# Tag Info

14

In the early days of radio, the resonance of the antenna in combination with its associated inductive and capacitive properties was indeed the item which "dialed in" the frequency you wanted to listen to. You didn't actually change the length of the antenna, but by changing the inductor (a coil) or capacitor connected to the antenna you tuned the resonance. ...

13

QM already made a big change to our lives: Without QM no transistors. Without transistors, no modern computers. Without modern computers, you wouldn't have been able to ask your question here. Before an integrated circuit (that encapsulates an array of transistors in a piece of computer equipment, say) is mass produced, one must do a huge number of ...

8

It is impossible to generate momentum in a closed object without emitting something, so the drive is either not generating thrust, or throwing something backwards. There is no doubt about this. Assuming that the thrust measurement is accurate, that something could be radiation. This explanation is exceedingly unlikely, since to get mN of radiation pressure ...

6

There are several different levels of advanced in quantum mechanics. I will try to answer using these levels of quantum mechanics: Basic: single particle or single particles interacting with a single atom/nucleus, or classical field picture--- anything Einstein would have been comfortable with. Advanced: highly entangled many-body quantum mechanics, ...

6

In theory, yes, this could be done. Pretty much exactly as much gravitational energy is lost by the water coming down as is gained by the water going up, so you could then supply the water while hardly using any energy at all. (Just enough to offset the heat generated by friction in the pipes.) One way in which it can be done in theory is simply to connect ...

6

Similar situations can arrise in experimental work on large scale machines, and there is a body of knowledge that gets passed around. The "can it hold pressure" test suggested by other answer works best if you can apply a pretty good over-pressure and have either a sensitive pressure gauge in the system or can afford to wait over night to see how tight ...

5

You've seen biplanes. Almost anything can fly if it has enough area, an angle of attack, and is nose-heavy. I've seen a model airplane in the shape of Snoopy's doghouse! It flew just fine. If you're wondering why nose-heavy it's this. Look at a normal plane with a main wing and a tail. It's nose heavy. The main wing holds the weight by lifting up. The tail ...

5

Ingo, when you consider the couple, you may put one of the "spouses" at the origin, so his torque is $P\times d_0$ for $d_0=0$, so his torque vanishes. Meanwhile, she is located at a nonzero $d$ so her contribution is $P\times d$ and nonzero. Because his torque is zero, it doesn't matter whether you add him or not. The only difference between the whole ...

5

It is just easier, i.e. less expensive, to build and maintain them that way. There exist alternative designs that are more efficient but also more difficult (= more expensive) to build, put up and maintain. You can check those out via this link.

4

If you look at a stress-strain diagram, the difference becomes clearer. The initial slope is where stress is directly proportional to strain (like a spring) and the material behaves like this up to its elastic limit where it reaches its yield strength. Beyond this the material deforms permanently (like an overstretched spring that won't return to its ...

4

http://www.physicsforums.com/archive/index.php/t-37701.html says "Most of the strength of a cylinder comes from the outer portions. I think the contribution goes like the cube of the radial position. So, if you took a solid rod and drilled out a half the volume from the center, you do not lose half the strength. Strength to weight ratio is ...

4

Shawyer's "analysis" is a mess, incoherent and deeply confused about fundamental aspects of relativity: he mixes up frames, assumes a universal rest frame, etc. The EmDrive supposedly works best when "stationary relative to the thrust", whatever that means, and Shawyer goes on to suggest using it for levitating vehicles with some kind of conventional ...

4

In principle, yes, you can create any such process. After all, you can control the volume of a gas sample more or less arbitrarily by changing the size of its container, and then you can add or remove heat to change the temperature and thus set the pressure to whatever value you want. (Obviously you have to stay within the boundaries of the gaseous region on ...

3

Because they travel on the same surface with the same roughness maintained to the same level. If cycle paths were precision machine flat steel rails you could have very small wheels, but since you must manage the same ruts and potholes you need a wheel of similar diameter. There are folding bikes with very small (<20cm) wheels - but they are useless on ...

3

The reason for the distinction is that a couple has no net force in some direction. If you had a rod floating in space and applied a couple, it would spin in place. This is because you applied the same force in opposite directions, so the overall force in any direction is 0. However, since you didn't apply the force directly in the center, you impart some ...

