Split Thread The validity of classical physics (split from: DWFTTW)

[humber]

Humber, somehow your explanation reminds me of someone pushing a rope.

Keep trying, humber. It isn't that hard. John explained it for you and he didn't need to google any "references".

This picture isn't all that clear but the centre of the contact patch is in front of the axle:

http://images.google.ca/imgres?imgu...firefox-a&rls=org.mozilla:en-US:official&sa=N

SCZ is Subduction Zone (SDZ). He used to decry the DDWFFTW cart. Seeing the treadmill videos changed his mind.

Do you think that a truck doing a wheely is the same thing, and is that the best you can do. I know I am right, otherwise I would have been showered with textbook examples that contradicted my claim.
The last remark concerning SZ is of course religions in intent.

 

[Furcifer]

Out of curiosity, is this how boat propellers work as well? They compress the water to a higher density behind them?

Tunny

Pick up a book, water isn't compressible.

 

[humber]

See? The bleedin' thing is hovering!

Tunny

That's right Tunny it is, but your contribution is as usual, no more than to hover and fly away. If you think that the contact point argument is wrong, then you can show it can't you?

 

[semper]

For an open system like this ("open" means mass can enter or leave the system) thrust is the exhaust velocity of the air relative to the cart times the differential change in mass wrt to time.

The cart is getting thrust by pushing more air (mass) out the back of the prop than it takes in the front of the prop. This is a differential in mass occuring over time. You've set the velocity of the wind as constant, it's a scalar quantity, big deal!

If you want to get an idea of what I think is happening I found this

Granted, this is based on single particles, but the idea is the same. If you consider the air as a point particle (well two joined) and the regions on either side of the blades as the bifurcating point it makes more sense.

Gee 3body, you got it bad, are you trying the old "out Humber the Humber" routine, you're encroaching on his turf.
Anyway, if you want to learn about propellers (your link was interesting but quite irrelevant), try this one; http://web.mit.edu/16.unified/www/SPRING/propulsion/UnifiedPropulsion7/UnifiedPropulsion7.htm

 

[semper]

The force must develop behinf the axle, if the wheel is to move forward. All references will confirm this.

Again, the force reacts against the friction. Forward motion means to the rear of the axle.

Only to the rear . Wheels can spin only in one direction at a time.

Yes, everybody thinks they understand wheels.

A driven wheel is different from a slave wheel, BTW.
When driven forward by the axle, the contact patch is to the rear of the axle.
On the treadmill, the wheels are driven. Because there is no effective thrust from the propeller there is no contact patch, and the belt cannot make one. It is busy going nowhere.

Just to clarify then Humbler, the powered wheels on a normal car, the ones connected to the gear train and engine, while driving (not braking) you would call those "slave" wheels. And when braking, all the wheels will be "driven" wheels. Is that your definition?

 

[Furcifer]

When you set up an experiment you should know what you are modeling. spork and JB's cart on a treadmill is a near perfect model of a cart already going at the speed of the wind outdoors where the speed of the treadmill is the speed of the wind. I saw in a later point you pointed out that the cart actually accelerates up the ramp. This shows that the cart under a similar situation outside would also accelerate.
What are you trying to model with your outboard motor in a barrel? If you don't know what you are testing for it pretty much automatically makes your test worthless.

And no velocity is never a scalar, by definition it is a vector if you want a scalar try speed.

Fine. Nevermind.

Walk me through "How it works"

Start with the thrust, use momentum theory and work your way up from there.

 

[ThinAirDesigns]

Fair enough. You present yourself as genuinely interested and don't act like a boob.

Besides the "boob" thing, there is one other excellent distinction between you and Mender -- he gets the physics right.

Listen to him closely and you'll shed some of that "boob" stigma.

JB

 

[Furcifer]

; http://web.mit.edu/16.unified/www/SPRING/propulsion/UnifiedPropulsion7/UnifiedPropulsion7.htm

Good link. However, the list of assumptions makes it irrelevant to the cart propeller. They assume that the mass flow in and out of the tube is the same. This can't be said for the cart, except possibly when the propeller is stationary. Do you agree?

