This is a common phenomenon and most of the people have encountered this while moving on ice:falling down! Why does this occur?  According to me, it is due to lack of static friction. But why will it be absent here? What is the cause? I have heard that there is a thin layer of water above the ice . But why,unlike other solids,  is this thin layer present? Will it not cool down to ice?
60336
Some textbooks I came across, and a homework assignment I had to do several years ago, suggested that the reason we can skate on ice is the peculiar $p(T)$-curve of the ice-water boundary. The reasoning is that due to the high pressure the skates put on the ice, it will melt at temperaturs below $273 K$ and thus provide a thin film of liquid on which we can skate. It was then mentioned as fun fact that you could ice-skate on a planet with lakes of frozen dioxide because that gas has the $p(T)$-curve the other way round. My calculations at that time told me that this was, pardon my french, bollocks. The pressure wasn't nearly high enough to lower the melting point to even something like $-0.5$ degrees Celsius. I suppose it is some other mechanism, probably related to the crystal structure of ice, but I'd really appreciate if someone more knowledgeable could tell something about it.
65210
I am curious what causes the charge build up in a cloud before lightning occurs. I have seen in a few places such as , that the process is not fully understood. Is this some form of static electricity? Here are a few of my thoughts:   It seems that rain or ice particles occur are present in the clouds where this happens since thunderstorms are usually accompanied by rain. These clouds tend to be cumulus or cumulonimbus clouds which are much taller than other types that are commonly encountered. Is the physical size of the cloud related to this type of buildup of charge or is it a result of the moisture content?  If the charge buildup is caused by static electricity which two surfaces are contacting to create the charge? Is this just rain and ice particles? Can it be something else? (I think I recall something about lightning storms without rain in the desert) Why do some clouds with lightning have rain and others do not if the rain drops are the source of the static? Is there a minimum solid or liquid particle density needed for static electricity within a cloud?    Any thoughts would be interesting to hear!
30798
This is a question that's been bothering me for the last couple of days:  How is it that the hobbits of the Fellowship were able to survive such cold temperatures throughout their travels without having any foot protection?   I know it's said that they have thick "leather like" soles, with ample amounts of hair for warmth but still, I don't understand how that could provide much more warmth than for instance a regular leather shoe. On top of this I know that multiple days (or even weeks) of their journey was spent crossing mountain ranges, which, near a summit would have a temperature far below freezing. This combined with the assumption that they only have the warmth of shoes on there feet would have presumably resulted in some level of frostbite. But to my knowledge there is no such mention of these problems in the novels or movies aside from the odd "It's cold".  Anyways, I'm just wondering if anyone has some form of explanation to this phenomenon or do hobbits just have really durable feet?
48047
See below gif image taken from .    Or see  about 30 sec in.    Is this a real effect?  Why does it seem to turn periodically? Can it be explained by classical mechanics alone? Is there a simple equation that models this behaviour?
121349
Why is latent heat required for a system to change its state? (I.e. liquid water to ice) Why the "extra" energy?
112197
When it rains and water flows down an inclined street, ripples may form that are carried along with the current. Here's a picture with an example of what I'm talking about    I'd like to know what the fluid mechanical mechanism is for the formation of these waves. A quick google search reveals that they've been identified in some scientific literature as "rain waves" (if you have a jstor subscription you can see ) but I've only been able to find phenomenological descriptions of their properties without any details of the physics giving rise to them. In particular, I'm curious if people think that the surface on which the water is flowing must have a particular texture in order to give rise to the waves.
56154
I've come across a couple of videos where some interesting sounds are produced using ice.  (Click on the images to see the video.)      Here, they drop a block of ice into a deep crevice and as the block falls, you can hear some strange sounds. These sounds remind me of some sound effects in old cartoons and sci-fi movies. The pitch of the sounds start high and end up low. Is this some kind of doppler effect?      Here, there's a man performing 'nordic skating', making the same sounds as he skates on top of a thin ice sheet. Maybe they are caused by the cracking ice. But why does that sound like this?    There are some other videos too, where they skip stones on frozen lakes.    Here the ice makes some high-frequency sounds which correspond to the chirping of birds.  What are these sounds and why do they sound the way they sound?
35012
since the Christmas season is here, I would like to ask a question about the movie, "A Christmas Story." In one of the subplots of the movie, Ralphie's friends were betting each other that their tongue would stick to to a frozen pole. Finally, the kid did it and it stuck to the pole.   Why does this happen? I believe that there is a physics explanation for this.
84391
Let's take a bottle (no cap) with half its volume filled with water. If we rotate the bottle at some slower average velocity, water does not drop out of it. The same principle works on . Wikipedia says that the phenomenon of  is used here. What is the physics behind this? (I mean, Which opposes or equilibrates gravity during the vertical loop?)
55786
When you have a drink with an ice cube and twirl the glass, the liquid itself seems to twirl but the ice cube stays roughly in the same place. Why is this?
