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Oct
27
comment Intuitively, how can the work done on an object be equal to zero?
Your English is probably better than mine. But sometimes people on this site struggle with English. It takes courage to do that, and I always try to help them if I can. I studied Spanish in school, but I'm not at all good at it.
Oct
27
comment Intuitively, how can the work done on an object be equal to zero?
@EduardoGuerrasValera: You're right. I just like to keep it tactile.
Oct
27
comment Intuitively, how can the work done on an object be equal to zero?
+1 for the photo !
Oct
25
comment Intuitively, how can the work done on an object be equal to zero?
If you push on something and it doesn't move at all in the direction you're pushing, you've done no work on it. Push as hard as you want, as long as you want, against a brick wall. There is no energy transfer. But if you lift it up - that's a different story.
Oct
25
comment Pressure difference in water tanks at different heights
@soumyadeep: Right, unless you want split hairs about gravity being less strong at the higher elevation :)
Oct
25
comment Why does a bicycle (without any support of stand) falls down being at rest, but not under motion?
@Andrey: In a two-wheel bike, as MSalters said the Delft people showed, there is more going on than just gyroscopic precession, that's true. However, the strength of the gyroscopic effect depends on the angular momentum of the wheel, and can be quite strong. (On a bike it is the front wheel, because that's the one that's free to precess.)
Oct
25
comment Why does a bicycle (without any support of stand) falls down being at rest, but not under motion?
@Andrey: Consider a front wheel on a car, moving at highway speed. You can turn left of right without fighting any gyroscopic tendency, right? When you yaw the wheel left, it wants to roll right (aircraft axes). The frame pushes back against that roll, and that pushing back causes the wheel to yaw left, exactly as you originally moved it. It doesn't fight you, but only because there are strong forces happening in the wheel bearings. That's what happens when you physically prevent precession. BTW, MSalters is right, but then go back to the one-wheel argument.
Oct
25
comment Why does a bicycle (without any support of stand) falls down being at rest, but not under motion?
@Andrey: Correct.
Oct
25
answered Pressure difference in water tanks at different heights
Oct
25
revised Why does a bicycle (without any support of stand) falls down being at rest, but not under motion?
added 49 characters in body
Oct
25
answered Why does a bicycle (without any support of stand) falls down being at rest, but not under motion?
Oct
24
comment Airplane with banner in a windy day
@richard: Play with paper airplanes. Buy a balsa wood glider for a couple dollars and fly it. Get a copy of Stick and Rudder. Take an introductory flight lesson. Read this site. Learn about flight first-hand. Words will not be enough.
Oct
21
comment How can you calculate air resistances at different speeds?
That is only for low Reynolds number. For things like autos, aircraft, etc. Reynolds number is high, and drag is proportional to the square of velocity.
Oct
19
comment Airplane with banner in a windy day
@richard: Right. If the pilot cannot see the ground, like if she is in a cloud, there is no way to tell how fast the air is moving over the ground or in which direction (unless they have a way to tell motion w.r.t the ground). Like if my airspeed is 100, but my GPS or DME says my ground speed toward my target is 60, that tells me I have a headwind component of 40. But without the electronics or ground view, there is no way to tell. Any more than you can be aware of rotational speed of the earth, or orbital speed through space.
Oct
17
comment Airplane with banner in a windy day
@aufkag: in reality, it's not an issue. The only time you see sudden changes in wind speed is if you encounter low-level wind shear (like if there are big trees near the runway, or an inversion). Vertical changes, on the other hand, are quite common, making the ride bumpy. Near a thunderstorm, they are dangerously severe. Needless to say, you're not even flying near those, let alone towing a banner.
Oct
17
comment Airplane with banner in a windy day
@aufkag: Generally you assume that winds are more or less constant (at a particular location and altitude), that you get from a winds aloft forecast. If your route is long enough that there will be changes, you solve it by breaking up the route into shorter "legs". It's not a super-precise science.
Oct
17
revised Airplane with banner in a windy day
added 738 characters in body
Oct
17
revised Airplane with banner in a windy day
added 248 characters in body
Oct
16
comment Airplane with banner in a windy day
@User58220: You got it.
Oct
16
answered Airplane with banner in a windy day