 

[humber]

Just to clarify then Humbler, the powered wheels on a normal car, the ones connected to the gear train and engine, while driving (not braking) you would call those "slave" wheels. And when braking, all the wheels will be "driven" wheels. Is that your definition?

Yes. The front wheels of a rear wheel drive are not driven but 'slave'. These can and do rotate at a different velocity than the rear wheels even for the same diameter. For one reason, there is some reduction in effective diameter of the rear wheel as it deforms. This difference in angular veloicties is also called 'slip', so that can be confusing.
The rear wheel may deform so it looks like the patch is to the front, but the effective force will come from behind the axle. It is not possible for friction to 'suck" the wheel forward. Be aware of the difference between an axle-driven wheel, and one driven from the rim.
It is obvious. The wheel provides thrust to drive the car forward, and so must be to the rear of the axle.
Braking differs is all or only a set of wheels have brakes, but that is not really the issue.

 

[John Freestone]

Do you think that a truck doing a wheely is the same thing, and is that the best you can do.

Well, it was a start.

I know I am right, otherwise I would have been showered with textbook examples that contradicted my claim.

I wonder if you can see the problem with your method of discerning truth, there. I gained confidence that big-mass falls faster than little-mass because no-one stepped in and corrected me, apart from you, but you turned out to be right.

I am looking for more evidence, but haven't found anything at all. The balloon search began like that, too. A lot is coming up about rail wheel research, which I haven't looked much at. I guess it's much more vital, and relates to railway safety, and is probably more about lateral movements and derailing than whatever forward-backward position the contact takes w.r.t. the axle.

I hope you might find time and post any you find supporting your version of events. Perhaps it would also be fun if you explained the mechanics of your version, too, rather than just asserting it. I thought the patch would be in front if anything, and when mender said you had it backwards, I decided to dare to discuss what I felt should happen. I gave clear reasons, if non-technical ones, and even physical experiments that one might do with a piece of rubber. Imagine a piece of rubber is a wheel fixed to a vehicle with some fair amount of resistance from friction. Find a roundish rubber (er, do I need to say "eraser" for you guys over the pond?). Pretend your hand is the vehicle and start turning the rubber against your desk. Does it not tend to pile up in front?

What's your version? Do you think that the driven wheel moves the contact patch behind the axle first to initiate motion? Does the wheel drive its axle forward w.r.t. ground, leaving the contact position behind?

Good luck!

 

[Subduction Zone]

Fine. Nevermind.

Walk me through "How it works"

Start with the thrust, use momentum theory and work your way up from there.

You have to be a bit more specific in your requests. You are already busted for your inane outboard motor in a barrel experiment. You have also misunderstood where thrust comes from, trying to create matter out of nothing. If you want to ask specific questions about how a propeller works I would refer you to spork, aero was his major after all. If you canot tell from my screen name I am but a humble geologist/geophysicist. This is not my area of expertise, but even I can recognize when someone is way off base.

 

[Furcifer]

They sometimes get slightly short with people who raise very stupid objections, or jump in all guns blazing before they've got any kind of idea what the cart does, or constantly gripe about some minor detail, or just become trolls on the subject, etc. I made a couple of those mistakes myself. I took one look at the thing (Goodman's video), read a few posts, and made a sweeping judgement from a position of almost total ignorance. I, like so many, thought that the prop was a turbine, for a start. Getting over that was quite a big step to understanding it, but my schoolboy physics of 25 years ago and gut feelings still kept me thinking I had found reasons why it couldn't work. The treadmill being equivalent (near as is necessary) was another minor glitch, but I soon remembered some of that from earlier, applied myself to it, and now it's about as confusing as seeing someone skydive indoors over a big fan.

