Today, I was watching an ABC education program about friction. Three statements on the program, not necessarily wrong, seem worthy of debate or at least discussion.

1. Without friction, you couldn’t fly in an indoor skydiving centre.

2. Without friction, there would be no glues.

3. Without friction, trees would all fall over.

mollwollfumble said:

Today, I was watching an ABC education program about friction. Three statements on the program, not necessarily wrong, seem worthy of debate or at least discussion.

1. Without friction, you couldn’t fly in an indoor skydiving centre.

2. Without friction, there would be no glues.

3. Without friction, trees would all fall over.

Good questions.

I’ll have a think.

The Rev Dodgson said:

Good questions.

we only have 1

Where ¿

mollwollfumble said:

Today, I was watching an ABC education program about friction. Three statements on the program, not necessarily wrong, seem worthy of debate or at least discussion.

1. Without friction, you couldn’t fly in an indoor skydiving centre.

2. Without friction, there would be no glues.

3. Without friction, trees would all fall over.

subject friction, I remember way way back, maybe was a brief product review, or patent or something, regard a low friction nut, as in nuts that go on bolts, anyway they did sort of ask in the review of what keeps the nut on, that that’s sort of the idea, friction keeps it in place, keeps whatever tight

transition said:

mollwollfumble said:

Today, I was watching an ABC education program about friction. Three statements on the program, not necessarily wrong, seem worthy of debate or at least discussion.

1. Without friction, you couldn’t fly in an indoor skydiving centre.

2. Without friction, there would be no glues.

3. Without friction, trees would all fall over.

subject friction, I remember way way back, maybe was a brief product review, or patent or something, regard a low friction nut, as in nuts that go on bolts, anyway they did sort of ask in the review of what keeps the nut on, that that’s sort of the idea, friction keeps it in place, keeps whatever tight

in NS it was, as recall

transition said:

mollwollfumble said:

Today, I was watching an ABC education program about friction. Three statements on the program, not necessarily wrong, seem worthy of debate or at least discussion.

1. Without friction, you couldn’t fly in an indoor skydiving centre.

2. Without friction, there would be no glues.

3. Without friction, trees would all fall over.

subject friction, I remember way way back, maybe was a brief product review, or patent or something, regard a low friction nut, as in nuts that go on bolts, anyway they did sort of ask in the review of what keeps the nut on, that that’s sort of the idea, friction keeps it in place, keeps whatever tight

Think about what would have to change to eliminate friction

transition said:

transition said:

mollwollfumble said:

Today, I was watching an ABC education program about friction. Three statements on the program, not necessarily wrong, seem worthy of debate or at least discussion.

1. Without friction, you couldn’t fly in an indoor skydiving centre.

2. Without friction, there would be no glues.

3. Without friction, trees would all fall over.

subject friction, I remember way way back, maybe was a brief product review, or patent or something, regard a low friction nut, as in nuts that go on bolts, anyway they did sort of ask in the review of what keeps the nut on, that that’s sort of the idea, friction keeps it in place, keeps whatever tight

in NS it was, as recall

> 1. Without friction, you couldn’t fly in an indoor skydiving centre.

The first one is subtle because without friction the fluid dynamics equations admit two solutions. In one solution the indoor skydiver can fly, in the other solution the skydiver can’t fly. It’s like an example from catastrophe theory.

To highlight the paradox, if we take a situation with fixed air speed with the skydiver in flight and slowly reduce the friction to zero then the skydiver will remain in flight even when the friction has dropped to zero.

But if we reduce the friction to zero first and then very slowly increase the air speed then the skydiver can’t fly. But disturb this air stream and the separation point jumps to a new location and the skydiver can fly.

> 3. Without friction, trees would all fall over.

Would it be like a tree (with roots) floating in dense water? Or not?

If so, then a key factor in stability is the location of the metacentre. A higher metacentre implies greater initial stability against overturning. But again there’s an application of catastrophe theory to be considered here. A tree (like a ship) may be stable against small displacements, but is likely to be unstable when subjected to large displacements.

But if it isn’t like a tree floating in dense water – then what?

> 2. Without friction, there would be no glues.

