# Tag Info

187

Suppose the leg spacing for a square and triangular chair is the same then the positions of the legs look like: If we call the leg spacing $2d$ then for the square chair the distance from the centre to the edge is $d$ while for the triangular chair it's $d\tan 30^\circ$ or about $0.58d$. That means on the triangular chair you can only lean half as far ...

97

The real question is why not 2 legged chairs? First, our legs are single points that contact the floor. How they reach the floor is not important. If you have $n$ legs evenly spaced, then the ratio between the radius of the legs and the length of the shortest axis is $\cos(\pi/n)$: 2 legs: 0 3 legs: 0.5 4 legs: ~0.71 5 legs: ~0.81 6 legs: ~0.87 7 legs: ...

32

Five legged chairs are common too. Sometime in the 1980s a lot of office chairs were replaced - on the grounds that four-legged swivel chairs became quite unstable when the tilting/reclining feature shifted the centre of gravity. One reference I've found is this, which mentions an Australian safety standard from 1990 I'm pretty sure the mass replacement ...

26

Consolidating some of the points made in the answers to the question you linked, and comments: When constructing a chair, 4 legs is easy when you use traditional (wooden) construction - 90 degree angles, and easy to make stackable. A little bit harder than three legs because you have to make sure they are all the same length (or the chair will wobble). ...

23

I'll bet you've never sat in a three-legged chair. They are not very stable. It is easy to tumble out if you lean over too much. I think the problem is that the axis of rotation for tumbling (line between two legs) is closer to the center in a three-legged chair (compared to a four-legged chair) unless the chair is very wide. Leaning over can put your ...

9

John Rennie provides an excellent explanation as to why a higher amount of legs provides greater stability and improved balance. As for the question of why not greater than 4, it can be answered by considering the environment the chair will be in. It's rational to assume that not all surfaces are equal and flat. As geometry proves, a chair can make contact ...

8

The diagram looks like the Fat Man bomb that was dropped on Nagasaki. The Wikipedia article gives lots of info on the design if you're intereted in pursuing it further. The casing is just to make it aerodynamically stable so it fell in a controlled way. The bomb itself is spherical so the case could be spherical as well if it weren't for aerodynamic ...

6

There will be no attempt to utilize the energy released from fusion reactions at NIF. NIF's goal is to demonstrate that a sustained fusion reaction is possible. Such a demonstration is termed "ignition" and there is a good chance it will be achieved within the next three years. The next step is a project called LIFE which involves a prototype power plant ...

4

I was going to comment, but am making it into an answer. The experimental proof that fusion works with small amounts of matter ( no need of gravity to work) is the H-bomb. The engineering problem is large and people are working on it with various designs. The world community has put its money on ITER which is currently being built in France. The design ...

4

This should be more of an engineering/economics question than a physics one. The statement "... the simpler, cheaper, three-legged chair?" Is false. 120 degree angles have historically much harder to manufacture than 90 degree angles, the small cost of an extra leg is easily offset by the increase in manufacturing costs have having to cut innumerable 120 ...

3

Stability is a valid point, but probably not really the real reason. People think and design in 90 degree angles. Everything around us is squarish. Rooms, houses, windows, doors, even books and screens. Orthogonality makes calculations and manufacturing simple. It's all about parallel and perpendicular lines, easy cutting, going along the wood grain, zero ...

3

The difference between an office chair's 5 wheels/supports and a regular chair's 4 legs is that the latter has all of its load going straight down. The legs only need to be strong enough not to shatter. In fact, a chair could easily get away with 3 legs but for the stability. In contrast the office chairs legs support load perpendicular to their ...

3

There are no proofs, there are only disproofs. The history of fusion is littered with designs that seem promising on the back of the envelope, but upon closer inspection turn out to have unforeseen difficulties--usually some kind of instability, or energy loss mechanism. For example, in magnetic confinement devices, the strength of the magnetic field is ...

