What I want to know is the relationship that lubricants have with viscosity. Is viscosity the cause of lubrication? The higher the viscosity, the more lubrication? And why not water instead of oil?
35233
How come mineral oil is a better lubricant than water, even though water has a lower viscosity?    When two surfaces slide over each other with a gap filled with a fluid, the different layers of the fluid are dragged at different speeds. The very top layer touching the top metal surface will have the same speed as the surface itself, while the bottommost layer is stationary. The speed in the layers between is distributed linearly and there exist friction forces between those layers that slow the movement. Those frictional forces should be reduces however, if a fluid with a lower viscosity is chosen.   How come this is not so?   Does it have to do with water's polarity, so that it sticks to surfaces in a different way than oil?
53833
When I got more into physics, I started asking myself if just like acceleration represents the growth of speed, something else could also represent the growth of acceleration itself. And it came that is exists and is called jerk. Before I thought about this, I thought that acceleration was the highest derivative that made sense on physics.  My question is, why can we perform accurate calculations without account for jerk? I never needed it in a single physics class! I've also read that it is seldom used.  Also, while position, speed and acceleration seem familiar to me due to everyday experience, why doesn't jerk seem familiar to me? I never tell someone when driving a car "you're jerking too much", whereas I could say "you are accelerating too much".  Is it that jerk has little influence because it is almost constant? Even if it is not, what about higher order derivatives? Why don't derivatives higher than acceleration have much influence on calculations? We perform a lot of reasoning in everyday life without resorting to it!
75171
I am confused by rolling friction.  Suppose you have a cylinder rolling which starts at rest at the top of an incline plane and begins to roll down the plane without slipping. Is work done by the incline on the cylinder?   I know from doing some problems that the total kinetic energy (translational and rotational) is $mgh$, which is true only if the only work done on the cylinder is by gravity. But also, the cylinder must have nonzero angular acceleration, so friction must be exerting torque on the cylinder, so work must be done by friction. One of these statements is wrong.
55231
When you add heat to a liquid (or a fluid), can it be solidified? If not, why in the world does an egg's stuffs become solid (or at least no more a liquid) when you 'boil' it in water?
13577
It is commonly told that, hydrophobic/ lipophilic/ nonpolar molecules can quite easily pass phospholipid bilayer, and hydrophilic (polar or ionic) molecules can't pass (when no protein aid that); because hydrophobic nature of the lipid.  But in the same logic, hydrophobic molecules shouldn't pass through the bilayer. Because there are 2 hydrophilic layers in the membrane. i.e.      A and A' .  Then how the hydrophobic molecules can pass through A and A'?   I guess, it is due to thin diameter of A and A' . Is that?
40972
What are some guidelines or rules with regard to filtering and reusing (vegetable/canola/sunflower) oil that has been used for deep frying?  Is it safe and acceptable under some circumstances to filter and keep oil? If so, does it depend on which foods were fried? How long can the filtered oil for be kept for?  Or, is it never appropriate? If not, are the reasons health or flavour related? or both?
31717
We know that $T_c$ is the temperature above which no amount of pressure could force a gas to liquefy. But why is this? Somehow I don't buy the point that the gas molecules exert too much pressure back to get close and turn into a liquid. If we had tens of thousands of atmosphere pressure (such as on the inside of hot planets), we should be able to liquefy any gas at any temperature.
5490
I'd like answers both in the more intuitive side an on the more precise side.   Thinking of water as "cubes" of water, for example, would allow pressure in the z axis to be independent of the y or x axis. Choosing other "shapes", like a  triangular prism, yield different results. Will this fact, then, be dependent on the format of the molecules of the liquid ?  I've heard of the rotational symmetry of liquids. What is its precise statement ? Why is it true ?
65072
Why does differentiability implies continuity, but continuity does not imply differentiability?   I am more interested in the part about a continuous function not being differentiable.  Well, all I could find in regards to why continuous functions can not be differentiable were counter- examples...   I just wanted to know if there was a more detailed way of explaining this.
9398
I've been looking for questions about dark matter, and I've read some very interesting answers. However, I desire too look into it deeply.   This is not actually a question. I'm asking the community to recommend interesting references to understanding dark matter and dark energy.   I accept all sort of references: notes, books, scientific papers etc.  Let us assume some background on classical physics, thermodynamics and basics about quantum theory.
106215
What's the theory on using water vs oil for chicken marinades?  I ask because of this recipe:    After multiplying the recipe by a lot, it makes a good marinade, but almost all of the other marinades I've seen online involve oil.  Why does this one use water?    (In case the link goes bad, the recipe is:   1 Tbsp Honey  1 Tsp Yellow Mustard  1 Tsp Sriracha 1 Tbsp Water)
56362
What is the difference between Speed, Velocity and Acceleration? Could any one describe it pictorially?. I am more over confused even after investigating many times. I am unable to relate myself practically.
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?)
123321
I read that:  If you take a rough surface and make it smooth, the coefficient of friction decreases. But if you make it super smooth, then the coefficient of friction increases. How come?
8610
When dragging an object, there is a greater start-up force than the force it takes to keep it moving. Why is this? Why are there two different values for friction?
69088
Having read the Wikipedia page on  I'm still not sure if stuff like liquid helium at the lambda point actually have surface tension or not. Is superfluidity the same thing? And are there any liquids with no surface tension at room temperature?
112135
What I Think I Know: Hydrophilic and hydrophobic things repel each other. Since the cell membrane contains hydrophobic tails, it is difficult for hydrophilic molecules to pass through the cell membrane.  Question: Why don't the heads of phospholipid bilayers repel hydrophobic molecules? In other words, if the phospholipid membrane has both hydrophilic and hydrophobic portions, why do only the hydrophobic portions act as repellents?  Stipulation: I need a quotation from a textbook, university, or other credible source.  Evidence of Prior Research:  : This question has a comment that links a graph from a textbook. I can't understand the graph by itself. There are no other references.  : All references discuss the topic, but none answer the question.  I understand that this topic is addressed quite frequently, but the question I'm asking is never answered. Texts tend to say something like, "Ions can't get through the core because of their charge" and completely gloss over the concept of lipids getting through the heads. Please don't dismiss this as a homework question that I didn't take the time to research. This is my second try asking this question (on this site); I have spent hours trying to find an answer.