Um. It’s easier to concoct situations where glue would not work, such as flat surface on flat surface. But when one of the surfaces is concave then what? This also draws in the molecular mechanism behind friction. Nil friction would nullify hydrogen bonds but not covalent bonds.

But wouldn’t that mean that you can get geometric friction without any friction in shear, because of surface unevenness. And if there is no friction because of nil surface unevenness, then there’s a continuum between microscopic unevenness that we ascribe to friction and macroscopic unevenness such as metre-scale protrusions.

To get nil friction would we have to specify an arbitrary cut-off length between the microscopic and macroscopic? I think so.

If we do specify an arbitrary cut-off length and allow covalent bonding to exist in a nil-friction environment then there are glues and adhesives.

i’d love to know what keeps engine big-end cap bolts from coming loose, must be deformation in various surfaces

transition said:

i’d love to know what keeps engine big-end cap bolts from coming loose, must be deformation in various surfaces

The bolts are torqued to the point where they are safely in the middle of the elastic deformation zone. That applies a huge load on the threads & under the bolt head.

Spiny Norman said:

transition said:

i’d love to know what keeps engine big-end cap bolts from coming loose, must be deformation in various surfaces

The bolts are torqued to the point where they are safely in the middle of the elastic deformation zone. That applies a huge load on the threads & under the bolt head.

about what I imagined, yeah

cheers

transition said:

Spiny Norman said:

transition said:

i’d love to know what keeps engine big-end cap bolts from coming loose, must be deformation in various surfaces

The bolts are torqued to the point where they are safely in the middle of the elastic deformation zone. That applies a huge load on the threads & under the bolt head.

about what I imagined, yeah

cheers

That makes sense. Does that mean that even without friction, the bolts will stay in place?

transition said:

Spiny Norman said:

transition said:

i’d love to know what keeps engine big-end cap bolts from coming loose, must be deformation in various surfaces

The bolts are torqued to the point where they are safely in the middle of the elastic deformation zone. That applies a huge load on the threads & under the bolt head.

about what I imagined, yeah

cheers

> That makes sense. Does that mean that even without friction, the bolts will stay in place?

Ooh, I have an answer to that and the answer is fascinating. If the bolts and nuts are perfectly elastic (linear or nonlinear elastic) then the bolts will not stay in place but will start to move and continue to move with the slightest push.

But if the bolt or nut material has some hysteresis, even a tiny amount, then the bolt will stay where it is. Every metallic material has some hysteresis, from the movement of internal dislocations, so the bolt will stay in place.

But if the bolt and nut material are not metallic but perfect crystals then perfect elasticity (no hysteresis) is possible, and the bolt will undo itself unless the bolt geometry is imperfect in such a way (tapered) that it automatically tightens itself.

mollwollfumble said:

transition said:

Spiny Norman said:

The bolts are torqued to the point where they are safely in the middle of the elastic deformation zone. That applies a huge load on the threads & under the bolt head.

about what I imagined, yeah

cheers

> That makes sense. Does that mean that even without friction, the bolts will stay in place?

Ooh, I have an answer to that and the answer is fascinating. If the bolts and nuts are perfectly elastic (linear or nonlinear elastic) then the bolts will not stay in place but will start to move and continue to move with the slightest push.

But if the bolt or nut material has some hysteresis, even a tiny amount, then the bolt will stay where it is. Every metallic material has some hysteresis, from the movement of internal dislocations, so the bolt will stay in place.

But if the bolt and nut material are not metallic but perfect crystals then perfect elasticity (no hysteresis) is possible, and the bolt will undo itself unless the bolt geometry is imperfect in such a way (tapered) that it automatically tightens itself.

the compression force energy goes into fitting the two surfaces in their stopped position, both on threaded surfaces and nut faces or bolt head faces against whatever

i’d guess a force in the direction of undoing initially has to overcome a hill of resistance

the hill of resistance are caused by deformations (variations) of material compression, and stretching too on the thread surfaces

transition said:

mollwollfumble said:

transition said:

about what I imagined, yeah

cheers

> That makes sense. Does that mean that even without friction, the bolts will stay in place?

Ooh, I have an answer to that and the answer is fascinating. If the bolts and nuts are perfectly elastic (linear or nonlinear elastic) then the bolts will not stay in place but will start to move and continue to move with the slightest push.

