Upward Force from Form Drag

[gordonjudd]

Gordy, did you watch it with the sound off, he explains the mechanics. It no more generates lift than a lasso does and would work perfectly well in a vacuum

Vince,
I assume this is the explanation he gave that you thought was a complete explanation of the mechanics of the levitating loop in the string shooter:

There is an old saying that you can’t push string, but in this case that is exactly what we are doing. The wheels push it forward. It has inertia that keeps it pushing forward until the loop is pulled back towards the machine and then it gets pushed again.

As stated in Wikipedia in today's thinking inertia is referring to an objects to mass.

Inertia comes from the Latin word, iners, meaning idle, sluggish. Inertia is one of the primary manifestations of mass, which is a quantitative property of physical systems.

However Wikipedia also notes that:

In common usage, the term "inertia" may refer to an object's "amount of resistance to change in velocity" (which is quantified by its mass), or sometimes to its momentum, depending on the context. .

So let us assume he meant "The line has "momentum" that keeps it pushing forward". If we assume that "pushing" is referring to a force then he is making the common mistake of equating momentum with a force.

Thus if Newton was correct and it takes change in momentum to produce a force his explanation of the mechanics of the string shooter does not wash.

It might work in a vacuum, but that does not explain the difference in the width of the legs and the shape of the loop when he changes the launch angle of the shooter. I would think those changes are only effected by the angle of the forces produced by the skin drag on the line.

If it was just the momentum change at the loop that was producing the tension in the string, then I would think the axis of loop would rotate in the plane that was created at the source of the shooter. It doesn't do that as you can see the loop remains aligned in a vertical plane even though the angle of the shooter was changed as shown here:


Maybe this is another interesting experiment for Dr. John Biggins to explain based on some real physics as he did with the chain fountain.

So what force (or forces) do you think is offsetting the force of gravity on the mass of the spinning loop? It can't be momentum.

Gordy

[Graeme H]

Thanks to Dirk for finding this example that shows how skin drag (form drag on an inclined section of line is many times larger) and the tension that comes from the change in the momentum of the string going around the loop is sufficient to offset the gravitational force on a length of rotating string.

Really?

I would have thought this experiment would be the "nail in the coffin" for your theory of skin drag lift, but you still manage to see it here Gordy. How do you do that? I'm impressed.

When he shoots the string nearly vertically (at 2:45), why is the skin drag in the loop not pulling it all the way over the top? Is the loop just too narrow to do that? How is the string staying up at 8'? Skin drag on the upward path must be about the same as on the downward path, so if it's there at all, skin drag must be pulling it down too, right?

Or maybe the momentum in the upper leg has an effect. Maybe even the majority of the "lift"? The upper leg does indeed seem to go where ever he points it (or deflects it).

Notice the nice parabolas the upper leg makes throughout the experiment? Just like a projectile in motion. I've seen that in a piece of string somewhere before.

Notice how the upper leg is prevented from completing its flight by the tension in the lower leg? There is not a lot of tension in the lower leg (see 3:20), perhaps a similar amount as if the length of string in the lower leg were hanging under gravity as a catenary without being in motion at all. But it is enough to stop the upper leg finishing its parabolic trajectory.

Would you like to predict the outcome of him pointing his string shooter at the wall, destroying the loop? Would the string just fall to the ground straight out of the motors without a loop? Or would the upper leg maintain trajectory initially imparted by the string shooter?

If this experiment does not convince you that Newton's Laws of Motion are in effect in the fly cast, I don't think you're ever going to see it.

I think you should make one of these shooters for yourself so you can run your own experiments. Wheels are just the wheels from a remote control car. Maybe I will instead ...

Cheers,
Graeme

[VGB]

Gordy

Wikipedia isn’t a recognised science journal. I also don’t understand why you posted the video if you think it’s hokum, surely you should be writing to the high school science teacher that produced it.

Thus if Newton was correct and it takes change in momentum to produce a force

I’m not sure that he did and there’s another authority that doesn’t think that Newton is a suitable tool for analysing flexible distributed masses:

http://www.sexyloops.co.uk/theboard/vie ... 481#p41481

Maybe this is another interesting experiment for Dr. John Biggins to explain based on some real physics as he did with the chain fountain.

And maybe he’ll post it in a real physics forum.

So what force (or forces) do you think is offsetting the force of gravity on the mass of the spinning loop? It can't be momentum.

What happens if I remove the external force provided by the motor?

What force ( or forces) are offsetting the force of gravity and making the water go up this child’s nose, it can’t be skin drag?

https://youtu.be/oUgELRJO6QU

Vince

[gordonjudd]

What happens if I remove the external force provided by the motor?

Vince,
It stops levitating since there is no tangential velocity on the line and thus no tension at the loop produced by a momentum change?

What kind of question is that?

The work energy provided by the wheels is what is making up for the skin drag losses and potential energy differences on the two legs of the line. No motor, no levitating loop.

Gordy

[Graeme H]

Astounding.

[James9118]

but maybe James found an error in it and removed it.

Cheers,
Graeme

No error (I think), I just thought it opened up a whole load of opportunities for pedantry from certain contributors, so I removed it.

[VGB]

Astounding.

Science is a wonderful thing

[Graeme H]

No error (I think), I just thought it opened up a whole load of opportunities for pedantry from certain contributors, so I removed it.

Fair enough. The well is already pretty deep.

[Graeme H]

I think I'm starting to get the picture now.

The guy flicks a switch. That sound startles a pixie and she runs away. She is trapped within the string but keeps running like a hamster on a wheel, dragging the string in the top leg through the rotors on the wooden device. That generates electricity and charges the three batteries in the box.

It's not a string shooter: it's a pixie-powered battery charger. The guy is a genius!

[Graeme H]

I'll repeat these questions Gordy. Please try an answer them:

Q1: When he shoots the string nearly vertically (at 2:45), why is the skin drag in the loop not pulling it all the way over the top? Is the loop just too narrow to do that?

Q2: When he aims the string straight up, how is the string maintaining a height of ~8'? Skin drag on the upward path must be about the same as on the downward path, so if it's there at all, skin drag must be pulling it down too, right?

Q3: Would you like to predict the outcome of him pointing his string shooter at the wall, destroying the loop? Would the string just fall to the ground straight out of the motors without a loop? Or would the upper leg maintain trajectory initially imparted by the string shooter?

Cheers,
Graeme