Merged DDWFTTW in a “Real” Wind

[ThinAirDesigns]

That is not what 69dodge said at all.

I believe it to be an exact or at least relevant analogy.

It would be more like "when I carry a briefcase across a room by the handle, the handle does no work on the rest of the briefcase", which is a more interesting question.

If my hand is securely gripping the handle of the briefcase (in other words, no relative motion), why would the above be any more interesting?

JB

 

[humber]

I'll call you a fraud when you start taking money, for now your just a crank and liar.

In your case, the money is totally unnecessary.

 

[Dr. Keith]

Really, all snarkiness about your previous failures and lies aside, this is the most interesting thing about your stance to me. You deny that a craft could gain power from a substantial steady state differential between air speed and ground speed, but find it obvious that a massively fluctuating wind is the source of power. I have never seen a device that gains power from massively fluctuating wind directions, but I would love to.

Hummer: I enjoy the side stuff, but I really would love to see you expand on this.

 

[windgrins]

Twisting in the wind (DDW I might add!)

Static force does no work in any frame.

Static in one frame means by definition it isn't static in a frame moving wrt to the static frame. Why do you think PPG wants to work in the frame of the ground and not the cart. The work is not the same in different frames. If it was, why would he care?

Does this one apply here:

#8. Your concept of reference frames is too complicated especially when you use more than one.

And you can stand in a train. Perhaps you can market a car with seats, but no engine.

Just because a train has low acceleration doesn't mean it has no acceleration. If the train jams on the brakes, try just standing there and see how that works out!

Sailcarts have no engines and some companies market them just fine!

And this one here:

15. Airplanes have engines and boats move on the water so no abstracted principle applies to the cart at all, period, unless I want to use it poorly in my example.

Then that is the force of acceleration, and so not static.

My point exactly-imposed on the driver by his seat!

OK, enough batting trivially easy floaters. Time for a beer!

WindGrins

 

[humber]

I have never seen a device that gains power from massively fluctuating wind directions, but I would love to.

A ddw trial where the change in wind direction is more than 60 degrees...

 

[humber]

Static in one frame means by definition it isn't static in a frame moving wrt to the static frame. Why do you think PPG wants to work in the frame of the ground and not the cart. The work is not the same in different frames. If it was, why would he care?

http://www.lightandmatter.com/html_books/2cl/ch03/ch03.html
There are many examples where the transfer of energy out of an object cancels out the transfer of energy in. When the tractor pulls the plow with a rope, the rope does negative work on the tractor and positive work on the plow. The total work done by the rope is zero, which makes sense, since it is not changing its energy.

And in both the cases of the tree and the cars on the trailer, there is only static force.

 

[Dr. Keith]

A ddw trial where the change in wind direction is more than 60 degrees...

I'm sorry, maybe I wasn't clear: can you show me a device that is powered by a fluctuation in wind direction?

Seriously. I don't know why you think the fluctuations add to the performance.

 

[Llyricist]

I'm sorry, maybe I wasn't clear: can you show me a device that is powered by a fluctuation in wind direction?

Seriously. I don't know why you think the fluctuations add to the performance.

He read a NASA site that explained an airfoil's lift by the fact that it changed the direction of the air's flow. In humber logic, that means that air changing direction causes lift.

 

[windgrins]

Hard to call it a device!

I'm sorry, maybe I wasn't clear: can you show me a device that is powered by a fluctuation in wind direction?

A tornado maybe!

 

[humber]

I'm sorry, maybe I wasn't clear: can you show me a device that is powered by a fluctuation in wind direction?

Seriously. I don't know why you think the fluctuations add to the performance.

The chart show you such a device. Another is a windvane. The power to move that, comes from wind direction change, and vanes too have a lift/drag ratio.

But cart supporter Steve Smith of NASA says:

One other possibility is worth discussing — that it could be the time-variation of the wind direction, rather than simply the fact that the wind is not (also..my edit) DDW. There are examples of similar effects in nature that would give some reason to consider this. For example, what is called “dynamic soaring” -- the maneuvering in a wind gradient field (regions where there is a consistent pattern of different wind speed) that allows birds (and gliders) to soar by extracting energy out of the wind gradient field."

Not small, but big changes, as seen in the chart.

 

[BowlOfRed]

Consider pushing an object across a table with a force of 1N and it moves away from you at a steady 1ms^-1. There is a force of 1N acting on the object and it is travelling at 1ms^-1 so the power applied is force x distance / time = 1 joule per second or 1 watt.