But if the bolt or nut material has some hysteresis, even a tiny amount, then the bolt will stay where it is. Every metallic material has some hysteresis, from the movement of internal dislocations, so the bolt will stay in place.

But if the bolt and nut material are not metallic but perfect crystals then perfect elasticity (no hysteresis) is possible, and the bolt will undo itself unless the bolt geometry is imperfect in such a way (tapered) that it automatically tightens itself.

the compression force energy goes into fitting the two surfaces in their stopped position, both on threaded surfaces and nut faces or bolt head faces against whatever

i’d guess a force in the direction of undoing initially has to overcome a hill of resistance

the hill of resistance are caused by deformations (variations) of material compression, and stretching too on the thread surfaces

you notice the hill of resistance when you undo a wheel nut, those sorts of things, you need crack it from the bedded position, so to speak

so, it could be said, the compression force causes the faces to fit, to bed with each other, surface imperfections and deformations tend to familiarize, match each other with and opposite surface, they self-lock

transition said:

transition said:

mollwollfumble said:

> That makes sense. Does that mean that even without friction, the bolts will stay in place?

Ooh, I have an answer to that and the answer is fascinating. If the bolts and nuts are perfectly elastic (linear or nonlinear elastic) then the bolts will not stay in place but will start to move and continue to move with the slightest push.

But if the bolt or nut material has some hysteresis, even a tiny amount, then the bolt will stay where it is. Every metallic material has some hysteresis, from the movement of internal dislocations, so the bolt will stay in place.

But if the bolt and nut material are not metallic but perfect crystals then perfect elasticity (no hysteresis) is possible, and the bolt will undo itself unless the bolt geometry is imperfect in such a way (tapered) that it automatically tightens itself.

the compression force energy goes into fitting the two surfaces in their stopped position, both on threaded surfaces and nut faces or bolt head faces against whatever

i’d guess a force in the direction of undoing initially has to overcome a hill of resistance

the hill of resistance are caused by deformations (variations) of material compression, and stretching too on the thread surfaces

you notice the hill of resistance when you undo a wheel nut, those sorts of things, you need crack it from the bedded position, so to speak

so, it could be said, the compression force causes the faces to fit, to bed with each other, surface imperfections and deformations tend to familiarize, match each other with and opposite surface, they self-lock

Yes. Self-locking is forbidden if there’s no friction, though, because that counts as static friction.

A fluid without friction is a superfluid, like cryogenic helium.

Without friction, surface tension would still work, so the surface tension from the lubricating oil around a bolt could end up holding a loose bolt in.

> 3. Without friction, trees would all fall over.

A tree whose roots are wedged in a crack in a rock would not fall over, even without friction. I’m thinking particularly of those trees whose roots penetrate through small holes in limestone into the caverns below.

I’m wondering now whether:

4) Would a human being without friction disintegrate into a pile of bones and a puddle of dead cells on the floor?

transition said:

mollwollfumble said:

transition said:

about what I imagined, yeah

cheers

> That makes sense. Does that mean that even without friction, the bolts will stay in place?

Ooh, I have an answer to that and the answer is fascinating. If the bolts and nuts are perfectly elastic (linear or nonlinear elastic) then the bolts will not stay in place but will start to move and continue to move with the slightest push.

But if the bolt or nut material has some hysteresis, even a tiny amount, then the bolt will stay where it is. Every metallic material has some hysteresis, from the movement of internal dislocations, so the bolt will stay in place.

But if the bolt and nut material are not metallic but perfect crystals then perfect elasticity (no hysteresis) is possible, and the bolt will undo itself unless the bolt geometry is imperfect in such a way (tapered) that it automatically tightens itself.

the compression force energy goes into fitting the two surfaces in their stopped position, both on threaded surfaces and nut faces or bolt head faces against whatever

i’d guess a force in the direction of undoing initially has to overcome a hill of resistance

the hill of resistance are caused by deformations (variations) of material compression, and stretching too on the thread surfaces

Doesn’t overheating change all of this?

The Rev Dodgson said:

mollwollfumble said:

Today, I was watching an ABC education program about friction. Three statements on the program, not necessarily wrong, seem worthy of debate or at least discussion.

