The physics of floating screwdrivers [video] | Hacker News Reader

The physics of floating screwdrivers [video]

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137 pointslisper23 comments

Whats super interesting to me is how this system becomes dynamically stable. In the pure thick shaft version, it looks like the jet is centered on about 1/4 of the way down the shaft - so it should be imparting torque on the whole thing, but the fluid dynamics are enough to keep it at what appears to be a pretty constant angle.

Also, as a note about the golf ball comment (this seems to happen quite a lot), nothing he said was wrong, but there's slightly a bit more nuance going on. For a sphere (since we're talking golf balls) in a given flow, there will be a point along the ball will the flow will detach and form eddies and the mentioned low pressure zone behind the ball causing additional drag. By adding dimples, you introduce turbulence into the boundary flow that basically gives it enough momentum to keep up with the ball for longer, letting it stick the ball and therefore separate later - therefore lesser losses through that pressure drag.

The reason why we don't dimple everything flying through the air is that adding turbulence to the boundary flow also increases the amount of drag on the object (so called skin friction drag). The act of adding dimples may either increase or decrease total net drag depending on the exact parameters of the object and the flow.

It just happens that for golf balls flying through atmosphere, typical drive velocities are right in the range where adding dimples causes noticeable net reduction in drag.

It works for cars too:

https://www.youtube.com/watch?v=eR5SlwNf4K0

Diffusers do a similar (but different) thing while looking slightly cooler than dimples :)

https://en.wikipedia.org/wiki/Diffuser_%28automotive%29

Then why the fuselage of an airplane is smooth?

http://physics.stackexchange.com/questions/109395/why-arent-...

In the pure thick shaft version, it looks like the jet is centered on about 1/4 of the way down the shaft - so it should be imparting torque on the whole thing, but the fluid dynamics are enough to keep it at what appears to be a pretty constant angle.

The angle is caused by the dynamic pressure of the jet; if you had a circumferential jet (like a Dyson fan), you could get it to float vertically.

Images in this article are very nice for visualizing types of drag: https://en.wikipedia.org/wiki/Drag_(physics)

Very interesting, consider how heavy that screwdriver is.

What would be a ratio of a cost of electricty (to blow an air) to the weight of an object it can raise?

Could this be used as initial propeler to raise a rocket high enough to use this technique as a "first stage engine" in flying object out into space?

> Could this be used as initial propeler to raise a rocket high enough to use this technique as a "first stage engine" in flying object out into space?

I don't really see how. You'd need some kind of gigantic and fairly stable platform to shoot the jet from - if you have that, it would be easier to just launch the rocket from that.

And you're forgetting the fact that we seldom just launch stuff "into space" - we want it in orbit. Only about 20% of the delta-v budget of a typical launch to orbit is the "vertical" component, the large majority is spent on attaining angular ("sideways") velocity. So you'd be doing an awful lot of work just to cut down that 20% by, say, a quarter - it's simply not worth it.

A question and a comment:

1. How much of the lift is caused just by the air pushing up?

2. The turbulence caused by a square back is (I think) why rear windows should be closed in a pickup - prevents the exhaust from coming in.

1. All of it. A difference in air pressure is just an abstraction for talking about forces on air particles. The description of what's going on with the screwdriver would be equivalent physically and much more intuitive if air pressure wasn't mentioned at all.

What's going on is the curved back pulls air around it, which deflects it downward, and that downward force on the air implies an upward force on the screwdriver. With the sharp edge back, this detaches the air flow instead, meaning there's much less deflection, and thus less force.

The air-deflection model is a much better intuition pump than the air pressure model. I think the historical dominance of the air pressure model is because it's easier to measure - just stick some air pressure gauges on surfaces, and you can calculate what that means for lift.

I guess it differs between people, what is easier to grasp :) . Pressure of air makes a real physical force (actually, pressure is defined as that total force divided by area). At the same time deflection downward, which implies an upward force, is correct but doesn't show the mechanism how actually that implied force is born. It is born out of the difference of pressure between top of the screwdriver and the bottom - at the bottom you have more or less undisturbed ambient pressure, at the top the flow reduces the pressure.

What was interesting to me is Ben's experiment with long enough cone which failed to get lift. Shouldn't be so, I think. The problem could be that his air jet is relatively narrow - so with a long cone, which makes an angle with the stream not the whole cone gets flown over, and not the whole area is under the reduced pressure. It would be very interesting to see if a wider air jet would still fail to lift the long tapered cone.

The largest forces here are gravity and the air directly hitting at the screwdriver. Other forces (Coanda effect) stabilize the screwdriver. Different air hose angles would send the screwdriver flying or let it drop.

Technically, all of it?

Lift is cause by higher pressure air pushing into a lower pressure area.

To rephrase the question, how much of the lift is generated by high-pressure air below the screwdriver, and how much is generated by low-pressure air above?

Pressure is relative, and it is this pressure difference that causes the lifting force. So what is "high pressure" relative to the top of the screwdriver is "low pressure" relative to the bottom of it (and the atmosphere.)

Relative to 1 atm.

Then there is no high pressure at the bottom, only low pressure at the top.

The air coming out of the nozzle is high pressure, is it not?

It depends what type of pressure you mean; the stream of air has a high dynamic pressure, but its static pressure is the same as atmospheric.[1] In any case, the air is not directed at the bottom of the screwdriver but at its side. It follows the surface (Coandă effect[2]) and then creates a region of low (static) pressure near the tip when it separates. This is lower than the atmospheric pressure at the bottom; thus there is lift.

[1] http://www-stud.rbi.informatik.uni-frankfurt.de/~plass/MIS/m...

[2] https://en.wikipedia.org/wiki/Coand%C4%83_effect

The air at the nozzle mouth is at 100 psi, and there is a high pressure region around the nozzle. It is not correct to say that the static pressure is the same as atmospheric.

It is correct to say that the static pressure is atmospheric pressure. Inside the nozzle the pressure was 100psi when stationary, the static pressure was traded for dynamic pressure when it exited the confinement of the pressure vessel.

Okay so how much of the lift is generated by high-dynamic-pressure air at the side of the screwdriver? None?

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