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        riddles >> general problem-solving / chatting / whatever >> Diving diagram
        
(Message started by: Noke Lieu on Jun 20^th, 2006, 8:56pm)

Title: Diving diagram
Post by Noke Lieu on Jun 20^th, 2006, 8:56pm

Sorry about the other posts.. just after a definate response. And ther is a slight emphasis on time.

If you were to overlay forces-diagram-style diagram on a somersaulting diver for a poster... what would you include? Remember it's good to be accurate, but it's also important to look impressive.

I have gone with
arrow from Centre of mass pointing up labelled

Drag= 1/2.cpAv²
( I guess that's more important if they're a sky diver somersaulting- couldn't find a guide as to when to use that rather than the more simple version. But it looks more impressive.) (maybe drag should be labelled F up- or not... :o)

arrow from centre of mass pointing down labelled F_down=mg

rotation depicted by arrow curved around the body labelled
F= mvr

although perhaps

T= r F sin(theta).
But struggling to think of a way of displaying theta, so maybe not so appropriate.

I welcome suggestions/comments/correction

Title: Re: Diving diagram
Post by towr on Jun 21^st, 2006, 1:04am

You might consider modeling the diver as a stickfigure rather than just a centre of mass. Because if (s)he pulls the limbs closer to the CoM, rotation will be faster. If you just have CoM you can't model that.
On the other hand, it might become needlessly complicated for a poster.

You might also consider, instead of overlaying the model and the diver, to put them side by side. If you use a stickfigure as model, in the same position as the diver it'd be instantly clear that they're the same, and any forces in the model apply to the diver. It'd provide a nice dichotomy between reality and model.

Aside from forces, you might also want to include a few values like mass, momentum, angular momentum etc. Maybe even 'constants' like gravity and air's drag coefficient.
Numbers often make things look impressive.

btw, if it's just a diver from a high board, I wouldn't use a quadratic drag formula, I think it's mostly linear at that speed.

Title: Re: Diving diagram
Post by Icarus on Jun 22^nd, 2006, 5:07pm

The only other thing I can think of (particularly for a sky diver) is to note that drag does not act on the C.M. Instead, it acts on a "Center of Drag", which may be offset from the center of mass, producing a torque. This is how a skydiver somersaults in the first place.

Title: Re: Diving diagram
Post by Noke Lieu on Jun 23^rd, 2006, 12:29am

Thanks guys.

There is a *slight* embellishment, in that it's not your standard poster... it's actually one element in the livery of some vans. As such, the amount of space available is limited. We've obtained photos of a diver in a piked somersault and by the time she's on there with equations, room is becoming an issue.

I really like the idea of adding in the numerical value of some of the constants... am trying to convince our graphics department of the value of it. (heh. pun.)

Yeah, the linear formula is probably better. I remember reading somewhere that one uses the quadratic when the object is travvelling fast, or non-spherical. And the somersaulting would (I think) increase the apparent sphericality.

It's just h at the quadratic looks better. Maybe introducing a few of the constants will balance it out.

Title: Re: Diving diagram
Post by towr on Jun 23^rd, 2006, 12:38am

You could use a more complicated drag formula. Because either the linear or quadratic version is just a simplification where you show the dominant term. At low velocities the quadratic term is dwarved by the linear one, and at higher velocities vice versa. And at even higher velocities third or fourth power terms start to take over, afaik.