# Tag Info

317

I did the experiment. (dipping wins) H2O ice bath canning jar thermometer pot of boiling water stop watch There were four trials, each lasting 10 minutes. Boiling water was poured into the canning jar, and the spoon was taken from the ice bath and placed into the jar. A temperature reading was taken once a minute. After each trial the water was ...

103

Stirring will win, hands down, every time. This is why physicists need to talk to chemists once in a while. As Georg correctly remarks, the latent heat of vaporization of water is enormous - but he's wrong about waving the spoon; stirring is the champion here. Why? Temperature is really the average kinetic energy of the molecules in the bulk substance, ...

65

Actually, what you've read about the production of nuclei is not quite correct. There are several different processes by which atomic nuclei are produced: Big Bang nucleosynthesis is the fusion of hydrogen nuclei to form heavier elements in the early stages of the universe, as it cooled from the big bang. There are rather specific thermal requirements for ...

40

The problem was solved by a certain Albert Einstein, who is probably not unknown to users of this web site. Actually he also calculated the viscosity of particle dispersions (Einstein (1906). A. Ann. Phys. 19: 289 - not on the net!). Presumably he only did special relativity after getting bored with fluid dynamics. Anyhow, it's because the motion of the ...

35

How to cool a cup of coffee with the help of a spoon. Hmm... Empty the cup using the spoon, discarding the hot coffee. Strike the cup with the spoon, shattering it and forcing it to release the hot coffee. Drink the coffee with the spoon. Use the spoon to carve a cup-shaped hole in a large block of ice, put the cup in there. Put the spoon in the coffee, ...

34

The following fact lies at the heart of this and many similar issues with sizes of things: Not all physical quantities scale with the same power of linear size. Some quantities, like mass, go as the cube of your scaling - double every dimension of an animal, and it will weigh eight times as much. Other quantities only go as the square of the scaling. ...

31

Your understanding is correct. There cannot be a geostationary satellite at the poles, basically because it would have to be at rest, which cannot happen as it would get pulled by the earth's gravity and eventually crash to the surface. In fact, there cannot be a geostationary satellite anywhere else, except above the equator(in an equatorial orbit). ...

29

Explaining the Higgs mechanism properly is a fair bit beyond the level of the Feynman lectures, but here's an attempt. Spontaneous symmetry breaking In order to understand the Higgs mechanism in detail, you need to know about two concepts that are involved in quantum field theory. The first is spontaneous symmetry breaking. This is actually a pretty simple ...

28

The key word in what you've heard is "available" because there is quite a lot of lithium in the earth that is not so easy to obtain. The notion of "available Lithium" probably means known land reserves, which according to this page amount to 14 million tons. The amount dissolved in seawater is estimated at 230 billion tons (which is enough for lots of ...

27

Spencer's comment is right: we never "prove" anything in science. This may sound like a minor point, but it's worth being careful about. I might rephrase the question like this: What's the smallest size of the Universe for which we have substantial observational evidence in support of the standard big-bang picture? People can disagree about what ...

27

Summary: I find a formula for the diameter of a bubble large enough to support one human and plug in known values to get $d=400m$. I'll have a quantitative stab at the answer to the question of how large an air bubble has to be for the carbon dioxide concentration to be in a breathable steady state, whilst a human is continuously producing carbon dioxide ...

23

Since general relativity is a local theory just like any good classical field theory, the Earth will respond to the local curvature which can change only once the information about the disappearance of the Sun has been communicated to the Earth's position (through the propagation of gravitational waves). So yes, the Earth would continue to orbit what ...

23

The reason fluids flow off your hand while solids don't, is that fluids can change shape and solids can't. The molecules in a fluid want to stay together, but they don't care about the shape they're in, so gravity will cause them to spread out over your hand and flow off the sides. Solids can't change shape so they just stay on top off your hand, held in ...

22

With respect to the content in the cup, all Your hampering with the spoon is irrelevant. Cooling of a hot coffee is achieved by vaporisation of water. At temperatures between 100 and say 50 °C the vapor pressure is so big, that the heat carried away by convection of the hot (and much less dense than air!) vapor dominates all other heat transfer ...

22

There is an additional loss of energy when driving through puddles on a wet road, because the tire treads have to exert work in order to eject water. One way to look at it is that the keeps trying to glide on top of the water, but is continuously sinking into it to meet the pavement, which is equivalent to driving slightly uphill.

