# Tag Info

107

Your derivation is composed of correct statements and indeed, if something is known to act as a lubricant, we want the viscosity to be as low as possible because the friction will be reduced in this way. For example, honey is a bad lubricant because it's too viscous. However, your derivation isn't the whole story. The second condition is that the two ...

27

A good lubricant tends to effectively minimize direct contact among components of any device that need it Keeping this in mind, viscosity is not the only factor involved. Grind a graphite pencil lead, and it makes a mighty fine lubricant. It might be that in the case of water placed between two surfaces, a water droplet which was supposed to act as an ...

19

The parallel plate situation that you describe is not the typical condition encountered in practical lubrication operations. In addition to facilitating the surfaces sliding over one another, the lubricated bearing must also support a normal load. To do this, the gap between the surfaces varies with location along the bearing. For example, in a journal ...

12

Why Oil is Slippery Explaining why oil is slippery requires a look at its chemical properties. First, oil is non-polar, which means it does not have a positive or negative charge. Some molecules, like water, have a “charge distribution,” which means the molecule acts almost like a battery, part of it has a positive charge and part of it has a ...

8

Hair, like fingernails and animal horn is made up mostly of a protein called Keratin. The strength and hardness of this polymer is caused by three types of chemical bonds: salt bonds, hydrogen bonds and disulphide bonds. Water can significantly break the first two types (but not the bisulphide ones). Significantly wetting hair thus leads to making it more ...

6

I can only give you a hand-waving explanation, but the fastest way to empty a bottle I have empirically found is to pour it upside down but rotate the bottle. The main problem is that the empty space above the water needs to be filled again with air which can not get through. If you rotate the bottle correctly, an air corridor inside the bottleneck opens and ...

6

My high school physics teacher was saying that “this is because of interference of sound waves. During the day, there are a lot of sounds and they cancel each other due to interference. But, during the night, there are few sounds and they can reach to our ears without canceling each other”. You need a better high school physics teacher. Temperatures tend ...

5

I should start with the disclaimer that I don't know the answer to this, however I have seen very similar patterns in flocculating systems and I would guess that the same principles are involved. The patterns are produced by adding a drop of pigment to a layer of slip. Both of these are colloidal suspensions. Slip is a suspension of aluminosilicate ...

4

@tbf is right; lubrication, and tribology in general, is complicated. That's why there is that high effort to understand it and to design advanced materials. There are several phenomena that cause the friction force exist and the ones you have neglected causes that oils are superior to water in most industral applications. In dry sliding we can identify ...

4

It's definitely related to a surface tension That is correct. Surface tension means that the total surface area wants to be minimised. Although the water surfaces aren't very well drawn, the surface is slightly concave in the half filled glass, slightly convex in the brim filled glass. The presence of the ball reduces the total surface area. As a ...

3

As a rough estimate: we can approximate the shape of the can with a cylinder of radius $R=3,2$ cm and height $h=15,5$ cm (those are the dimensions of a Cola can). Its total area can be found by using $$A_T=A_L+2A_B= 2\pi R h + 2\pi R^2$$ which gives us $A_T$=$375,8$ cm$^2$. Assuming that water vapor condensation forms a 1 mm thick, continuous sheet of ...

3

I would tend to agree that background noise is a factor, but rather than reducing, adding to the sound you are trying to make sense of. So part of that may be how your brain is able to filter the information from the background noise. But at night the temperature is lower and according to this tutorial on sound propagation (which does cite reliable ...

3

Option 3: Squeezing the bottle. Of course it depends how much pressure you are able to do, the strongest you are the fastest, and if you compress the bottle in an industrial press, the water will leave in a split second without breaking the bottle (well, it gets deformed, but that was an option).

3

What I have observed is that if we turn the filled bottle (open) upside down and plug in a straw then the water starts to flow out faster. This happens because by plugging in a straw we make a way for air to come inside the bottle and fill empty space. Another observation that I've made is that when water is flowing out of a bottle (upside down) just shake ...