1. Without friction, you couldn’t fly in an indoor skydiving centre.

2. Without friction, there would be no glues.

3. Without friction, trees would all fall over.

Good questions.

I’ll have a think.

OK, I have thunk.

1. I don’t think friction is a requirement for lift from an aerofoil. Differential compression across the upper and lower surfaces will do that.

2. This seems to be saying you can’t have cohesion without friction. I don’t see why not. I don’t see why in principle you couldn’t have a molecular bond between a perfectly flat surface and a glue.

3. Again, this seems to be ignoring cohesion, but let’s suppose a tree with frictionless roots is planted in a granular soil with no cohesion and no friction between the particles. In this case the soil would act like a liquid, but the soil would not support the “root ball”, it would pass through the sand just like the roots of a tree with no soil would pass through water.

So yes, the tree would fall over if there was no friction and no cohesion.

If there was cohesion the soil would act just like a clayey soil, and the tree would not fall over.

mollwollfumble said:

transition said:

Spiny Norman said:

The bolts are torqued to the point where they are safely in the middle of the elastic deformation zone. That applies a huge load on the threads & under the bolt head.

about what I imagined, yeah

cheers

> That makes sense. Does that mean that even without friction, the bolts will stay in place?

Ooh, I have an answer to that and the answer is fascinating. If the bolts and nuts are perfectly elastic (linear or nonlinear elastic) then the bolts will not stay in place but will start to move and continue to move with the slightest push.

But if the bolt or nut material has some hysteresis, even a tiny amount, then the bolt will stay where it is. Every metallic material has some hysteresis, from the movement of internal dislocations, so the bolt will stay in place.

But if the bolt and nut material are not metallic but perfect crystals then perfect elasticity (no hysteresis) is possible, and the bolt will undo itself unless the bolt geometry is imperfect in such a way (tapered) that it automatically tightens itself.

You are going to have to provide some evidence for the statement:

“But if the bolt or nut material has some hysteresis, even a tiny amount, then the bolt will stay where it is”

For the nut not to move there has to be some force parallel to the slope of the interface, so if we are saying there is no cohesion there must be some friction.

For exactly zero friction the surfaces would have to have exactly equal slope everywhere, and they would have to be infinitely stiff, so they couldn’t have hysteresis anyway, but it’s not the hysteresis causing the friction, irregularities caused by elastic deformation would result in some friction.

The Rev Dodgson said:

mollwollfumble said:

transition said:

about what I imagined, yeah

cheers

> That makes sense. Does that mean that even without friction, the bolts will stay in place?

Ooh, I have an answer to that and the answer is fascinating. If the bolts and nuts are perfectly elastic (linear or nonlinear elastic) then the bolts will not stay in place but will start to move and continue to move with the slightest push.

But if the bolt or nut material has some hysteresis, even a tiny amount, then the bolt will stay where it is. Every metallic material has some hysteresis, from the movement of internal dislocations, so the bolt will stay in place.

But if the bolt and nut material are not metallic but perfect crystals then perfect elasticity (no hysteresis) is possible, and the bolt will undo itself unless the bolt geometry is imperfect in such a way (tapered) that it automatically tightens itself.

You are going to have to provide some evidence for the statement:

“But if the bolt or nut material has some hysteresis, even a tiny amount, then the bolt will stay where it is”

For the nut not to move there has to be some force parallel to the slope of the interface, so if we are saying there is no cohesion there must be some friction.

For exactly zero friction the surfaces would have to have exactly equal slope everywhere, and they would have to be infinitely stiff, so they couldn’t have hysteresis anyway, but it’s not the hysteresis causing the friction, irregularities caused by elastic deformation would result in some friction.

So trains would work as there is deformation of the rails caused by the wheel.

The Rev Dodgson said:

The Rev Dodgson said:

mollwollfumble said:

Today, I was watching an ABC education program about friction. Three statements on the program, not necessarily wrong, seem worthy of debate or at least discussion.

1. Without friction, you couldn’t fly in an indoor skydiving centre.

2. Without friction, there would be no glues.

3. Without friction, trees would all fall over.

Good questions.

I’ll have a think.

OK, I have thunk.

1. I don’t think friction is a requirement for lift from an aerofoil. Differential compression across the upper and lower surfaces will do that.

