# Tag Info

45

You have created a rather poor pinhole camera (camera obscura). You can see an "image" of the sky, a green space (trees) and even a reddish brown blur that is your driveway. This is not diffraction or refraction - it's geometrical (classical) optics. Because the hole is pretty big, you see a very blurry image. But basically, the light from the sky falling ...

14

This is difficult to answer, since you seem to have a major misconception about forces. The best answer is to go back your freshman, or even high school, physics book and read the section on forces. However, briefly: The pulling force isn't somehow split between the train and the ground. The rope will pull with the same force on whatever is holding each ...

13

I'm going to respond to (v1) of the question, which asks why the night sky is dark (black and unlit) compared to the day sky even though there are many light sources at night. The updated question references Olber's paradox, which has been answered many times before. Like most things we see in everyday life, there are a number of reasons contributing to ...

7

Who is interested can find detailed information at wiki, or here The problem is known (as you added in your edit) as Olbers' paradox, and was posed already in the mid 1500's, by Johannes Kepler in 1610 and even later by Edmond Halley in the eighteen century, and curiously, even the novelist an poet Edgar Allen Poe anticipated possible explanations as to why ...

7

Steam is caused when water vapor condenses. This is caused by the air having too much water vapor for it to hold. When you have a lot of heat under the pan, the air above the pan is quite hot and can hold a lot of water. The water evaporating from the pan disperses into the atmosphere and doesn't condense. When you turn off the heat, the pan and food ...

6

Paper is opaque because there are mineral fillers in between the cellulose fibers, which are in a crystalline form. When surrounded by air, they appear opaque due to the higher difference in the index of refraction between the two. When wet, the paper appears translucent because there is a lower difference in the index of refraction between the crystals and ...

5

Thumbs up for a interesting and curious question. Yes there is worn out due to friction for sure when two surfaces rub along with each other, like you said worn out keyboards are a good example of it. Also, hardness does play an important role in it. Hardness is a physical property and there are different measurements of hardness: scratch hardness, ...

5

Don't even think about this!! REALLY. The risk is probably low, but the outcome of a broken rope is pretty severe in climbing. Too severe for the sake of a tug of war game. This is an advanced materials engineering question. 9kN is presumably the rope's rating, right? 50-60 grownups will easily impart this force onto the rope (30 at each end, each pulling ...

4

A breeze at 35°C and 90% humidity (typical conditions in Houston, Texas) doesn't cool you off. It just makes you feel even more miserable. A breeze at 40°C and 20% humidity (typical conditions in Phoenix, Arizona) doesn't cool you off, either. It, too, just makes you feel even more miserable. Your body cooled because the air velocity was much lower than the ...

4

The following passage is extracted from Stephen Hawking's book "A Brief History of Time": In fact, various contemporaries of Newton had raised the problem, and the Olbers article was not even the first to contain plausible arguments against it. It was, however, the first to be widely noted. The difficulty is that in an infinite static universe ...

4

The front of the train compresses air which can blow you away, while at the back of the train air rushes back in after the train has displaced it. This backdraft is especially troublesome in closed areas such as subways, where a train exits a small tunnel near a platform and the displaced air rushes back into the vacated tunnel. Next time you see a big truck ...

3

Firstly, I suspect that oxidation will throw a spanner into any such plan. For metals like aluminum, which have a very high affinity for oxygen, a "virgin" surface will begin to tarnish almost immediately. The second problem is that metals have microstructure. Neighboring crystals in a polycrystalline aggregate such as a metal piece have to satisfy certain ...

3

You have two subproblems here. One is the motion of the train and the other is how to grab the rope. Let's focus on the train. It has a mass $m$ and there is some friction with the ground (assumed low because it is on rolls) $\mu N$ , being $\mu << 1$. You apply a force $F$, assumed modulo constant. So, the horizontal acceleration is: \sum F = m ...

2

Well, one sub-question at a time. Suppose you are exerting a force of 300 N, how much KE is going to the train and how much is wasted to the ground? Applying a force to an object doesn't necessarily involve energy expenditure. If there is a weight lying on the ground, then the weight is exerting force on the ground, but the weight isn't expending any ...

2

The reason that a sticker curls when it is peeled, is that the force needed to peel it off, stretches the the bottom (the glued side) of the sticker beyond its elastic limit. This causes the bottom to be longer than the top, causing it to curl up. The same principle applies to a ribbon that is being bent over a sharp edge.

