Potatoes Stick to Knife Blade

[Shalmanese]

Not really. If you read what I have pointed out several times, and if you read Physics 101, slicing is horizontal work, it take no horizontal force to work against vertical force (atmospheric pressure).

 

I am more than happy to keep explaining.

 

dcarch

If this were true, things would stick to a mandoline if you rotated it 90 degrees and potatoes would fall off knives if you just held it sideways.

Atmospheric pressure pushes on all sides equally (well, almost, there's a slightly higher pressure on the top side due to gravity but it's miniscule).

[dcarch]

If this were true, things would stick to a mandoline if you rotated it 90 degrees and potatoes would fall off knives if you just held it sideways.

Atmospheric pressure pushes on all sides equally (well, almost, there's a slightly higher pressure on the top side due to gravity but it's miniscule).

 

Depends on the mandoline, the first slice may stick, the second one will push the first  one forward away from the blade and falls off. If you hold the mandoline sideways, if the slice is thin, friction and weak water surface tension will hold it in place. If it is thick, gravity will pull it down. If you try to pull it off the blade vertically, then the laws of force and motion will need 14 lb /sq in to overcome (assuming the potato slice is not leaking air). 

 

Second statement is very correct. Not only Air pressure pushes on all sides, same pressure also pushes from inside out. That's why an open soda can is not crushed by pressure, and you don't feel any pressure on your body.

 

dcarch

[JoNorvelleWalker]

Because tonight's dinner is gratin dauphinois, and because I am a quondam scientist, and because I have a chamber vacuum sealer, I did an experiment:

 

Using a flat bladed knife I cut a thin slice of potato, which of course stuck.  I placed the knife over a bowl in the vacuum chamber, such that the potato slice was facing downward.  When I evacuated the chamber, near maximum vacuum, the slice of potato fell off into the bowl.  I repeated this experiment five times.  Just like clockwork each slice fell off at the same time of the vacuum cycle.

 

Make of this what you will.

 

In addition I noted that when cut with the flat bladed knife the potato slices reproducibly stuck.  However using my highwest chef's knife that has dimples on the blade, not one potato slice would stick.

[dcarch]

Nice experiment. Thanks.

 

dcarch

[gfweb]

The water "glue" evaporated at near max vac.

 

That it didn't fall off at lesser vacuums effectively  rules out air pressure as the cause of adherence.

 

Nice experiment.

[pbear]

With respect, I think Jo's experiments confirm what dcarch has been arguing.  That said, Norm Matthews explained how to solve this problem back in Post #3.  Edward J's solution also may work., but Norm's is what I've been using for a very long time.

[lordratner]

"it take no horizontal force to work against vertical force (atmospheric pressure)."

you can stop now. not sure how to put this politely, but you have no idea of what you're talking about.

The irony with this statement is that you are wrong. Just because the concept is beyond your ability to grasp, does not make it false.

Dcartch isn't great at explaining scientific concepts, but he's done good on this one.

[lordratner]

The water "glue" evaporated at near max vac.

That it didn't fall off at lesser vacuums effectively rules out air pressure as the cause of adherence.

Nice experiment.

In my experience with vacuum boiling, it takes a bit of time for the moisture to completely evaporate.

I think the two likely answers are:

Dcarch is right (I believe he is)

The boiling liquid is putting air between the potato and knife, which supports both your theory and Dcarch's.

[gfweb]

I'd say that the boiling water is putting water vapor between the plates first, then air perhaps. 

 

 

 

How much force does it take to open a newspaper? Next to none, even though air is pressing on it.

 

Now how about if the sheets are wet? significantly more, yet air pressure is the same.

[AlaMoi]

if you accept that atmospheric pressure is a vertical force:

 

"it take no horizontal force to work against vertical force (atmospheric pressure)."

 

there's nothing I can add.

Edited November 10, 2014 by AlaMoi (log)

[dcarch]

I'd say that the boiling water is putting water vapor between the plates first, then air perhaps. 

 

How much force does it take to open a newspaper? Next to none, even though air is pressing on it.

 

Now how about if the sheets are wet? significantly more, yet air pressure is the same.

 

On the other hand, the two videos I linked, and the experiment I proposed, Which I have tried, none requires water, nor potato starch.

 

And your newspaper example, the moment you peel one layer off, air immediately fill in to equalize the atmospheric pressure. That's how you open a can lid, if you have done food canning, and how you remove a suction cup, by peeling from the edge.

 

dcarch

[dcarch]

if you accept that atmospheric pressure is a vertical force:

 

"it take no horizontal force to work against vertical force (atmospheric pressure)."

 

there's nothing I can add.

 

No. Relative to atmospheric pressure, which is everywhere, horizontal and vertical, up and down ( based on hydraulic theory), the force you apply to slide (not pull) an object will be horizontal perpendicular to the atmospheric pressure.

