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# I don't understand how this car works.

Sun 6 Jun, 2010 08:04 pm
Wind-powered car goes down wind faster than the wind

Quote:
(PhysOrg.com) -- A wind-powered car has been clocked in the US traveling down wind faster than the wind. In a recent run at New Jerusalem in Tracy, California, the car reached a top speed of more than 2.85 times faster than the wind blowing at the time (13.5 mph) powered by the wind itself. The run should now settle the DWFTTW (down wind faster than the wind) debate that has been raging for some time on the Internet about whether or not such a feat was possible.

...

Cavallaro explained the car is able to move faster than the wind because the propeller is not turned by the wind. The wind pushes the vehicle forward, and once moving the wheels turn the propeller. The propeller spins in the opposite direction to that expected, pushing the wind backwards, which in turn pushes the car forwards, turning the wheels, and thus turning the propeller faster still

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Type: Discussion • Score: 5 • Views: 23,543 • Replies: 795

roger

1
Sun 6 Jun, 2010 11:58 pm
I've heard of ice boats and some sailboats that could outrun the wind, which of course they must do when tacking into the wind.

This one looks like perpetual motion, but maybe someone can explain it a little better than the article.

1
Mon 7 Jun, 2010 06:48 am
@roger,
Yeah, I understand why sailboats go faster when tacking than directly down wind.

Well, I can't do the math, but I get the basic concept.

Like you said though, this looks like getting more energy out than is put in. Maybe it's water fueled.
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rosborne979

1
Mon 7 Jun, 2010 08:37 am
I don't understand it either, and I'm dubious that it's possible (unless I misunderstand what they are claiming).

0 Replies

ebrown p

1
Mon 7 Jun, 2010 09:10 am
It doesn't violate the conservation of energy.

Remember it takes energy to accelerate a car. Constant speed takes zero net energy.

BillRM

1
Mon 7 Jun, 2010 09:23 am
@ebrown p,
On earth there is a little thing call friction and it does take energy a lot of energy to maintain a constant vel.

1
Mon 7 Jun, 2010 09:25 am
OK, I did some more reading.

The claim is that the source of power is the difference between the speed of the air and the speed of the ground; much like how a thermocouple generates electricity because of temperature differential.

I'm guessing then, that the wind vanes then act like a tacking sail, because they're hitting the air at a different angle of attack.

I'm not qualified to speak to the accuracy of the claim, though.

I would like to see Mythbusters do a show on it.

1
Mon 7 Jun, 2010 09:26 am
@ebrown p,
ebrown p wrote:
Remember it takes energy to accelerate a car. Constant speed takes zero net energy.

So turn off your engine when you get up to speed; you'll save a lot of gas.
0 Replies

Cycloptichorn

1
Mon 7 Jun, 2010 09:27 am

OK, I did some more reading.

The claim is that the source of power is the difference between the speed of the air and the speed of the ground; much like how a thermocouple generates electricity because of temperature differential.

I'm guessing then, that the wind vanes then act like a tacking sail, because they're hitting the air at a different angle of attack.

I'm not qualified to speak to the accuracy of the claim, though.

I would like to see Mythbusters do a show on it.

This is correct.

Cycloptichorn
0 Replies

ebrown p

2
Mon 7 Jun, 2010 09:36 am
@BillRM,
Sure there is friction. So to accelerate the car (and they only have to accelerate it a little bit) all they have to do is a little tiny bit better then friction.

The problem with this discussion is that no one has explained why it wouldn't work.

What's wrong with the simple explanation (which is the way the people who built the thing explain it)? The wheels are connected to the propeller. The wheels move the propeller and the propeller pushes air backwards, which by Newton's third Law pushes the car forward.

They are taking kinetic energy from the wind (by slowing down air molecules) and transferring it to the car. They are obviously taking enough energy from the wind to compensate for friction and a little more (to get the small amount needed to accelerate the car).

I don't know why you all are having so much trouble with this--- considering that no one has shown why they think any law of physics is being broken.

This car is cool, but it ain't magic.

1
Mon 7 Jun, 2010 09:44 am
@ebrown p,
ebrown p wrote:
What's wrong with the simple explanation (which is the way the people who built the thing explain it)? The wheels are connected to the propeller. The wheels move the propeller and the propeller pushes air backwards, which by Newton's third Law pushes the car forward.