22

You would feel weightless if every part of your body of mass $m$ would be subject to an upward force equal to $m$ times the local gravitational acceleration $g$. Such an exact part-by-part cancellation is not going to happen via diamagnetic levitation as utilized on the frog in your example. Not only does this levitation couple according to magnetic ...

21

I can't believe the density or material of the spoon hasn't been considered. If the spoon is very dense you can take it and wave it in the air in a 15° arc and say, "Dear waiter, if you don't put some cold milk in my coffee I will hit you between the eyes with this abnormally dense spoon". On the other hand, if its made of gold or silver you hold it ...

21

No, the water would not be sucked up. Even if you take a pipe with vacuum, closed the top and dipped the open end of that pipe in water then the water would only rise 10 meters. After that the 'pull' from your vacuum is in balance with the force of gravity acting on a 10 meter water column. Maybe needless to say: The top of the atmosphere is way higher ...

20

Friction is indeed necessary for a car to push off the ground. However, it does not matter how much friction there is so long as your tires are not skidding. You only waste fuel if you put energy into spinning the wheels that is dissipated as heat before ever contributing to the car's motion. Otherwise, Newton's third law holds and the ground pushes you ...

20

The accepted answer by udiboy is completely right; however theoretically it is not only gravity that acts on a satellite, but also light pressure from the Sun and Earth. Given a sufficiently light and large solar sail (implausible at the current technological level), it is possible to counteract acceleration due to gravity and enter a totally non-Keplerian ...

19

Special Relativity derives from two basic ideas: The speed of light (in a vacuum) is always c. The laws of physics are the same in all inertial reference frames (basically, points of view that aren't accelerating, that is, they obey Newton's Laws.) With these two points and a little math, various proven conclusions may be derived: Time Dilation: When ...

18

Well, if you are only allowed to use a spoon, the fastest way to cool the coffee for drinking is to get a spoonful, blow on it, drink it from the spoon, take a next spoonful. Convection does wonders. If you are allowed a saucer instead of a spoon, pour a bit of coffee in the saucer, blow on it and drink it.

17

From a purely temperature point of view, not human perceived level of hotness, it is better to point the fan outward. This is because the fan motor will dissipate some heat, and when the air is blown outwards, this heat goes outside. This is all assuming the room has enough ventillation cracks and the like that the pressure inside is still effectively the ...

16

Gravitational influences do propagate at the speed of light, not instantaneously. The question of what would happen if the Sun instantly disappeared is actually a funny one in general relativity. The equations of general relativity imply as a mathematical consequence that energy must be locally conserved. Therefore, there is no valid solution to the ...

16

Maybe I should turn the comment to an answer. The physics of the situation is the same as when one can upturn a water glass with the water not falling out. The atmospheric pressure keeps it in. There exist the diving bells with open bottoms . As they are lowered the pressure in the air goes up to balance the water pressure, because the lower in the water ...

16

Phrased for a child: Water and honey and gasoline and oil are liquids. "Liquids" means that the little molecules we would see if we had very good vision, stick to each other and roll down an incline or over your hand. As another said, like children in a chain holding hands, they can push and pull and can move as a chain wherever the force takes them. ...

15

The answer may depend slightly on the humidity in the room (as that will determine the evaporative cooling rate), but basically your best bet is to increase the surface area of your coffee as much as possible and increase the rate of airflow over the coffee as much as possible (so that the local gradient of partial pressure of water vapor is as steep as ...

15

The basic tragedy of space travel is expressed by the Tsiolkovsky rocket equation, which says that the amount of reaction mass you need grows exponentially with your $\Delta v/v_e$, where $v_e$ is the exhaust velocity. The advantage of antimatter propulsion is high energy density, but energy density doesn't have any direct, major effect on the amount of ...

15

Ask yourself this: why doesn't the vacuum of space just suck away our atmosphere? The reason is because of the earth's gravity, which pulls on the gas envelope around the planet to keep it in place. The phenomenon we call 'air pressure' is also the result of this. The tube will only fill until its contents are being pulled down by gravity with the same ...

14

No one has discovered it. Dark matter is a proposed explanation to some observed phenomena. In particular, Galaxies rotate at a speed that implies they are quite heavy, especially towards the outer edges - but when we look at the mass from stars and interstellar gas, there isn't enough to make them spin the way they do. Gravitational lensing is a ...

Only top voted, non community-wiki answers of a minimum length are eligible