I certainly didn't feel ganged up on, but it was frustrating. I now see that a good deal of the frustration was that I was unable to explain what a particular theoretical objection was precisely enough. It seemed that people misunderstood, or answered it with some other point that I thought was irrelevant to the one I raised. Now I see that this was because I was talking to people who knew much more about physics and especially way more about aerodynamics, and their answers were almost always on the money, and it was my problem not being able to connect the dots.

My worst feeling at certain points was that they did not seem to teach clearly how the cart worked, but rather sat back and waited for objections, and then corrected or criticised the objection. But that was partly because it was set as a brainteaser. If you hold everyone by the hand and walk them through it all in nice neat steps, well, it's just a series of lectures on some fairly trivial quirk of mechanical engineering.

I think I agree with you there to some extent. I think that sometimes the frustration of not being believed by people like you causes some exaggeration - or indeed that might be said of some parts of the problem. And then if you've a mind to think it's all a scam, you may miss the context of that. But I'm not sure what "first year" is these days, let along "first year physics". It depends on your location maybe.

But you can deal with accelerations of objects in constant-velocity inertial frames just fine, I believe. It's not an accelerating frame of reference. It is a little like someone jumping out of an aircraft to skydive in still air, and saying "Now, if you think about it, we could have a ruddy big fan blowing air upwards at that speed and float about in it!" ...or all the endless analogues that have been proposed, some more accurately fitting than others. Those who dispute the treadmill proof on those grounds are just like people telling us not to waste our money trying to make an indoor skydiving rig; you'll never fly! In case you don't know, you do.

Well maybe you should work out how that business plan goes in theory. These guys seem to have been genuinely ready to bet on it, have independent tests, and I think spork even said at one point that he'd put up way more than the disbelievers, and - and here's a nice little thing to ponder on - whoever lost would pay JREF. Now there's a shrewd, greedy, selfish blogger if ever I saw one.

I agree with most of what you said here. I never took a tone until spork got ignorant with me. That sort of arrogance is uncalled for. I simply pointed out there was no video of a cart starting from rest on a treadmill. SCZ (SubduCtionZone) pointed out very quickly why that was, but the tone was already set. I'd still be interested to see it, I'm curious how long it would take.
I don't think it's a scam or immpossible to explain without violating the Laws of Thermodynamics. I just think it is very complex and I have yet to see anything substantial. I'm not saying it hasn't been presented, just not here. I've barely got the time for this thread.
It is a brainteaser, and I'm not well versed in aerodynamics. I've no objections to learning.
As for the cart accelerating, yes you can use the cart as a frame of reference but is unecessarily complex. If I remember correctly you end up with "fictious" forces and ugly numbers. It's certainly not an inertial reference frame.

 

[Furcifer]

You have to be a bit more specific in your requests. You are already busted for your inane outboard motor in a barrel experiment. You have also misunderstood where thrust comes from, trying to create matter out of nothing. If you want to ask specific questions about how a propeller works I would refer you to spork, aero was his major after all. If you canot tell from my screen name I am but a humble geologist/geophysicist. This is not my area of expertise, but even I can recognize when someone is way off base.

Not out of no where, from the air in front of the prop. I'm saying you have to expel mass at a greater velocity out than it comes in to generate thrust. With a tail wind, you have a mass of air building at the back of the prop, as the expelled air meets the incoming wind. That point, where the two meet, would be compressd wouldn't it? More mas per unit area behind the prop than in front.

 

[humber]

Well, it was a start.

I wonder if you can see the problem with your method of discerning truth, there. I gained confidence that big-mass falls faster than little-mass because no-one stepped in and corrected me, apart from you, but you turned out to be right.

No contradiction, I think, John. Usually, if I get responses that are said to contradict my claims, they turn out to be false, or apples and pears.

I am looking for more evidence, but haven't found anything at all. The balloon search began like that, too. A lot is coming up about rail wheel research, which I haven't looked much at. I guess it's much more vital, and relates to railway safety, and is probably more about lateral movements and derailing than whatever forward-backward position the contact takes w.r.t. the axle.