2. This seems to be saying you can’t have cohesion without friction. I don’t see why not. I don’t see why in principle you couldn’t have a molecular bond between a perfectly flat surface and a glue.

3. Again, this seems to be ignoring cohesion, but let’s suppose a tree with frictionless roots is planted in a granular soil with no cohesion and no friction between the particles. In this case the soil would act like a liquid, but the soil would not support the “root ball”, it would pass through the sand just like the roots of a tree with no soil would pass through water.

So yes, the tree would fall over if there was no friction and no cohesion.

If there was cohesion the soil would act just like a clayey soil, and the tree would not fall over.

Ccouple of comments:

1…. He was not talking about aerofoils. It was indoor sky-diving. Where high-velocity air from below keeps a person airborne.

3…. Trees fall over here in high winds, but without the wind they seen to stay upright. (Perhaps trees are well-balanced).

There is no clay in the “soil” here. Only very fine-grained sand and 1-3% very fine grained organic matter. It is incohesive near the surface when dry. Below the surface it is surprisingly cohesive when even just slightly damp.

I suppose that’s in part why the sand-hills can stand to 220 m high, and the sand cliffs on their seaward side can sustain 70+ degree slopes, despite constant wind and water erosion.

Tamb said:

The Rev Dodgson said:

mollwollfumble said:

> That makes sense. Does that mean that even without friction, the bolts will stay in place?

Ooh, I have an answer to that and the answer is fascinating. If the bolts and nuts are perfectly elastic (linear or nonlinear elastic) then the bolts will not stay in place but will start to move and continue to move with the slightest push.

But if the bolt or nut material has some hysteresis, even a tiny amount, then the bolt will stay where it is. Every metallic material has some hysteresis, from the movement of internal dislocations, so the bolt will stay in place.

But if the bolt and nut material are not metallic but perfect crystals then perfect elasticity (no hysteresis) is possible, and the bolt will undo itself unless the bolt geometry is imperfect in such a way (tapered) that it automatically tightens itself.

You are going to have to provide some evidence for the statement:

“But if the bolt or nut material has some hysteresis, even a tiny amount, then the bolt will stay where it is”

For the nut not to move there has to be some force parallel to the slope of the interface, so if we are saying there is no cohesion there must be some friction.

For exactly zero friction the surfaces would have to have exactly equal slope everywhere, and they would have to be infinitely stiff, so they couldn’t have hysteresis anyway, but it’s not the hysteresis causing the friction, irregularities caused by elastic deformation would result in some friction.

So trains would work as there is deformation of the rails caused by the wheel.

I suppose a train on a perfectly flat track might work if you were happy to accept near zero acceleration.

Michael V said:

The Rev Dodgson said:

The Rev Dodgson said:

Good questions.

I’ll have a think.

OK, I have thunk.

1. I don’t think friction is a requirement for lift from an aerofoil. Differential compression across the upper and lower surfaces will do that.

2. This seems to be saying you can’t have cohesion without friction. I don’t see why not. I don’t see why in principle you couldn’t have a molecular bond between a perfectly flat surface and a glue.

3. Again, this seems to be ignoring cohesion, but let’s suppose a tree with frictionless roots is planted in a granular soil with no cohesion and no friction between the particles. In this case the soil would act like a liquid, but the soil would not support the “root ball”, it would pass through the sand just like the roots of a tree with no soil would pass through water.

So yes, the tree would fall over if there was no friction and no cohesion.

If there was cohesion the soil would act just like a clayey soil, and the tree would not fall over.

Ccouple of comments:

1…. He was not talking about aerofoils. It was indoor sky-diving. Where high-velocity air from below keeps a person airborne.

OK, but the same argument applies. It’s the pressure difference that reduces the rate of fall, and that would still be there without friction.

Michael V said:

3…. Trees fall over here in high winds, but without the wind they seen to stay upright. (Perhaps trees are well-balanced).

There is no clay in the “soil” here. Only very fine-grained sand and 1-3% very fine grained organic matter. It is incohesive near the surface when dry. Below the surface it is surprisingly cohesive when even just slightly damp.

