misc.fitness.misc Google Group

Re: the physics of situps

2009-11-21T00:40:06Z

By altering one's shape, the c.o.g. appears to
shift, relative to the body, but in absolute space
(the swing's reference frame), the c.o.g. is fixed;
Newton's 3rd, remember? An isolated body
cannot translate its center.
I don't think so...
A pendulum can overcome its frictional losses,
through an external kick - e.g. a clock - but in

Re: the physics of situps

2009-11-20T02:09:59Z

by changing the body's position the centre of gravity is altered. try
this very slowly on the swing and you will see the position the swing
hangs is slightly altered. by changing the c.o.g the other way the
reverse is noted. NOW...and this is the clever part...if you do this
at ..and only at...or at least very very close to...the resonant

Re: the physics of situps

2009-11-19T19:51:41Z

When you throw your arms forward, your shoulders are still in contact
with the ground. Because your shoulders push down on the ground, the
ground pushes up on the shoulders, exerting a torque on your torso
with your hips as the pivot point.
Another way of saying the same thing is that swinging your arms

Re: the physics of situps

2009-11-19T06:46:47Z

Its all about the muscles moving in sync. the first movement will be
the arms. since you are not floating in space whatever force it
creates can be transfered to an object you can push away from-in this
case the floor.
if you will move only the arms, the shoulder will be the fulcrum and
you will achieve , well nothing. but since right after the arm flail,

Re: the physics of situps

2009-11-18T01:35:18Z

um, it doesn't PROVE anything... I said it
FEELS easier, when throwing the arms.
That's the point of the question.
You think all your subjective perceptions
correspond to objective reality?
Could be... did they revoke Newton's 3rd,
since my school days?
Sure you do - your arm strength. If you pull
up on your shoes really hard, that opposes

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Re: the physics of situps

2009-11-15T05:26:26Z

The gravitational force is the chi in this discussion.
By the way, I think I have learned enough BJJ basics so that I can
benefit
from watching BJJ training videos. I now have got some from Renzo
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Can you please give me some good names in BJJ so I will see if I can
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Re: the physics of situps

2009-11-15T04:12:23Z

Slides easier when lifted. Also it takes more force to
"break" static friction.

Re: the physics of situps

2009-11-15T03:00:31Z

What...no chi?
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Re: the physics of situps

2009-11-15T01:35:35Z

Mabe...
The arms connect at the shoulder joint,
they do not push off the ground. So I do
not find this explanation satisfactory.
You read my mind! I was thinking the same thing...
Swinging the legs back and forth certainly produces
energy, but how is it directed? At the bottom, feet
forward: forward momentum, feet back: bacward momentum.

Re: the physics of situps

2009-11-15T01:27:34Z

On Sat, 14 Nov 2009 14:48:07 -0800 (PST), RichD
No it doesn't. You admitted it's easier to sit up that way. That
proves theres a net force pulling your body towards your feet.
It causes momentum in your arms which is transferred to your
shoulders.
Absurd. You have nothing available to overcome the force of gravity.

Re: the physics of situps

2009-11-15T01:27:18Z

No. How would that explain it? The tailbone is the
only point in contact with the floor.

Re: the physics of situps

2009-11-14T22:48:07Z

No, that's the naive "arms pull you forward"
explanation, which fails; the boostrap fallacy.
Throwing the arms forward pushes backward
against the shoulders. It doesn't create net motion.
If you reach down and grab your shoelaces,
can you pull yourself to the ceiling?
I wonder if there's some other action occurring,