Not easy to find, I agree.

I hope you might find time and post any you find supporting your version of events. Perhaps it would also be fun if you explained the mechanics of your version, too, rather than just asserting it. I thought the patch would be in front if anything, and when mender said you had it backwards, I decided to dare to discuss what I felt should happen. I gave clear reasons, if non-technical ones, and even physical experiments that one might do with a piece of rubber. Imagine a piece of rubber is a wheel fixed to a vehicle with some fair amount of resistance from friction. Find a roundish rubber (er, do I need to say "eraser" for you guys over the pond?). Pretend your hand is the vehicle and start turning the rubber against your desk. Does it not tend to pile up in front?

You are confusing the distortion of the rubber with where the driving force is, and you need to drive it from the axle not the rim. Tyres are complex, and have controlled modes of flexure, because they must also handle cornering forces etc.
(As an side, an engineer told me that marketing would often reject tyre designs, because they did no look 'aggressive enough'. I found that disquieting.)

ETA: The forward distortion that you see in soft rubber is the 'plough' of the rear acting force.
Think of a car in soft and or mud. A belt moving belt under the wheels cannot do that, without moving the cart along with it, let alone create a force from the rear, to be in the opposite direction.

If your wheel has sticky surface, and you roll it over a sheet of paper, then the paper will shear a way from the rear.

What's your version? Do you think that the driven wheel moves the contact patch behind the axle first to initiate motion? Does the wheel drive its axle forward w.r.t. ground, leaving the contact position behind?
Good luck!

The car is in forward motion, so the driving force must come as thrust from behind the driving axle. The contact patch, the reactive frictional force of the ground, must also be from behind the axle.
It is not possible for for the wheel to drag the vehicle forward in front of the axle, because the first point of contact lies directly under the axle. That is why the wheel must deform to some degree for driven motion to occur. The only option for forward motion, is behind the axle.

This is also why locomotives can pull more than their weight. The static friction of the driving wheels against steel is low and due to gravity, and lying under the axle. Force of acceleration distorts the wheel and track to produce a higher rolling friction, allowing greater loads to be hauled, because the hauled wheels will still have something like the lower static friction.
Early railway engineers did not know this, and built geared tracks and even legs. (Brunton's and Blackinson's engines.)
There, something that has been learned about classical mechanics in the last 200 years.

 

[mender]

3dp(3bodyproblem), the way I originally thought the cart worked was by essentially using the air pushed back by the prop as a virtual sail. The wind pushed against the virtual sail/air cushion, making the cart move forward. Since the air pushed back by the prop has velocity towards the rear, the final speed of the cart can be and will be faster than just the speed of the wind along the ground.

I figured that the final speed of the cart would be made up of the speed of the wind plus the speed of the air being pushed back, minus the change in the wind's speed needed to harness enough energy to roll the cart over the ground at the higher speed. The cart would reach a steady state speed when the amount of energy taken from the wind (by slowing the wind down just like a wind turbine) matched the amount of drag that the cart generated when traveling along the ground. A simple energy balance that I worked out several ways.

I have since found out that the air cushion/virtual sail thing isn't how the cart works but it helped me get past the initial "can't work because it moves faster than the power source" issue.

Spork, I promise to point out the errors once we get past this part.

 

[humber]

3dp(3bodyproblem), the way I originally thought the cart worked was by essentially using the air pushed back by the prop as a virtual sail. The wind pushed against the virtual sail/air cushion, making the cart move forward. Since the air pushed back by the prop has velocity towards the rear, the final speed of the cart can be and will be faster than just the speed of the wind along the ground.

I figured that the final speed of the cart would be made up of the speed of the wind plus the speed of the air being pushed back, minus the change in the wind's speed needed to harness enough energy to roll the cart over the ground at the higher speed. The cart would reach a steady state speed when the amount of energy taken from the wind (by slowing the wind down just like a wind turbine) matched the amount of drag that the cart generated when traveling along the ground. A simple energy balance that I worked out several ways.