Because that explanation creates energy out of nothing. That explanation would work even if there were no wind, so long as you gave the car a little push, the car would continue to accelerate.

If I attach a thermocouple to a refrigerator, so that the thermocouple powers it, the system will eventually run down. The refrigerator will not get colder and colder and colder.

ebrown p wrote:
This car is cool, but it ain't magic.

I'm not claiming it's magic; I'm saying that their explanation violates the laws of physics, and I don't understand how they got the results they claim to have gotten.

This leaves me with two possibilities:

1) They did not get the results they claim to have gotten.
2) Their explanation is wrong, and there is an alternate explanation.
ebrown p

2
Mon 7 Jun, 2010 09:47 am
Quote:

Because that explanation creates energy out of nothing. That explanation would work even if there were no wind, so long as you gave the car a little push.

This is your mistake. It is not creating energy out of nothing.

The energy is being taken from the kinetic energy of the wind. If you slow down moving air molecules, you reduce their kinetic energy-- this energy is being transfered to the cart.

It might help you understand if you consider that key part of this system is the road (which is interacting with the wheels). This means you should be analyzing this system from the road's frame of reference. This is why the apparatus would not work with no wind (which, by the way would not break the conservation of energy, but would break the second law of thermodynamics). The difference between the speed of the wind, and the road is key to this system working.

You are simply wrong that their explanation violates the laws of physics.

1
Mon 7 Jun, 2010 09:52 am
@ebrown p,
ebrown p wrote:

Because that explanation creates energy out of nothing. That explanation would work even if there were no wind, so long as you gave the car a little push.

This is your mistake. It is not creating energy out of nothing.

I'm not claiming that they're making energy out of nothing, so I haven't made that mistake. I'm saying their explanation is wrong.

ebrown p wrote:
The energy is being taken from the kinetic energy of the wind. If you slow down moving air molecules, you reduce their kinetic energy-- this energy is being transfered to the cart.

But that's not how they explained it, is it?

You're conflating two things: the actual reason for why the car works, and their explanation for why the car works.

Their explanation would create perpetual motion, so it must be wrong, and there must be a different reason for why the car works.

1
Mon 7 Jun, 2010 09:55 am
@ebrown p,
ebrown p wrote:
This means you should be analyzing this system from the roads frame of reference.

I'm not quite sure what you mean. The road's "frame of reference" is the same as a stationary observer's "frame of reference". The car is hardly reaching relativistic velocities, so why are we even discussing frames of reference?

I think your physics might be a little rusty.
ebrown p

2
Mon 7 Jun, 2010 09:56 am
Quote:
Their explanation would create perpetual motion,

You are stating this as an assertion without any evidence.

There is an external energy source (namely the wind). This has nothing to do with perpetual motion.
ebrown p

1
Mon 7 Jun, 2010 10:00 am
Frames of reference were around long before relativity and are a part of classical physics. Discussions of Newton's laws often involve frames of reference.

Let's agree to discuss this from the road frame of reference (which you correctly state is the same as a "stationary" observer). There are other possible frames (i.e. the wind or the cart)... but we will ignore them for now.

Let's assume the wind is going at 13 mph (in the road frame of reference). Do you agree that the wind has kinetic energy?

1
Mon 7 Jun, 2010 10:02 am
@ebrown p,
Again, you're conflating their explanation (with which I have a problem) and why the car works (which I am trying to determine).
ebrown p

1
Mon 7 Jun, 2010 10:04 am
No I am not. You asserted that their explanation "violates the laws of physics".

I am asking you to support this assertion. If we can establish that wind travelling at 13 mph has kinetic energy, it means that the law of conservation of energy is not being violated and it means that this has nothing to do with "perpetual motion" (which implies no source of energy).
0 Replies

1
Mon 7 Jun, 2010 10:04 am
@ebrown p,
I've already addressed why I think the car works. If you wish to propose an alternate explanation, I have no objection.
0 Replies

1
Mon 7 Jun, 2010 10:05 am
@ebrown p,
ebrown p wrote:
What's wrong with the simple explanation (which is the way the people who built the thing explain it)? The wheels are connected to the propeller. The wheels move the propeller and the propeller pushes air backwards, which by Newton's third Law pushes the car forward.

I'd love to see the math on this.

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