Re: [Phys-L] Water Slide Video & drag force

[John Denker <jsd@av8n.com>]

In the context of:
  https://www.youtube.com/watch?v=RBPGcE9nD1w 

On 8/13/19 1:16 PM, Folkerts, Timothy J wrote:

> it seems an odd perspective

That's a polite way of putting it.

I might have gone with "arrant idiocy".
We could send the presenters a T-shirt:
  https://www.av8n.com/physics/dunning-kruger-shirt.jpg

Almost everything they say in that video is ridiculous.

On 8/13/19 2:43 PM, Paul Nord wrote:

> > Right off the bat, the example of the Indy race car is wrong.

Agreed.

> > Those are designed with a huge drag force that aerodynamically 
> > pushes the car down to the ground.

Yes, true and important.

> > It's they only way they stay on the ground at those speeds.

Not exactly.  Zero lift (plus plain old gravity) suffices to keep
a car on the ground -- even a F1 car, if you're driving it in a
straight line.  The strong negative lift is to improve cornering.
More normal force allows greater transverse force, for any given
coefficient of friction, in accordance with the usual high-school
notion of friction.

> > A passenger car is way more aerodynamic.

That's misleading in context.  Usually when people say "aerodynamic"
they mean streamlined.  Streamlining reduces /parasite/ drag.  The
parasite drag of a Formula One car is not great, but it's not
terrible either.  More than half of the aforementioned large drag
comes from *induced* drag, as a byproduct of the downward lift.
Lift and drag are two very different things.  Completely different
directions.  But lift always comes at the cost of some drag, i.e.
induced drag, and that's an especially big deal when the wing is
short. That's why high-performance gliders have long skinny wings.

The formula for induced drag is different from the formula for
parasite drag; the former involves the area of the *wing* whereas
the latter involves the frontal area.

Also note that the frontal area is the total front-facing cross-
sectional area, which for the situation in question would be set
by the torso, not the feet.  It's not the area of the front-most
part.  You can't lower the frontal area of a car by sticking an
elephant-snout out the front.

On 8/13/19 2:35 PM, bernard cleyet wrote:

> Tibia/fib. partially out of the water presenting a more streamlined 
> figure. (feet completely part of the time)
Indeed!  How can they possibly think that pointing the feet makes a
difference when the feet are completely out of the water?

Continuing down that line of reasoning:  All the simple lift and drag
formulas assume the body is entirely in the medium. Our hero is not a
submarine, and he's not an aircraft.  He's not even a surface ship,
since a great deal of his weight is supported by the solid structure
of the slide.

> Some big rigs have plates mounted on the rear, so it presents a
> pyramid instead of the usual vertical flat plate.

It's a lot more complicated than that.  A low coefficient of drag
requires streamlining the front end, the back end, and everything
in between.

> trout

Yes, the trout shape is very good, especially at low Reynolds
number.  People like to buy cars that are pointy in the front
and rounded at the back, because they "look" streamlined and
"look" fast, but that's because most people are not as smart
as a trout.

================

The magic word that hasn't yet been mentioned is /hydroplaning/.
This is why a sailboard or a good catamaran can literally sail
circles around an ordinary displacement-hull boat.

Ordinary fluid dynamics is complicated.  It's 100x more complicated
than your average Dunning-Kruger alumnus imagines it to be.
Feynman said it was more complicated than quantum electrodynamics.
Hydroplaning is fun, but the physics of it is reeeeally complicated,
even worse than the basic lift and drag ideas that apply to aircraft,
cars, sports balls, et cetera.

I am not in the mood to discuss the other hundred things the video
gets wrong.
  https://m.xkcd.com/386/