I suppose that’s in part why the sand-hills can stand to 220 m high, and the sand cliffs on their seaward side can sustain 70+ degree slopes, despite constant wind and water erosion.

With no cohesion and no friction they wouldn’t even stand up even without any wind.

As for real soils, I agree. All soils (even clays) have quite a bit of friction, and even sands have some cohesion, even if we engineers like to pretend that soils are either cohesive or frictional.

The Rev Dodgson said:

The Rev Dodgson said:

mollwollfumble said:

Today, I was watching an ABC education program about friction. Three statements on the program, not necessarily wrong, seem worthy of debate or at least discussion.

1. Without friction, you couldn’t fly in an indoor skydiving centre.

2. Without friction, there would be no glues.

3. Without friction, trees would all fall over.

Good questions.

I’ll have a think.

OK, I have thunk.

1. I don’t think friction is a requirement for lift from an aerofoil. Differential compression across the upper and lower surfaces will do that.

Yes there’s the pressure differential that helps cause lift but also simple Newtonian reaction – The air molecules hitting the bottom of the wing/skydiver and thus causing that body to have an upward(ish, in the case of a wing) reaction.

Spiny Norman said:

The Rev Dodgson said:

The Rev Dodgson said:

Good questions.

I’ll have a think.

OK, I have thunk.

1. I don’t think friction is a requirement for lift from an aerofoil. Differential compression across the upper and lower surfaces will do that.

Yes there’s the pressure differential that helps cause lift but also simple Newtonian reaction – The air molecules hitting the bottom of the wing/skydiver and thus causing that body to have an upward(ish, in the case of a wing) reaction.

But wouldn’t the air molecules also hit the top of the wing?

Tamb said:

Spiny Norman said:

The Rev Dodgson said:

OK, I have thunk.

1. I don’t think friction is a requirement for lift from an aerofoil. Differential compression across the upper and lower surfaces will do that.

Yes there’s the pressure differential that helps cause lift but also simple Newtonian reaction – The air molecules hitting the bottom of the wing/skydiver and thus causing that body to have an upward(ish, in the case of a wing) reaction.

But wouldn’t the air molecules also hit the top of the wing?

Yes but not so much as the air that goes over the top is trying to pull away from the top, thus helping create a region of low-pressure. My explanations are a bit simplistic but close enough.

Spiny Norman said:

Tamb said:

Spiny Norman said:

Yes there’s the pressure differential that helps cause lift but also simple Newtonian reaction – The air molecules hitting the bottom of the wing/skydiver and thus causing that body to have an upward(ish, in the case of a wing) reaction.

But wouldn’t the air molecules also hit the top of the wing?

Yes but not so much as the air that goes over the top is trying to pull away from the top, thus helping create a region of low-pressure. My explanations are a bit simplistic but close enough.

I see what you are saying. Sounds logical.

1. Without friction, you couldn’t fly in an indoor skydiving centre.

Without friction there would be no natural forces, no gravity would mean all atomic particles in the universe would not attract each other.

There would be no galaxies, black holes, stars, planets and life. All atomic particles would be free to drift around in space time.

Creating an extreme entropic universe.

I keep seeing this title as Children’s Science fiction…. and get a bit interested.

Arts said:

I keep seeing this title as Children’s Science fiction…. and get a bit interested.

+1

Tau.Neutrino said:

1. Without friction, you couldn’t fly in an indoor skydiving centre.

Without friction there would be no natural forces, no gravity would mean all atomic particles in the universe would not attract each other.

There would be no galaxies, black holes, stars, planets and life. All atomic particles would be free to drift around in space time.

Creating an extreme entropic universe.

I think you should submit your proof that gravity requires friction for immediate peer review.

If accepted there could be a Nobel prize or two in this.

Sex would be pretty ordinary.

sibeen said:

Arts said:

I keep seeing this title as Children’s Science fiction…. and get a bit interested.

+1

A new science show mixing Dr Who with Professor Julius Sumner Miller

:)

Arts said:

I keep seeing this title as Children’s Science fiction…. and get a bit interested.

I hope the lack of fiction isn’t causing friction.

Peak Warming Man said:

Sex would be pretty ordinary.

Yes, most people would lose interest all altogether

meaning less washing hands and cleaner people.