3

Yield stress is the stress at which that the material deforms permanently, ultimate tensile stress is the stress at which it breaks. There is probably some official ISO/ASME definition of how much it has to deform for it to count as having yielded. Materials first deform elastically - when you release the stress they return to their original shape, this is ...

3

Choosing an appropriate coordinate system often vastly simplifies a problem. Anyone who wants to solve a problem expediently will try to find a coordinate system that simplifies the problem. If your professors told you that physicists do not do this, then your professors told you a falsehood.

3

As Manishearth says, for engines with more than one cylinder the firing of the other cylinders rotates the crankshaft. However, as any fan of vintage motorcycles will know, you can have four stroke engines with a single cylinder. In this case the engine has a heavy flywheel attached to the crankshaft and the momentum of the flywheel keeps the crankshaft ...

3

I'll begin by answering the easier questions. No, Aluminium is not up to the mark. As mentioned in the comments, the spacecraft we create would probably have several stages that are jettisoned at various points in the journey. However, given that this question seems fairly hypothetical. If we were to design a single-stage craft for interstellar travel to an ...

2

Harbors have solved the problem of waves for centuries. It is walls. The same is true of the Netherlands who live below sea level. Solution is walls. A series of artificial islands/walls could be geo-engineered, at great cost, where the tsunami waves could break. The permanent solution is to forbid any buildings below the height of the maximum recorded ...

2

Best aerodynamic shape depends mainly on the application i.e. the desired speed at which it should work, interact with the air. If you have heard, asymmetrical wings and snout nose are helpful in case of supersonic flights; since you want to minimise drag which is a killer at those speeds, while normal wings and pointed nose is better in subsonic flight. ...

2

Though I generally agree with whoplisp's answer, it is worth to note that obtaining the (lower) limit of the thickness is rather tricky as long as it is defined by stability under 'strong' deformations. Where 'strong' is compared with the tube wall thickness. Which is obvious from common sense point of view: thin rod is easier to deform than the hollow ...

2

The concept of phase space was probably floating around for ages, but it only really became central to modern physics in the mid 19th century, with Boltzmann and Maxwell's formulation of statistical mechanics, Hamilton's reformulation of mechanics, and Liouville's theorem. The concept of phase space is already implicit in the work immediately following ...

2

Application: A strain gauge is a device used to measured the strain (change in length as a proportion of the original length) in an object as a result of an applied load. Most strain gauges are designed to measure strains in only one direction. How it works: A common type of strain gauge consists of thin metallic foil cut into a pattern such that most of ...

2

It looks like any other modern signal processed multi-beam radar system - I think the "holographic" probably has more to do with their marketing dept than anything to do with holograms. With phased array radars, instead of having a single rotating dish which transmits in a single direction and listens for a return form that direction, you have a series of ...

2

Lift is a function of the speed of the air from the leading edge to the trailing edge. In a flat turn, the inner wing is moving slower than the outer wing therefore there will be a difference in the amount of lift produced. But in fact, an airplane can not change direction by flat turning this way. Rolling into the turn by the use of the ailerons is the ...

2

I don't quite get it - you can achieve ping-pong ball levitation using nothing else but an air stream, say, from a hair dryer (maybe with a funnel), without any control systems (see, e.g., http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/the-aerodynamics-of-a-ping-pong-ball/ ).

2

The RPM is restricted by frictional losses in the engine and the tendancy of the engine to explode if you rotate it too fast. In that sense the RPM limit is down to how well the engine is made and what it's made of rather than any fundamental EM properties. The torque is dependant on how large a magnet field the engine generates, and this is mostly ...

2

This is in addition to @JohnRennie's excellent answer. First thing, the "more this less that" depends upon if the motor is being operated under constant voltage or current. I'm talking about torque only, RPM will increase if torque increases generally, but other friction forces and self-induction make it a headache to solve. Condition Const ...

2

In an internal combustion engine, we have multiple cylinders. They are attached to a shaft in an alternating manner such that when one set of the cylinders have combustion, they drive the shaft to move down in the other set. See http://commons.wikimedia.org/wiki/File:Cshaft.gif

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