Downwind faster than the wind. Poll

[RAI]

Another big bang theory

When you clap your hands is the sound made by the right hand or the left?

A very good question, worthy of another thread. Maybe it comes from the air trapped between your hands going supersonic. I speculate

 

[mikemonty]

So let me get this right. You're saying that you can see how the cart can use energy from the wind to travel faster than the wind, but not how it can use energy from the ground to travel faster than the ground?

Let ME get it right. You adroitly misdirected this to avoid the point that was made and made a leap to the sort of irritating device I've seen you use before that I can only assume was intended to distract Halfway.
Kindofa verbal "Look at the stick, doggy, look at the stick, now... FETCH!"

The energy comes from the velocity difference between the wind and the ground. If you you an axis system attached to the ground, that difference appears as wind speed. If you use an axis system attached to the wind, that difference appears as the ground speed. If you use an axis system attached to the cart, you need to consider both speeds.

This is all terribly basic kinematics and dynamics, you know.

So do what we all said from the start - stick to the frame of reference that makes most sense - the ground.
You've had great fun jumping from one F.O.R. to another and using bizarre observers viewpoints in what seems to me to be an effort to avoid a conclusion (The conclusion?)

And always finish on a patronising line...
This is all very basic trolling, you know.

Atlp5 kept this going for YEARS on the physorg forum "plane on a conveyor" thread using similar techniques - you're still only amateur status, but working on it.

 

[Ubergeekian]

Let ME get it right. You adroitly misdirected this to avoid the point that was made and made a leap to the sort of irritating device I've seen you use before that I can only assume was intended to distract Halfway.

Not at all. I merely pointed out - as I have done before and, sadly, will probably have to do again - an obvious corollary of what someone said.

So do what we all said from the start - stick to the frame of reference that makes most sense - the ground.
You've had great fun jumping from one F.O.R. to another and using bizarre observers viewpoints in what seems to me to be an effort to avoid a conclusion (The conclusion?)

Not at all. Thinking of the problem - thinking of any problem - in different reference frames is not only a good way of understanding what's going on; it's also a very good way of checking models for consistency. Because, you see, the results have to be the same no matter what reference frame you use.

In this case the results are always the same: after the cart (boat) has passed, the velocity difference between the ground (some of the water) and some of the air has reduced. Kinetic energy has been extracted from the system and used to overcome friction and drag.

I'm sorry that Halfway hasn't felt able to answer the three simple questions I asked. Nevertheless, here's a simple one for you: relative to a reference frame fixed to the cart (or boat) how much kinetic energy does it gain as its ground (or water) speed increases?

 

[mikemonty]

Not at all. I merely pointed out - as I have done before and, sadly, will probably have to do again - an obvious corollary of what someone said.
...

I'm sorry that Halfway hasn't felt able to answer the three simple questions I asked. Nevertheless, here's a simple one for you: relative to a reference frame fixed to the cart (or boat) how much kinetic energy does it gain as its ground (or water) speed increases?

Valliant attempt at a rebound shot, but as the Mythbusters guys t-shirt says,
"I reject your reality and substitute one of my own"

I'm quite comfortable with my frame of reference and with my own handle on the world. (as are at least 49% on an upward trend on this poll)
I've had a go and have not been able to convince you - you have had a go and have not been able to convince me, I think we should leave it there.

 

[snowleopard]

Well, the manned cart has now done a successful trial - see the new thread.

 

[Ubergeekian]

I'm quite comfortable with my frame of reference and with my own handle on the world.

Perhaps that's how we differ. I'm always questioning and doubting my understanding of the world, and trying to improve it. Comes of being a professional scientist, I expect.

How do you feel about the video of the San Jose State University team's manned cart whizzing downwind with telltales streaming behind it? Is your comfort at all shaken, or would that take more than mere evidence?

 

[[2068]]

Ahh that old chestnut, the power supply is the motor that makes the running machine work, some of this power is captured by the wheels of the device that feeds it via a gearbox to a shaft onto which there is a propellor. The thrust of the prop is arranged so it pushes the device along the running machine in the opposite direction to that which it is travelling. IT looks like the device is clawing its way upwind under its own power, but turn off the running machine and the party is over - no more power input.

Great for year 10 science thanks for the link

Wow.
We had the correct answer to this on page one of the thread.
Why so many bizzarre and incorrect theories?

 

[RAI]

Wow.
We had the correct answer to this on page one of the thread.
Why so many bizzarre and incorrect theories?