Peak Warming Man said:

Sex would be pretty ordinary.

Probably require application of bindings at appropriate positions to work at all.

How you would stop the bed sliding all over the floor, I don’t know.

The Rev Dodgson said:

Peak Warming Man said:

Sex would be pretty ordinary.

Probably require application of bindings at appropriate positions to work at all.

How you would stop the bed sliding all over the floor, I don’t know.

You could nail it to the floor.

Tau.Neutrino said:

The Rev Dodgson said:

Peak Warming Man said:

Sex would be pretty ordinary.

Probably require application of bindings at appropriate positions to work at all.

How you would stop the bed sliding all over the floor, I don’t know.

You could nail it to the floor.

Would surface tension still work?

Tamb said:

Tau.Neutrino said:

The Rev Dodgson said:

Probably require application of bindings at appropriate positions to work at all.

How you would stop the bed sliding all over the floor, I don’t know.

You could nail it to the floor.

Would surface tension still work?

I don’t see why not.

Others may disagree.

No friction

No universe

Spiny Norman said:

The Rev Dodgson said:

The Rev Dodgson said:

Good questions.

I’ll have a think.

OK, I have thunk.

1. I don’t think friction is a requirement for lift from an aerofoil. Differential compression across the upper and lower surfaces will do that.

Yes there’s the pressure differential that helps cause lift but also simple Newtonian reaction – The air molecules hitting the bottom of the wing/skydiver and thus causing that body to have an upward(ish, in the case of a wing) reaction.

Yep. I agree with you there, but this relates back to my earlier statement that without friction the fluid dynamics equation for flow around an aerofoil have two solutions. In one solution it has lift and in the other solution it doesn’t. The jump from one to the other depends on the location of the trailing stagnation point. If the trailing stagnation point is the aerofoil tip, as we normally see, then the aerofoil has lift in zero friction. But if the trailing stagnation point is one third of the way along the top chord, the other possible solution to the equations, then there is no lift.

> I keep seeing this as Children’s science – fiction and getting excited.

I was hoping you would ;-)

wookiemeister said:

No friction

No universe

Um, without friction you can still have an inflationary epoch, still have a gas and a temperature, still have condensation into stars and asteroids. But after that, things get a bit tricky. Stars wouldn’t behave the way we would want. Asteroids aren’t slowed by the extrasolar nebula enough to allow gravity to make them larger.

> So yes, the tree would fall over if there was no friction and no cohesion. If there was cohesion the soil would act just like a clayey soil, and the tree would not fall over.

Cohesion, I forgot all about cohesion. Good point.

4. What about people? Without friction would we all collapse into a pile of bones in a puddle of dead cells?

mollwollfumble said:

Spiny Norman said:

The Rev Dodgson said:

Yes there’s the pressure differential that helps cause lift but also simple Newtonian reaction – The air molecules hitting the bottom of the wing/skydiver and thus causing that body to have an upward(ish, in the case of a wing) reaction.

Yep. I agree with you there, but this relates back to my earlier statement that without friction the fluid dynamics equation for flow around an aerofoil have two solutions. In one solution it has lift and in the other solution it doesn’t. The jump from one to the other depends on the location of the trailing stagnation point. If the trailing stagnation point is the aerofoil tip, as we normally see, then the aerofoil has lift in zero friction. But if the trailing stagnation point is one third of the way along the top chord, the other possible solution to the equations, then there is no lift.

> I keep seeing this as Children’s science – fiction and getting excited.

I was hoping you would ;-)

wookiemeister said:

No friction

No universe

Um, without friction you can still have an inflationary epoch, still have a gas and a temperature, still have condensation into stars and asteroids. But after that, things get a bit tricky. Stars wouldn’t behave the way we would want. Asteroids aren’t slowed by the extrasolar nebula enough to allow gravity to make them larger.

> So yes, the tree would fall over if there was no friction and no cohesion. If there was cohesion the soil would act just like a clayey soil, and the tree would not fall over.

Cohesion, I forgot all about cohesion. Good point.

4. What about people? Without friction would we all collapse into a pile of bones in a puddle of dead cells?

I shouldn’t think so, but there must be a lot of things that wouldn’t work very well, from walking or even standing upright, down to the microscopic level.