on friction.

Unlike you, I don’t have 2 gogs to rub together. Am I missing something?

Title: Re: Frictionless Cogs
Post by: CPT ArkAngel on 24/08/2020 18:04:37

To speak of a frictionless floor or cogs is unphysical. Only a frictionless dark matter is possible. Matter would pass through this dark matter without any friction but gravity would still apply. If the floor stops you, it is not frictionless.

Title: Re: Frictionless Cogs
Post by: Bored chemist on 24/08/2020 19:18:00

A cog does not rely on friction to turn another cog.
It's a set of levers.
They push.
In the limit where one cog is very big it is the same as a rack and pinion like this one
https://en.wikipedia.org/wiki/Rack_railway#/media/File:Zahnradbahn_riggenbach.jpg
which is used to push trains up hillsides.

Imagine that the train is very heavy. It would slide back down the track if there was only friction driving it up.

But, as you can see in that picture, for the train to slide back, the teeth of the gear would have to pass through the metal of the track.
What pushes the train up is a normal force, not a frictional one. (apologies for the awful picture)

gear.jpg (46.39 kB . 820x580 - viewed 2295 times)
It's the push down by the force represented by the  red arrow and up by the force drawn in green.
They cut the teeth on gears so that the surfaces roll over one another rather than sliding.

https://en.wikipedia.org/wiki/Involute_gear

Title: Re: Frictionless Cogs
Post by: pzkpfw on 24/08/2020 23:19:14

All makes me think of the time I got to do ice skating on a frozen canal.

Horizontally, parallel with the low-friction ice, it was fun to skate along.

Vertically (falling!), perpendicular to the low-friction ice: this 8 year old boy shouted out in a New Zealand accented English among all these Dutch people: "b*****y hell, this stuff is as hard as concrete!".

Title: Re: Frictionless Cogs
Post by: alancalverd on 25/08/2020 00:18:01

Quote from: Bored chemist on 24/08/2020 09:06:38

movement between two gogs does not rely on friction.

The Gog Magog hills south of Cambridge are known locally as The Gogs. Most movement between them relies on tyre friction. With a maximum elevation of less than 250 ft, you don't need a rack and pinion: a bike with frictionless cogs is entirely adequate.   

Title: Re: Frictionless Cogs
Post by: Halc on 25/08/2020 01:06:07

Said youtuber and the OP, the latter having seemingly left the building immediately after his sole post, seem to be under the impression that lack of friction means lack of ability to apply force. This is wrong.
Static friction is a force preventing lateral motion between surfaces in mutual contact.  Kinetic friction is a force resisting lateral motion of two relatively moving such surfaces.  The former does no work and requires no energy. The later requires energy, converting it to heat.

Neither of these things prevents normal (as opposed to lateral) motion, so falling off a building kills you, frictionless surface or no, and frictionless cogs work just fine.
Interesting way to die is via kinetic friction. You jump off a 200m building and ride a high-friction slope all the way down. The friction either heats you up and cooks you or possibly just sands you away into nothing.

Title: Re: Frictionless Cogs
Post by: CPT ArkAngel on 25/08/2020 04:54:14

It depends on your definition of friction. If you look at the details, I'm right. Though I don't disagree with Bored Chemist with his practical point of view. But if you look down the rabbit hole, you will find that the classical definition hits a floor of contradictions...