Now repeat the experiment on a train travelling at 50ms^-1. The object is still experiencing a force of 1N but now it is travelling at 51ms^-1. So how much power is being transferred to the object? 51 watts or 1 watt?

Obviously it's still 1 watt. So why is the calculation of force x distance now wrong?

You've left the desk out of your calculation. The reason the object isn't accelerating is because friction is providing a force of 1N backward.

In the v(desk)=0 case, we can imagine that all the work done on the object is going into friction and dissipating. The desk is applying a force to an object moving at 1m/s, so it's doing work. The object is applying a force to the desk (moving at 0m/s), so it is doing no work (on the desk). The difference (1W) is dissipated by friction.

Work done by force on object (1W)
Work done by object on desk (0W)
Difference (dissipated by friction) (1W)

In the v(desk)=50m/s case, both objects are doing work on each other.

Work done by force on object (51W)
Work done by object on table (50W)
Difference (dissipated by friction) (1W)

 

[humber]

A tornado maybe!

A large windbag or windsock.

 

[windgrins]

An interesting question!

The chart show you such a device. Another is a windvane. The power to move that, comes from wind direction change, and vanes too have a lift/drag ratio.

But cart supporter Steve Smith of NASA says:

One other possibility is worth discussing — that it could be the time-variation of the wind direction, rather than simply the fact that the wind is not (also..my edit) DDW. There are examples of similar effects in nature that would give some reason to consider this. For example, what is called “dynamic soaring” -- the maneuvering in a wind gradient field (regions where there is a consistent pattern of different wind speed) that allows birds (and gliders) to soar by extracting energy out of the wind gradient field."

Not small, but big changes, as seen in the chart.

That's a reasonable question! But for the rest of us, your old enemy, the Treadmill test, nixes that hypothesis pretty handily!

Windgrins

 

[humber]

He read a NASA site that explained an airfoil's lift by the fact that it changed the direction of the air's flow. In humber logic, that means that air changing direction causes lift.

If you read more carefully, it is expressed as the turning of the air mass, and what do you know, Steve Smith agrees.

 

[humber]

You've left the desk out of your calculation. The reason the object isn't accelerating is because friction is providing a force of 1N backward.

In the v(desk)=0 case, we can imagine that all the work done on the object is going into friction and dissipating. The desk is applying a force to an object moving at 1m/s, so it's doing work. The object is applying a force to the desk (moving at 0m/s), so it is doing no work (on the desk). The difference (1W) is dissipated by friction.

Work done by force on object (1W)
Work done by object on desk (0W)
Difference (dissipated by friction) (1W)

In the v(desk)=50m/s case, both objects are doing work on each other.

Work done by force on object (51W)
Work done by object on table (50W)
Difference (dissipated by friction) (1W)

That is not quite right. In both cases the work against friction is 1W,
but the KE of the object has changed from 1/2m(50ms-1)^2 to 1/2m(51ms-1)^2, whereas on the ground, from 0 to 1/2m(1ms-1)^2.
The additional energy for that larger KE, comes from the train.

 

[Llyricist]

Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift.

Apparently, even kids are expected to comprehend it.

 

[humber]

Apparently, even kids are expected to comprehend it.

Originally Posted by http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html
Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow.

Galileo not working today?

 

[humber]

That's a reasonable question! But for the rest of us, your old enemy, the Treadmill test, nixes that hypothesis pretty handily!

Windgrins

Thus showing the TM has nothing to do with a cart in the wind. A simple device made from aluminum strip, and some parts from a construction set, is laughing at you.

 

[BowlOfRed]

That is not quite right. In both cases the work against friction is 1W,
but the KE of the object has changed from 1/2m(50ms-1)^2 to 1/2m(51ms-1)^2, whereas on the ground, from 0 to 1/2m(1ms-1)^2.
The additional energy for that larger KE, comes from the train.

Uh, no. The original statement of the problem was "it moves away from you at a steady 1m/s". So there can be no change in KE during the process (otherwise it wouldn't be steady).

The object is not accelerating.

 

[humber]

Uh, no. The original statement of the problem was "it moves away from you at a steady 1m/s". So there can be no change in KE during the process (otherwise it wouldn't be steady).

The object is not accelerating.

If you assume that the object is already at 1ms-1 wrt the train, then the 50ms-1 drops out. and both cases are 0W and 1W

Work = force*distance
= F*( ut + 1/2at^2)
With respect to the floor of the train u = 0, and for constant motion a = 0.
so
Work = f*d = F*v*t
v =1ms-1

There is no 50W, because the negative work cancels the positive work.