I guess because it's a strange concept, to be powered by the wind, to travel away from the wind, faster than the wind.

Now that the team has done a manned vehicle demonstration that so clearly goes faster than the wind, dead down wind, I hope the sceptics will reassess their opinions but, for some, admitting that they were wrong is just too hard.

As far as the poll goes, it shows that the majority are not always right, or maybe why they always get the government that they deserve.

 

[[2068]]

I'm not talking about the manned demo, just what was shown in the first video, which is not the same thing.
In the first video, it's "powered" by the treadmill with air resistance not being a factor because it's pretty much stationary.

 

[snowleopard]

I'm not talking about the manned demo, just what was shown in the first video, which is not the same thing.
In the first video, it's "powered" by the treadmill with air resistance not being a factor because it's pretty much stationary.

The difficulty with the treadmill for demonstrating to sceptics is that it isn't long enough to allow the cart to accelerate so it has to be started at the condition where ground speed = wind speed. The equivalent with the manned cart would be towing it up to wind speed then letting it continue under its own power. That would be a perfectly valid demonstration of the principle but unnecessary and less convincing.

 

[[2068]]

If you wanted to find out a lot more about what is happening in the later dune test video, the first thing would be to fit some strain gauges and find out what forces are actually acting, and where.

Assuming that it can be shown to work in steady state conditions (i.e. not fluctuating wind) then there is something very obvious that MUST be the case: it MUST be the case that the force of air resistance acting on the front of the rotating prop plus friction etc. is a bit smaller than the thrust force acting on the back.

At this point you hand it over to the aero engineers, who eventually might tell you something interesting about the behaviour of propellors in air at low speeds, but there's no physics being violated here that I can see, just something interesting about propellors.

 

[RAI]

Maybe all it is saying is that good aerofoils produce excellent Lift to Drag ratios.

 

[Ubergeekian]

Assuming that it can be shown to work in steady state conditions (i.e. not fluctuating wind) then there is something very obvious that MUST be the case: it MUST be the case that the force of air resistance acting on the front of the rotating prop plus friction etc. is a bit smaller than the thrust force acting on the back.

At this point you hand it over to the aero engineers, who eventually might tell you something interesting about the behaviour of propellors in air at low speeds...

That's yer actual Kelvin-Froude Actuator Disk Theory.

Here's a good basic introduction ... not for the faint-hearted, though.

http://www.esru.strath.ac.uk/EandE/Web_sites/05-06/marine_renewables/technology/Froude.htm

 

[saltwater_gypsy]

it's the wind/water speed differential that does it,.

If you say the same thing often enough you will begin to disbelieve it.

 

[snowleopard]

If you say the same thing often enough you will begin to disbelieve it.

But then trying to sail in a 5 knot wind with a 5 knot tide in the same direction would soon bring reality back. Are there any here who claim to be able to sail under those conditions?

 

[RAI]

Tacking again

But then trying to sail in a 5 knot wind with a 5 knot tide in the same direction would soon bring reality back. Are there any here who claim to be able to sail under those conditions?

Yea! Drop the anchor and tack back and forth. Up stream speed through the water = 5 knots. (Well, if you want to go that way, it's better than drifting backwards).

 

[snowleopard]

A bit of Fred Drift

You are in a sailing boat in a 5 knot current and want to reach a buoy 10 miles directly down tide. Will you get there quicker if it is flat calm or if there is a 5 knot wind blowing towards your destination?

 

[RAI]

A bit of Dred Thift

You are in a sailing boat in a 5 knot current and want to reach a buoy 10 miles directly down tide. Will you get there quicker if it is flat calm or if there is a 5 knot wind blowing towards your destination?

Ah! You want to go that way! Time for a 2 hour lunch.

(Must look up tide tables to see how long the favourable current will hold).

P.S. How about starting a new poll "Who believes DDWFTTW is possible now?"

 

[ianj99]

But then trying to sail in a 5 knot wind with a 5 knot tide in the same direction would soon bring reality back. Are there any here who claim to be able to sail under those conditions?

YES using my patented bimedia prop. A plank with a prop at either end linked with a frictionless shaft & bevel gears. Immerse one in the briny and point the other at the sail...
This way you can exploit the differential between water and boat, boat and air, air and solar wind and capitalise on geothermal exoclines.

 

[snowleopard]

(Must look up tide tables to see how long the favourable current will hold).

It's a non-tidal river