Slinky physics question
Discussion
JonRB said:That’s what I’m thinking but somehow I’m having trouble visualising the vertical momentum being turned into horizontal momentum, I think it’s probably being acted on by an internal momentary force to keep the conversion of horizontal potential energy to lateral motion. This conversion is certainly possible as a marble can role down a hill.
The key question is where the energy for the flipping motion of the slinky comes from. If it is a product of the initial 'flick' to get it started then this will decay over time due to losses and the slinky will eventually stop. However, if it is a product of the conversion of potential energy to kinetic energy then it should continue.
Naturally if the system was regarded as perfect and without losses you would not need steps, the initial shove would supply the energy needed so it could slink along a flat floor.
UKBob said:Always hated chaos theory, doesn’t seem to be underlying logic. Is there really such a thing as chaos at all?
Chaos theory (whatever its called) guarantees that the question will be turned on its head - nothing is perfectly made, and the slinky will eventually hit the side, sooner or later. Come on, agree with me. You know Im right
Any way my more immediate concern is that I could build an elevator without sides to stop this from happening (I mean one where the slinky wont fall off the edge).
I’m with the people who say the speed of the tread mill is absolutely irrelevant, it’s the airspeed that matters to generate lift and not groundspeed. This is of course assuming the scenario that the aircraft engines are more than capable of overcoming the wheels friction or the wheels are frictionless (Most of the friction is dynamic losses caused by the air any way). Groundspeed is irrelevant, but don’t tell the French that or they might start taking aircraft off into the wind (higher airspeed and lower groundspeed – result is less runway needed, but this is a dig at their [French ATC] mistake with concord). If the aircraft was in a wind tunnel however it would be an entirely different matter.
JonRB said:
Staying with the aerodynamic theme: wings are designed with curved upper surfaces to generate lift from the airflow over them, right? Fine. So explain to me how a wing generates lift when inverted, i.e. aerobatic plane flying upside down.
AoA is the main thing but it is also worth noting that the section of a sports aircraft’s wing is almost symmetrical and it has very low dihedral effect to fight the pilot and try to ”right” the aircraft.
wolves_wanderer said:Think about the air movement (circulation) in the box, when the helicopter gets flying it will start to move air… Consider this situation to be real with a radio controlled helicopter.
As you lot seem to like these conceptual questions here is another:
Can a helicopter fly in an enclosed box containing air?
This problem has very real implications actually, particularly on helicopters with two blades working in close proximity and those vertical take-off/landing aircraft when close to the ground.
BliarOut said:That’s one set of conditions but what about if the box was only slightly bigger than the helicopter needed to fly?
As you lot seem to like these conceptual questions here is another:
Can a helicopter fly in an enclosed box containing air?
>> Edited by JonRB on Friday 9th December 22:08
Jon, Jon, taking the SW view again. The box size isn't specified, ergo I say it's twenty miles across. Of course she'll fly
I’m thinking that a circular airflow pattern would be set up which would decrease lift until the helicopter’s rotor could not longer provide sufficient lift and it would come down to the ground. I believe a similar thing can happen with the American V-22 Osprey where air circulates between rotors causing a loss of lift on the latter rotor and can happen to jump jet's exhaust gasses are sucked into the air intake causing loss of lift.
JonRB said:I just cant bring myself to agree with you jon, while I can see your point it is not technically incorrect to teach people about “lift” this way. Getting your head around the Bernoulli principals is something most students at that level would find too difficult (possibly?) but you can understand the relationship between the models and that both can be simultaneously correct, this comes as a natural development any way I feel.
Yes, but as the article that I linked to shows, 11+ physics gives an over-simplified and misleading view of the dynamics of lift.
GreenV8S said:
i've often wondered that peter .. how does the air know it has to speed up? it doesn't! lol
When a plane cuts through the air, the same air molecules it cuts apart want to be together again at the other end of the wing. The molecules that go over the top have to travel quicker than the molecules that are pushed along the bottom of the wing. The two molecules meet each other at the other end of the wing. Because there is the same amount of air molecules on the top as the bottom, and the molecules on top are travelling quicker, they are further apart, so less dense than underneath. Therefore the wing is sucked up.
Sails on a boat work the same way.
I shouldn't try and explain things though, I'm rubbish at it! I had to learn this for my sailing instructor’s course.
Yes, that's the common pseudo-scientific explanation, which actually doesn't make sense.
I’ll just share the flowing with you if you really want to know how it works (trying to avoid mathematical complications):
“First and formost, the part of Bernoulli's principle about air traversing the airfoil in equal amounts of time over the upper vs. lower surface is absolutely false. Bernoulli's principle simply states the following:
Basically, it says that air pressure decreases when its speed increases, and there's a property called 'total pressure' that's conserved along a streamline. This is counterintuitive for most people, because we tend to think faster air means more pressure.
Let's do a thought experiment. Let's say that I blow air onto my hand. My hand feels a higher pressure when it hits my hand. Is it because the air is moving faster? No. The air is slowing down before it hits my hand, which increases its pressure. Here's how it works. I purse my lips and compress my diaphragm to increase the pressure in my lungs. The air in my lungs is moving slowly, so you can say that the pressure in my lungs is the stagnation (or total) pressure of all streamlines that originate in my lungs. It is greater than the ambient air because I'm squeezing my diaphragm. Now my throat is narrower than my lungs, so air moves faster through it, and it's even narrower between my lips. When the air exits, it matches the pressure with the ambient air because it's allowed to expand freely. So the 'stream tube' exiting my mouth is at a speed at which the static air pressure is equal to the ambient air pressure.
Now as air approaches my hand which represents a barrier, it is slowed down again. The pressure my hand will feel should be roughly equal to the pressure inside my lungs minus all of the frictional losses along its path to the lungs. So my hand feels a high pressure. My lips, however, feel a lower pressure.
The actual pressure-velocity 'field' is an interplay between pressure and velocity. If air experiences a decreasing pressure gradient, it is forced towards it, because the higher pressure pushes it toward the lower pressure. The converse happens because as air moves faster it evacuates its region more quickly. Pressure will decrease which allows air to come from other areas to 'fill the void' left behind. Since it is at a lower pressure, the air will have accelerated to that point, which perpetuates the situation.
I hope all of that gives you a better intuition about how Bernoulli's principle works. Now let's get back to that airfoil. In most situations the air over the wing reaches the trailing edge *before* air under the lower surface. What causes an airfoil to be effective is something called the 'Kutta condition', which says that air cannot flow around a sharp corner. The reason for this is that in order to change direction instantaneously, it needs an infinite force, which means it needs a *very* low pressure at the point which represents an infinite flow velocity. Instead, friction within the air (we call it 'shear' when it's between streamlines) slows it down. This means that at the pointed trailing edge of an airfoil, the air must flow past that point at the upper and lower surfaces. This creates a constraint that requires the streamlines to protrude behind the trailing edge. This effectively creates a barrier that prevents air below the wing from 'wrapping up' to fill the void left above the upper surface. Instead, the pressure in the upper surface decreases, which causes the airfoil to pull more air up around the *leading* edge which has a larger radius of curvature. The air moving quickly around the front part of the upper surface is what creates most of your lift.
This is just as easily explaned using the 'Newtonian' explanation whereby the upper and lower surfaces deflect air downward when the wing is at an angle of attack. This is an equally valid approach, just using a different perspective. It doesn't give an intuitive feel, though, as to how the wing deflects air over the upper surface.”
This would be the simplified explanation I would give, near close enough for the engineers? As you can see it clearly shows why both the Newtonian and Bernoulli models are applicable.
>> Edited by speedy_thrills on Monday 12th December 09:52
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