2

An intuitive way of thinking about boiling an egg, is that you start a pool with a bunch of balls of yarn floating in it. Then when you heat the egg, it causes all the balls of yarn to unravel, and rather than a pool with some balls in it now it's a tangled mess of yarn. Before, each ball was free to move around as a unit, now each segment of each strand can ...

2

Various types of apparent temperature have been developed to combine air temperature and air humidity. For higher temperatures, there are quantitative scales, such as the heat index. For lower temperatures, a related interplay was identified only qualitatively; e.g., in a 2012 textbook: High humidity and low temperatures cause the air to feel chilly Or ...

2

Water can not bear normal loads as well as oil. Water is bound to escape from high pressure bearings to lower presser places in an open lubrication loop leaving bear contacts. Water can create bubbles around cavities and corners and break the laminar flow which will compromise the separation of moving parts. Water will react chemically with surfaces. There ...

2

Cut the bottle base and squeeze the bottle The air pressure has a major role to play in this situation. If you keep the bottle vertical, there wouldn't be any room for the air to move in as the water falls through which is the reason why you see turbulence and interruptions. There are various ways to tackle the issue. The best method would be to punch a ...

2

The question is: what is the optimal way to pour the water so that it [the bottle] completely empties fastest? I conclude the aim is to have the empty bottle, not the water in another container. Solution: Create a centrifuge-like setup, bottle opening to the outside. The setup will generate artificial gravity for the water in non-inertial frame of reference ...

1

We must all keep in mind that for average atmospheric pressure, and assuming Zhang could pull a hard vacuum with his abdomen (which is probably not feasible, but serves to provide us with a bound), the maximum (negative) pressure he could achieve is only about 14.7 psia. Given 36 tonnes, you can back calculate what the diameter of the bowl would have to be. ...

1

The exact mechanism you describe for how suction cups work is how the rice bowl work. Instead of the bowl being flexible, though, it's his body (skin and muscles) that are providing the change in volume necessary for the suction. So, instead of the suction cup creating the volume change, it's the surface the suction cup (the bowl) is attached to, Mr. Zhang'...

1

Yes. It's is not so much the water is the beach sand reflecting light back to you like a parabolic mirror. The droplets of water on your skin can form more surface area to catch light creating a magnifying effect focusing light on your skin as well. The random texture in the beach sand will also give you even tan. Most sand is white in color even if not the ...

1

Area of a base of a spherical cap of a height $h$ is $S = \pi (2Rh - h^2)$. This is the value water surface decreases as the ball dives in. This means that according to the principle of virtual work the force water sucks the ball in is $$F_{suck} = \frac{d}{dh} \sigma S = 2 \pi \sigma (R - h),$$ where $R = 2$ cm is a ball's radius, $h \approx 1$ cm -- ...

1

I would tentatively say no. It could be a matter of geophysics. Without weak interactions there would be no weak decay of Potassium-40. This means the interior of the Earth would be cold, and as a result there would be no tectonic activity nor would there likely be a strong geomagnetic field. Tectonic activity cycles carbon and other elements. Without the ...

1

There are devices called Monochromators, which can produce narrow band illumination with pretty good accuracy, so if implemented correctly, one may measure the visual range of a person, I would believe.

1

BUT WHAT ABOUT FLY which is also moving along with train with high speed. HOW IT IS POSSIBLE? A child really wonders about the motion of fly? So in a way you have posed it correctly! If you are in a train with all windows closed/or draped and the train is really smoothly going on the tracks with uniform speed- you may be wondering whether the train is ...

1

The basic problem is that headphones are heavy and asymmetric, whereas there's nothing attached to the shoelace. What this means is that the headphone - cord system can get stuck in stationary, locally but not globally minimum energy configurations: the twisting of a cord raises the energy of the cord, but the torsion resulting from the twist is not enough ...

1

Yes. It's is not so much the water is the beach sand reflecting light back to you like a parabolic mirror. The droplets of water on your skin can form more surface area to catch light creating a magnifying effect focusing light on your skin as well. The random texture in the beach sand will also give you even tan. Most sand is white in color even if not the ...

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