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

[Paul Nord <Paul.Nord@valpo.edu>]

Right off the bat, the example of the Indy race car is wrong.  Those are
designed with a huge drag force that aerodynamically pushes the car down to
the ground.  A passenger car is way more aerodynamic.  It's they only way
they stay on the ground at those speeds.  It shouldn't be difficult to find
some crash footage of an Indy car running over a small scrap of something
on the track.  Once they launch into the air it's all over.  (A Formula 1
race car has a drag coefficient of between .7 and 1.  A Tesla 3 has a drag
coefficient of .23.)
https://en.wikipedia.org/wiki/Automobile_drag_coefficient

In the case of his run down the water slide, I think that pointing his feet
clearly lifts him up over the water.  Since less water has to move out of
his way, there is less drag.

The conclusion about "you go 20% farther with pointed feet" strains
credulity.  It appears that an arbitrary starting point was selected for
this particular slide.  And it's also going to depend on a bunch of
specific things about the design of this water slide.

The big trouble with this theory is that you're not really changing the
cross sectional area of your body by pointing your feet.  Yes, a cone and a
sphere have a different drag coefficient.  But that's way more subtile than
cross sectional area.  Is that too complex for a high school level physics
lesson?

Paul


On Tue, Aug 13, 2019 at 3:17 PM Folkerts, Timothy J <FolkertsT@bartonccc.edu>
wrote:

> There is a fun video from "Flipping Physics" about how foot position (flat
> feet vs toes pointed forward) affects drag force - and hence the distance
> you will go at the bottom, horizontal section of a waterslide.
>
> It is a high school level explanation, but I disagree with model he chose
> and wanted to get some more opinions.  In the analysis of the drag force,
> he focuses on the changing cross-sectional area (of the feet themselves),
> rather than the change in drag coefficient (for the body as a whole).  To
> my way of thinking, the cross-sectional area (of the body under the water
> level) would be the constant.  The changing foot position would be like
> changing the nose of a car or changing the bow of a ship.
>
> His model seems to more or less be treating a foot as having a mass of ~
> 50 kg and estimating how those disembodied feet behave.  I can see that
> working reasonably well, but it seems an odd perspective for explaining the
> effect (especially since in the video, the pointed feet are pretty much
> completely out of the water and all of the drag from the water would be due
> to the legs and torso).
>
> https://www.youtube.com/watch?v=RBPGcE9nD1w
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