58853
A bucket is rotating in a uniform circular motion. When the bucket is at the topmost point in the loop, the water does not fall. At this point both the tension and gravitational force is acting on the water, then it should be pulled with an even greater force. Then why does the water not fall?  The same question can be asked about a motorcyclist performing a loop de loop. Why does he not fall on the topmost point when both gravity and normal reaction are acting downwards?
117040
So this Youtube video has been around for a while now  . Also this  may be related.  I just wanted to know if there is any standard hydrodynamic terms to explain it. Derek clearly states it is not about Bernoulli principle, as in this case the ball rotation is important.  Has any recent paper/videos discussed what happens if you do the same experiment with hydrophobic spheres? Can it be predicted with laminar flow theories or does it need turbulence?  Too many questions have been posed, but not so many responses on the internet yet.  Edit: I said Derek stated "it wasn't Coandă effect", yet what he says is that it is not "just Bernoulli's principle". Coandă effect is the typical hairdryer ping-pong ball levitation which is explained by Bernoulli's principle.   Edit2: rephrasing.  Additional comment: There seems to be two explanations "water adhere to the sphere and is ejected conserving horizontal momentum" and "Magnus effect" (ME). The later is not so obvious to me as the system is in equilibrium and there are two fluids, water and air, (does ME means air pushes the ball horizontally?).   Edit3: DESCRIPTION To avoid rot links and people that do not want to see the video, I'll try to explain the phenomena. You have a sphere and a water jet. The water jet is vertical (opposite to gravity), and touches the sphere almost tangentially. The ball not only rotates due to the water jet but also stays at constant height from the ground. The angular velocity can be defined in the direction of the cross product between the position of the contact point between the jet and the sphere (origin in the center of the sphere) and the direction of the jet. Some water drops fly away at the top of the sphere away from the jet.  Edit4: New question, would this phenomenon occur if there was no air?
44210
The usual explanation for warm fluids rising past cooler ones is that the warmer fluid has a lower density.  I'm trying to understand what this looks like at a molecular scale.  The density seems to be a large-scale phenomenon, and I don't understand how it can affect whether a particular molecule rises or falls.  Consider a cylinder of fluid that is being heated at the bottom.  The molecules at the bottom have a higher average energy.  How does this result in the tendency of the warmer molecules at the bottom to move upwards past the cooler ones?
68782
So I was thinking... If heat I feel is just lots of particles going wild and transferring their energy to other bodies, why am I not burned by the wind?  When I thought about it more I figured out that wind usually carries some humidity, and since particles of liquid are moving same speed as the wind, they are basically static relative to each other, so no energy is transferred between them (wind and water particles). And if that water sticks to my skin and wind blows, it'll evaporate thus taking energy from my skin and make me feel cold.  Thing is, I don't think that's really the case but even if it is, if I somehow dry out the wind, will it burn me if it's strong enough? And winds can reach some pretty high velocities (though I must admit I'm not sure if they are comparable to movement of atoms in warm bodies etc...).  So. Bottom line. Can I be burned by wind in some perfect scenario?
96379
This has happened in a couple of different places on my server. Does anyone have any idea what causes it to happen?  Some facts about the phenomenon:   other players on the server can't see them, only I can other players can walk out onto the ground that's supposed to be there animals can also walk out onto the missing ground, but their movement becomes sporadic several holes can appear very close to one another after leaving for some time, and then returning, the hole may be gone mobs can sometimes be seen walking along the bottom of the recess there appears to be a room (possibly a dungeon) suspended in the middle of the second one these are incredibly creepy   I haven't been brave enough to venture out onto one yet, but will try and post results.  Edit: It turns out when you fully jump into one, you fall the length of a block, and then you're pushed back up again. This happens repeatedly, and very quickly, so that you get stuck midair and have to /tp to get out.  Also, the hole in the second picture came back, but there is no longer a big room at the bottom.
31480
This is a strange question I would like to have some explanation accompanied with:  We have an ice block, with in it a lead block. This entire block is put into a glass of water, resulting in a rise of water level until the edge of the glass. The block stays floating on the surface. What will happen with the water level when the ice melts?  The answer is that it will drop, but how is this even true?? My thoughts are: - block floats on water, so rho(block) &lt; rho(water) - Archimedes law, the Fg of block = Fg of water pushed away. - Since a smaller volume of water has equal mass (and so Fg) to block, and ice is less dense than water, when it melts, no mass is lost, so it will take in less space??? Please help me with this point of thinking.
92882
If you have a glass of water, say, three quarters full and you pour it at an angle of say, $45^{\circ}$ with respect to the the table, the water comes out of the glass and goes directly down towards the floor.  However, when the glass is more full, or even three quarters full and the 'angle of pouring' is far less with respect the table, when the water comes out of the glass, rather than going straight down it kind of stays stuck to the glass and travels down the outside of it.  Why does this happen? (I'm sure many of you have observed it yourself by accident and ended up making a mess).