I have since found out that the air cushion/virtual sail thing isn't how the cart works but it helped me get past the initial "can't work because it moves faster than the power source" issue.

Spork, I promise to point out the errors once we get past this part.

If you are pushed in a wheelchair, you may turn around and push against he pusher so as to increase your velocity. This will be of effect, only if the pusher can react by supplying the additional force necessary for your acceleration, if not and the pusher 'gives' by the same amount, then you will not increase your velocity. Additionally, you must provide the energy.for your motion.
You are suggesting that the wind can both increase its force, and supply the necessary energy. If it could do that, you would get to the desired speed anyway, by the direct force and power of the wind.

 

[spork]

They assume that the mass flow in and out of the tube is the same. This can't be said for the cart, except possibly when the propeller is stationary. Do you agree?

Of course we don't agree. We realize that propellers cannot create mass out of nothing. It's not even worth offering a reference.

I never took a tone until spork got ignorant with me.

I assure you the ignorance is all yours. I'm confident that anyone on this forum (with the possible exception of humber) will be happy to confirm this.

That sort of arrogance is uncalled for.

I never cease to be amazed at the irony that comes from the ignorant.

I simply pointed out there was no video of a cart starting from rest on a treadmill.

And I simply pointed out that you were (and ARE) wrong.

I don't think it's a scam or immpossible to explain without violating the Laws of Thermodynamics.

No - in fact you've made up some very entertaining explanations already.

I just think it is very complex and I have yet to see anything substantial.

It's important to distinguish between your inability to understand and our inability to explain. Plenty of others have grasped it - only because they were willing and interested. Not because they could decipher our inscrutible analyses.

I've barely got the time for this thread.

You really shouln't let this thread cut into the important business of your life. We'll understand if you have to stop insulting us here.

I'm not well versed in aerodynamics.

But you have no problem taking a condescending attitude with people such as myself with an M.S. and many years of experience in aero.

I've no objections to learning.

Pure B.S. But thanks for playing.

 

[spork]

Spork, I promise to point out the errors once we get past this part.

No worries on my part. If you get him to 1/2 of your level of understanding or 1/10th your attitude, you will have far surpassed any expectations I have of him.

 

[humber]

Of course we don't agree. We realize that propellers cannot create mass out of nothing. It's not even worth offering a reference.

A change of mass density does not imply a change of mass.

3bodyproblem
I simply pointed out there was no video of a cart starting from rest on a
treadmill.

And I simply pointed out that you were (and ARE) wrong.

Where ?

Pure B.S. But thanks for playing.

Drone.

 

[Furcifer]

3dp(3bodyproblem), the way I originally thought the cart worked was by essentially using the air pushed back by the prop as a virtual sail. The wind pushed against the virtual sail/air cushion, making the cart move forward. Since the air pushed back by the prop has velocity towards the rear, the final speed of the cart can be and will be faster than just the speed of the wind along the ground.

I figured that the final speed of the cart would be made up of the speed of the wind plus the speed of the air being pushed back, minus the change in the wind's speed needed to harness enough energy to roll the cart over the ground at the higher speed. The cart would reach a steady state speed when the amount of energy taken from the wind (by slowing the wind down just like a wind turbine) matched the amount of drag that the cart generated when traveling along the ground. A simple energy balance that I worked out several ways.

I have since found out that the air cushion/virtual sail thing isn't how the cart works but it helped me get past the initial "can't work because it moves faster than the power source" issue.
Spork, I promise to point out the errors once we get past this part.

The thrust from the wind has to translate into a linear motion. The distance the the tip of the prop travels in one rotation at a given windspeed is greater than the windspeed relative to the road. The shaft translates this into linear motion in the wheels. I guess as long as the losse aren't too great, the wheels travel further forward than the wind in the same amount of time due to the area swept out by the prop.