Physics question of the day

Hein · Since 08 Mar 2005 1313 Posts Possessed

Is 10 knots of wind apposing 4 knots of current equivalent to 14 knots of wind?

Support your answer.

-Hein

Wind Slither · Since 04 Mar 2005 2573 Posts The 503 METAL

Hmmm.

The "true" wind is always the same regardless of current or hull speed. The "apparent" wind should feel like 14 knots when just floating in the current. Not sure about once your under way, but from experience around here it sure feels like the current has a direct affect on power that is atleast equal to it's speed.

pkh · Since 27 Feb 2005 6548 Posts Couve / Hood Site Lackey

I'd say so, though I can't give you the physics to support it.

It's a trip when you are riding in an area with heavy current, and then you hit a spot on the water where there is a jetty or something causing a neutral or opposing current, and you feel the whole kite depower.

Guest · Sun May 15, 05 8:17 am

Why do I always get sucked into these types of questions? I can't resist. Trying not to get this too nerdy and I am not a physics person. My guess would be no. The friction of the board on the water plus the weight of the rider would be completely different compared to a stationary point and wind speed.

Hein · Since 08 Mar 2005 1313 Posts Possessed

some music while you contemplate the answer...

http://www.discoverynet.com/~ajsnead/theme/midi/ppanth2.mid

want a hint?

fokiten · Since 04 Mar 2005 188 Posts Stoked

Regards,

Once one considers that the state of physics today, unlike yesterday, when dudes said yeah, there is this eather stuff...and folk just bought into it, today we know we can't answer even the most basic of questions.

Gravity foe expample, we still have no clue what's up with that.

no gavitron, just wack theory,, einstien f-en puked,, he worked on it for fifteen years,, finally said f-uc this, I'm getting drunk instead.

,

turbulent flow, we can't do that, too f-en complicated.

We know very little, very well.

Same deal here, yeah sit in the current feel the true wind add the apperent wind indroduced by the flow, yup, we can do that.

start moving and it all goes out the window, too f-en complicated.

Hey,,,no sweat,,,take a tip from einstien,,,fo-get-a-bout it...

Hell,, you want weird? think about the conservation of angular momentum,,

The tip of a whip goes from lazy ordinary speed to brecking the sound barrier in the course of like three inches.

Why? fuc-it, we got no clue,,,,,we just call it the conservation of angular momentum....

Time dilation anyone? (fo knows)

fokiten

Guest · Sun May 15, 05 11:30 am

yes, it is exactly the same, as long as you disregard caring about land. but if your world is just the water, then this is exactly the same as stationary water with 14kts of current. it's just a relative frame of reference. i can't believe this isnt recognized. if you never looked at land, and if there weren't any weird rip effects from the actual current on the surface it would be impossible to know the difference in theory. this is very basic stuff. you know einstein on the train with the speeding bullet etc--that is an extension of this. also feynman apparently got a question like this(a much harder variant) right in some high school physics competition because he just ignored everything else and realized that the water was all that mattered and that was the frame of reference.

some people talk about losing a board in current. if you are in the same current as the board, there are no effects. of course you might get pushed away from the beach, but you and the board are riding on the same magic carpet, and you both have the same wind.

technically the 14kts is the apparent wind, and then your speed with add or subtract from this in the usual way. but you could easily just call the 14kts the true wind b/c it is constant for your world and is what you feel when stationary. then your motion through the water affects it as usual.

there was an article in one of the international kite magazines a couple months ago about some of the maui people kiting on a fjord or something in alaska. once they passed some sandbar the current increased 10kts opposing the wind. which meant they would go from powered to totally overpowered--just like a huge sustained gust. at the point of transition there must have been weird affects on the board--getting grabbed and yanked upwind--but that is just at the boundary. once in the new world, the increased wind is the reality.

Wind Slither · Since 04 Mar 2005 2573 Posts The 503 METAL

[quote="Anonymous"]
some people talk about losing a board in current. if you are in the same current as the board, there are no effects. of course you might get pushed away from the beach, but you and the board are riding on the same magic carpet, and you both have the same wind.

Yeah, but the board is much more areodynamic than a dragging person with a kite. If you get separated at Rufus when the currents really pumping, you'll know what I mean.

wheels · Since 14 May 2005 29 Posts

[quote="Anonymous"]
technically the 14kts is the apparent wind...but you could easily just call the 14kts the true wind b/c it is constant for your world and is what you feel when stationary. then your motion through the water affects it as usual.

But doesn't this assume that you'll never leave the water surface? If you do leave the water, then are you saying that the 'true wind' would be 14kts? Is it really a constant?

If a kiter launches off the water and is in the air for 6 seconds, will the wind speed in his/her world still be 14kts, just prior to landing back on the surface of the water. Assuming that one would launch off the surface, somehow this 14kts doesn't seem very 'constant'.

-bw-

Dr Sloth · Since 05 Mar 2005 27 Posts

How about 4 knots of wind opposing 10 knots of current? Would that be the same as 14 knots of wind with no current? I can't see how it would be.

Wind Slither · Since 04 Mar 2005 2573 Posts The 503 METAL

Doc, according to the apparent wind calculator, you would have 14 knots apparent wind.

http://www.sailingusa.info/true_wind_calculator.htm

I am assuming that a person floating in a 10 knot opposing current is no different than a boat traveling at 10 knots straight up wind (0 degrees in no current).

Also, I have noticed first hand that when jumping in an opposing current you seem to lose power in the air whereas jumping in a non-opposing current you seem to get more power after leaving the water. It makes sense.

PS were those SW'erlies sailable on the coast today?

Guest · Sun May 15, 05 6:00 pm

Guest · Sun May 15, 05 6:06 pm

[quote="wheels"]

But doesn't this assume that you'll never leave the water surface? If you do leave the water, then are you saying that the 'true wind' would be 14kts? Is it really a constant?

If a kiter launches off the water and is in the air for 6 seconds, will the wind speed in his/her world still be 14kts, just prior to landing back on the surface of the water. Assuming that one would launch off the surface, somehow this 14kts doesn't seem very 'constant'.

this is a good question. we might think that leaving the water and entering the air means we feel the windspeed that the land feels. however, we have inertia at 4kts upwind. if you jump off the water and are in the air you will still be doing 4kts upwind relative to land. yes, the kite at 12 o clock will create downwind drag and reduce this. but that is also true for normal kiting. in the air you will begin moving downwind and this will reduce the apparent wind.

objects in motion stay in motion unless acted upon by an outside force.

Guest · Sun May 15, 05 6:25 pm

pkh · Since 27 Feb 2005 6548 Posts Couve / Hood Site Lackey

Hey all you smartypants, login so we can tell our friends who you are and how smart you are! Thumb's Up

Guest · Sun May 15, 05 9:32 pm

the water is dangerous

Dr Sloth · Since 05 Mar 2005 27 Posts