2

Short answer: There is no way that is better in absolute: 2) if a man can pull more than his own weight the best position is that in picture 2. In this position one cannot exploit gravity, but almost all is pull is transmitted to the train. He is probably exerting a force of 1500N, he is not wasting energy on the ground, but we must remember that his ...

2

The white cloud you see in the water is steam bubbles. The grains of salt provide nucleation sites that allow the water to vaporize as they fall through the superheated liquid (so BowlOfRed had it right--although it's steam that is forming, not dissolved gasses coming out of solution). If you raise a pot of water to near boiling and toss in a handful of ...

2

I'm not sure that it makes sense to try to measure your body weight to a precision of 100 g. For example I was just thirsty and drank a 20 ounce bottle of water, which transferred about 600 g extra mass to my stomach. Even just breathing changes your mass: if you take ten half-liter breaths per minute and your exhalations contain 5% carbon dioxide by ...

2

When the cap bursts off the bottle the air inside it will expand rapidly and adiabatically, so its temperature will fall. If there is enough water vapour in the air inside the bottle, and if the temperature reduction takes the temperature below the dew point, the water vapour will condense giving the fine mist that you see. In this case it looks to me (it's ...

2

There are several major difference between the asteroid and you. You have moisture on the skin. As the air passes over the skin, it picks up the moisture, making you feel cooler. The asteroids are travelling hundreds of times faster. The compression of air increases the heat substantially. You would barely notice the heat increase from a car.

2

In martial arts it is a test of your fast punches. Hit it fast The fighter has to twist his hand in the instant before hitting the target. Impulse as well as turning moment enforce the punch. The counteracting force will be formed the aerodynamic drag of the paper. Be quick before it's deformation will reduce its effectice area. Otherwise the paper will ...

1

Momentum. It's all momentum, you want to use your legs, shoulder and hip all to be "behind" your punch so that it has more momentum. You also want your hand to be continuously accelerating from the start to the end of the punch, as this will create more momentum by increasing velocity. Increasing mass also helps with impact. So, increasing the size of your ...

1

Well, in the pot with less water you'll have more salt spread through a smaller volume than the other pot. That means the water will be saltier in the pot with less water. I don't know the salt transfer rate for a solution to dumplings, but I would assume the one with a higher salt-to-water ratio would be saltier.

1

For the answer I will make two assumptions: in both pots the amount of water is enough 1) to get all the salt into solution and 2) to not get absorbed completely by the dumplings. Say pot A has less water than pot B than the concentration of salt in pot A will be higher. If the dumplings are initially dry, they will absorb water and the dumplings in pot A ...

1

A full 3D model might be too complex for this case, because then you need to know everything about the wall, and the tree, and the interaction between the two in great detail. I think a simplified approach might be more suitable. I'm not sure if this is oversimplified, but let me take a swing at it: If we assume the tree is supported by both the ground and ...

1

This is admittedly a late answer. Hopefully it will clear up some of the confusion. If you ignore aerodynamic drag, ignore that the coefficient of rolling friction varies with load, and ignore a number of other factors such as friction between the axle and the bearings that support it, then yes, stopping time / stopping distance is independent of vehicle ...

1

There are several different types of adhesion. As pointed out by Ryan S. : Electrical, like when a balloon sticks to your head after a good rubbing. Chemical, like plastic cement will "melt" the plastic pieces to be bonded together. And the most common, physical, which is extremely hard to make an example of without a picture. Therefore, I give you a ...

1

We have to define "most efficient". I will define it as the technique that allows the greatest force to be applied to moving the train along the track. Then we have three considerations relative to the position of the person pulling: torque on the body, transmission of force to the rope, and force on the train. TorqueWhen you pull a train with a rope, and ...

1

I would say that the reason for such weight is material that is used, just like in jewelry. But that doesn't mean that weight can be completely ignored. The important part is weight distribution, so that the center of mass is closer to the top of the pen. Also, important thing is the grip that pen has, and the quality of the nib, as well as the ink. So my ...

1

I'm fairly sure that the ridges exist to hold an inventory of liquid. A simple way to argue this is to note that the ridged loop has a much larger total surface area. However, I'm sure there are more complicated physics with the ridged surfaces. Since it helps brace the liquid within a ridge, it's sure to hang on to more water than what a flat surface of ...

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