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the physics of situps



 
 
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  #1  
Old November 14th 09, 02:25 AM posted to sci.physics,misc.fitness.misc,rec.martial-arts,alt.martial-arts.karate
hal
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Posts: 7
Default the physics of situps

On Fri, 13 Nov 2009 17:18:15 -0800 (PST), RichD
wrote:

Long ago, in my wasted youth, I participated in
a weekend of sadomasochism, conducted
annually by my college karate club, called
'special training'. One part of this consisted
of a ton of situps.

We paired off, with locked legs, done by count.
It was impossible to do them all with good form,
so I came up with a trick: throw my arms forward
as hard as I could, on each one, to raise myself.

But Newton's physics tells us an isolated body
cannot generate net momentum; action = reaction.
But I swear I couldn't have completed that segment
without the arm swing. So the question is, did this
trick really work as a cheat, physically? Or was
it just an illusion?

Nah, it makes perfectly good sense. You are adding another set of
muscle groups to the movement: by using your forearm muscles to pull
you arms forward you accelerate the mass of your arms forward which in
turn is transferred to your upper body to assist in the lift. Makes
sense when you think of the muscle groups you are using for the
movement.

--
Rich


  #2  
Old November 15th 09, 02:27 AM posted to sci.physics,misc.fitness.misc,rec.martial-arts,alt.martial-arts.karate
hal
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Posts: 7
Default the physics of situps

On Sat, 14 Nov 2009 14:48:07 -0800 (PST), RichD
wrote:

On Nov 13, hal wrote:
One part of this consisted
of a ton of situps.


We paired off, with locked legs, done by count.
It was impossible to do them all with good form,
so I came up with a trick: =A0throw my arms forward
as hard as I could, on each one, to raise myself.


But Newton's physics tells us an isolated body
cannot generate net momentum; =A0action =3D reaction.
But I swear I couldn't have completed that segment
without the arm swing. =A0So the question is, did this
trick really work as a cheat, physically? =A0Or was
it just an illusion?


Nah, it makes perfectly good sense. =A0You are adding another
set of muscle groups to the movement: =A0by using your
forearm muscles to pull you arms forward you accelerate
the mass of your arms forward which in
turn is transferred to your upper body to assist in the lift. =A0


No, that's the naive "arms pull you forward"
explanation, which fails; the boostrap fallacy.
Throwing the arms forward pushes backward
against the shoulders.


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 doesn't create net motion.


It causes momentum in your arms which is transferred to your
shoulders.


If you reach down and grab your shoelaces,
can you pull yourself to the ceiling?


Absurd. You have nothing available to overcome the force of gravity.


I wonder if there's some other action occurring,
which does assist in the situp...


No, the action is kinetic energy build up in your arms by swinging
them. Like a flail, when the swinging end reaches it's limit of
travel the kinetic energy is transfer to the fulcrum and to the base
piece of the weapon. The energy in your arms is transferred to your
upper body through your shoulders and pulls your upper body up and
towards your feet.


--
Rich


  #3  
Old November 15th 09, 04:00 AM posted to sci.physics,misc.fitness.misc,rec.martial-arts,alt.martial-arts.karate
Greendistantstar
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Posts: 11
Default the physics of situps

Wannabe wrote:
hal wrote in message ...
On Fri, 13 Nov 2009 17:18:15 -0800 (PST), RichD
wrote:

Long ago, in my wasted youth, I participated in
a weekend of sadomasochism, conducted
annually by my college karate club, called
'special training'. One part of this consisted
of a ton of situps.

We paired off, with locked legs, done by count.
It was impossible to do them all with good form,
so I came up with a trick: throw my arms forward
as hard as I could, on each one, to raise myself.

But Newton's physics tells us an isolated body
cannot generate net momentum; action = reaction.
But I swear I couldn't have completed that segment
without the arm swing. So the question is, did this
trick really work as a cheat, physically? Or was
it just an illusion?

Nah, it makes perfectly good sense. You are adding another set of
muscle groups to the movement: by using your forearm muscles to pull
you arms forward you accelerate the mass of your arms forward which in
turn is transferred to your upper body to assist in the lift. Makes
sense when you think of the muscle groups you are using for the
movement.



Your belly muscles are pulling your torso up with your ass as the
fulcrum. By throwing your arms forward, you reduce the "moment = the
sum of (mass * distance perpendicular to the gravitational force)"
contributed by the mass of your arms, where the perpendicular distance
is just the horizontal distance when you are lying flat on the floor.
Likewise, you will make it harder for the belly muscles to pull your
torso up if you extend your arms horizontally above your head because
you increase the "moment" contribulted by the mass of your arms that
your belly muscles have to work agains.

http://en.wikipedia.org/wiki/Moment_(physics)


Wannabe
=======


What...no chi?

GDS

"Let's roll!"
 




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