 

Only gravitational force is always vertical, relative to objects on earth; therefore, it take no work to move horizontally ( as in riding a bicycle). The work you do is only to overcome mechanical friction and air flow resistance when riding a bike. A satellite rotates around the earth endlessly without additional energy horizontally against vertical gravitational force.

 

Isaac Newton realized the meaning of vertical force and horizontal work when he was under the apple tree, and figured out that if a canon ball got fired at speed high enough (centrifugal force) it would surround the earth endlessly.

 

 

dcarch

Edited November 10, 2014 by dcarch (log)

[rotuts]

adhesion's it still for me :

 

or ' water does not 'stretch' ' which might be Van de Wall- ish, what ever you want to call it.

 

at sea level, 1 atm is about 1 kg / cm²

 

lets use that number.   get a pipe, say a thick strong pipe so the pipe will not be an issue.

 

have some water in that pipe, but also carefully machined 'pistons' that slide along the inner pipe 'if they can.'

 

if the pipe is 1 cm² in surface area, you should be able to pull one of those pistons, or the only one if the pipe had one piston on the non-sealed end.

 

I can ( sometimes ) lift 1 kg.   it the seal around the movable piston does not leak, there is no way I can move that piston against the

 

forces that hold water together and make it non-elastic

 

as far as I can recall, no liquid is easy to stretch.

 

Im sure at some point the force pulling on the sealed water will allow the water to vaporize, which is a different issue.

 

i dont know how to look up the value of that force.  remember, is a 'special pipe'  a mighty strong one so the 

 

pipe wont be an issue, nor the seal around the piston to the pipe.

 

https://www.physics.umn.edu/outreach/pforce/circus/airpressure.html

 

take a look at the above, at the magdeberg 'swing'

 

500 lbs to pull that apart, but there is air between the disks and air is elastic

 

fluid much less so.

Edited November 10, 2014 by rotuts (log)

[dcarch]

adhesion's it still for me :

 

or ' water does not 'stretch' ' which might be Van de Wall- ish, what ever you want to call it.

 

at sea level, 1 atm is about 1 kg / cm²

 

lets use that number.   get a pipe, say a thick strong pipe so the pipe will not be an issue.

 

have some water in that pipe, but also carefully machined 'pistons' that slide along the inner pipe 'if they can.'

 

if the pipe is 1 cm² in surface area, you should be able to pull one of those pistons, or the only one if the pipe had one piston on the non-sealed end.

 

I can ( sometimes ) lift 1 kg.   it the seal around the movable piston does not leak, there is no way I can move that piston against the

 

forces that hold water together and make it non-elastic

 

as far as I can recall, no liquid is easy to stretch.

 

Im sure at some point the force pulling on the sealed water will allow the water to vaporize, which is a different issue.

 

i dont know how to look up the value of that force.  remember, is a 'special pipe'  a mighty strong one so the 

 

pipe wont be an issue, nor the seal around the piston to the pipe.

 

https://www.physics.umn.edu/outreach/pforce/circus/airpressure.html

 

take a look at the above, at the magdeberg 'swing'

 

500 lbs to pull that apart, but there is air between the disks and air is elastic

 

fluid much less so.

 

Everything you have said, and the link you provided shows the importance of 14.7 lbs of pressure on conditions when two surfaces are closely put together. A slice of potato on a shiny knife blade for example.

 

Water is not really elastic, otherwise rain will come down in the shape of silk, not droplets.

 

Air is not elastic, otherwise bubbles in your aquarium will look very different.

 

 

dcarch

[lordratner]

I'd say that the boiling water is putting water vapor between the plates first, then air perhaps.

How much force does it take to open a newspaper? Next to none, even though air is pressing on it.

Now how about if the sheets are wet? significantly more, yet air pressure is the same.

Not a great example, since the news paper doesn't keep air out well.

There are videos in this thread that show how strong the vacuum can be with no water on a simple plastic bag.

If the potato starch and water was so sticky, you wouldn't be able to slide it sideways

[rotuts]

""    shows the importance of 14.7 lbs of pressure on conditions when two surfaces are closely put together  ""

 

yes.  but pulling apart a gas is relatively easy compared to pulling apart a liquid. that's my only point

 

its the properties of a liquid in the potato case that makes 'sticky-ness'

 

"""    Air is not elastic  """ its very elastic compared to a liquid.   that elasticity allows you to pump it out of a 'sealed' chamber

 

thus creating a 'vacuum camber'   good luck pulling out a liquid from a 'sealed' chamber    my only point.

 

I cant recall what to look up that can tell us the forces needed to pull apart a ( polar ) liquid

 

much more than to pull apart a gas.   polar probably takes more energy to pull apart than a non polar liquid, but

 

the potato is mostly water.