3

Wifi is around 2.4GHz so a wavelength of around 12.5cm Anything metal with holes much smaller than 12cm will work - even a mesh. But if you don't have a parabolic dish, only a spherical one, you might do better with a can antennea see eg http://www.turnpoint.net/wireless/cantennahowto.html for details of the calculation

3

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

Perhaps because the surfaces chairs (and tables, beds, etc) are placed on are flat by design with respect to earth's gravitational field. This makes for much better living/working/playing/etc quarters as every current and historical example proves. Building rooms/floors/houses/etc with flat surfaces automatically accommodates any four legged ...

2

The WiFi frequency range is pretty much the same as a standard microwave oven. The reflective coefficient of the metal mesh in the front door is above 99% and it contains no special materials. On the other hand the shape of the reflector has a large influence. So either you can follow Martin's suggestion of a can antenna, which is widely used or buy a ...

2

Something to keep in mind is the difference between changing the direction that the aircraft is pointing, versus changing the direction that the aircraft is moving. In straight and level flight you can stomp on the rudder and fairly quickly cause the nose to point 5 degrees or 10 degrees away from where it was. But (at least initially) you will not have ...

2

There's a beautiful book on the subject of aerodynamics, Stick and Rudder. Either the book you are reading is not very smart about how airplanes work, or you are mis-reading it. Normally, the way you turn an airplane is the same way you turn a bicycle, motorcycle, or high-speed boat. You bank or tilt the vehicle in the direction you want to go. Since you ...

2

Because the seat is usually square. If it was round (or another shape) it would be harder to make and harder to attach the back (or extend the back legs to make a back). If it had no back, it would be a stool. Stools can have three legs. (Not all answers have to do with physics!)

1

Why are four-legged chairs so common? Couldn't be simpler: it's easier to tilt over on a three-legged chair. This is so obvious that I suspect you have never tried a three-legged chair! That is precisely why, four-legged chairs are more common than three-legged chairs.

1

it's not a question of physics. But I cannot resist to answer safely to a why question ! While modern engineers approve today the main choice of 4, it is not the reason why chairs are 4-legged since thousands years. it is because human have the animal model , cows , dogs , cats , etc , with 4 legs. Horses and donkeys, which are ridden seated , are ...

1

I won't address the details of the fluid dynamics that cause the cage to produce noise because it is well out of my area of expertise. But I have performed an experiment in my office. The small desk fan I bought was too noisy. Since it lives on the top of a tall bookcase and well out of reach, I removed the cage completely. It is a lot quieter now. My guess ...

1

I've been thinking how one can try to measure it. I suggest the following setting. First one should find out the dependency of tap water resistivity on salt concentration. Likely it should be made by experiment rather than consulting tables because the results would depend on the tap water. Having done the first part, one can conduct the experiment itself. ...

1

Your chamber's thermal cycling environment is indeed "very demanding". You have my sympathy... From O'Hanlon's A User's Guide to Vacuum Technology, Chapter 17, Joints, Seals and Components: Welds: It is important to make the weld on the surface which will be exposed to vacuum. If the weld is made on the atmospheric surface, a gap may be created which ...

1

Looking at the Thor Labs link, and clicking on the "drawings" tab, I see the following diagram: I think I can conclude that the height of one (reflecting) side of the rhomb is 16 mm, and the run distance is (36.1 - 9.5)=26.6 mm. This would make the angle $\tan^{-1}\frac{16}{26.6}=31.0˚$. The angle of incidence is then $90-31.0=59.0˚$.

1

Depending on size, you could have a suspended weight and pulley system connected to a small generator ie use gravitational potential energy. Another would be an ultracapacitor topped up by a solar cell. If it is a very small power device you could consider energy harvesting technology - just about everything is listed in the wiki article

1

You can use a flywheel. When spinning in a vacuum, with magnetic bearings, flywheels lose very little energy over time. While they are probably best for large-scale applications, e.g. as backup for other power sources like solar energy, they have even been proposed as a power source for cars - although I assume there could be serious problems caused by the ...

1

Found the formula for the use I was looking for [1].  \begin{aligned} T&=L\sqrt{SF\frac{K}{2}}\times\sqrt{\frac{P}{F_a\left(1+\left(\frac{L}{W}\right)^2\right)}} \\ Where \\ T&=\text{thickness} \\ P&=\text{pressure} \\ L&=\text{length} \\ W&=\text{width} \\ F_a&=\text{apparent elastic limit or rupture modulus} \\ ...

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