Edited November 10, 2014 by rotuts (log)

[rotuts]

good point.  but sliding is not pulling apart, its just using the liquid as a lubricant. it sort of 'rolls right along'

[dcarch]

""    shows the importance of 14.7 lbs of pressure on conditions when two surfaces are closely put together  ""

 

yes.  but pulling apart a gas is relatively easy compared to pulling apart a liquid. that's my only point

 

its the properties of a liquid in the potato case that makes 'sticky-ness'

 

"""    Air is not elastic  """ its very elastic compared to a liquid.   that elasticity allows you to pump it out of a 'sealed' chamber-------------------------

 

Elasticity and compress-ability are not the same. Water cannot be compressed. Air gets more concentrated if compressed, following Boyle's law.

 

As I said up thread water can provide some stickiness from surface tension, but that is no where near 14.7 lbs.

 

dcarch

[rotuts]

water cannot also be 'expanded' ( elasticity ) in a closed sealed chamber w a vent to 'pull' on it considering

 

reasonable forces.  it must take a lot lot more than 14.7 lbs to 'expand' water, I just cant find that figure.

 

 

 

air also can be both compressed and expanded in a chamber  as any gas can.

Edited November 10, 2014 by rotuts (log)

[rotuts]

one more thought :  in the ref. above I made mentioning suction cups, swings etc somehow energy was applied to the system in question

 

so there is a differential pressure across that cup  'the suck' and that 'swing'

 

in slicing a potato w a thin smooth surface  ( The Knife ) I see no mechanism to create any pressure differential. one would need that 

 

differential to create the 'suck'  

 

tale a peek at that suction cup above :  the rubber material is initially distorted, then the 'spring-back-ed-ness' of the material creates

 

the differential pressure.  work ( applied energy ) is the distortion work that reconfigures the rubber and the rubber somehow prefers its 

 

initial state, ie creating that pressure differential.

 

I cant see how a knife creates a pressure differential.  water 'seals' the potato to the knife and needs to be 'unsealed' either by a 

 

'flick of the wrist' or a channel on the knife to let some air get between the potato and steel or ceramic edge.

 

Time to Pull a Cork :

 

I note that TJ's lower shelf White has an air interface in the top of the bottle between the cork and the liquid.  the surface area

 

of these corks is exactly 1 cm² I use a levered cork screw to gip the cork ,an leverage to pull it out.  1 kg in no laughing matter

 

just think what it would be like if that cork was sealed right down to the Wine ( Very Special Water, with Unusual

 

and Delicious Properties !   )

 

with no air interface for me to work with and expand Id have a lot more trouble 'pulling a cork'

 

lucky or unlucky me !

Edited November 10, 2014 by rotuts (log)

[JoNorvelleWalker]

Now imagine if that special water, by some circumstance, could be under well higher than atmospheric pressure...

[dcarch]

"------so there is a differential pressure across that cup  'the suck' and that 'swing'

in slicing a potato w a thin smooth surface  ( The Knife ) I see no mechanism to create any pressure differential. one would need that 

differential to create the 'suck'  ---"

 

That part of the experiment can be misleading. The fact is, no force is required for the suction cup or the "swing" to be pressed together. You can find out easily by the experiment I suggested, and a few experiments in the video I linked. All you need is to come up with a way to prevent air from leaking into the space between the two surfaces, which can counteract the atmospheric pressure for you to experience the 14.7 lbs/sq in force.

 

dcarch

[rotuts]

"""    Now imagine if that special water, by some circumstance, could be under well higher than atmospheric pressure...  """

 

yes indeed. if that Special Water were, say Heavy Water, and that higher then atmospheric pressure were produced by intensely 

 

focused X-Rays, well, you'd have a hydrogen bomb right there in that Potato !

[rotuts]

""  no force is required for the suction cup or the "swing" to be pressed together "" 

 

yes, its fhe external force of 1 ATM.

 

" a way to prevent air from leaking into the space between the two surfaces "

 

yes again.  if the two 'vacuum' spheres never had the air pumped out, and were 'super welded' together

 

and no air could ever enter the sphere, they would never come apart, so the second statement simply serves itself.

[dcarch]

Permit me to go off  topic a little, although I think the thinking applies to many cooking situations. Many cooking myths happen because:

 

In a science lesson longtime ago, this terrible example was given in the lecture to illustrate observations and conclusions, so people, please don't send me hate mail.

 

A fellow throws a frisbee and commands his dog to fetch.  The dog obliged.

 

Then he cuts off the dog’s legs and again commands the dog to fetch a Frisbee. Obviously the dog was unable to do so.

 

The fellow therefore concludes that dogs use legs for hearing.

 

Apologies to all dog lovers.

 

dcarch