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Old 08-08-2011, 03:12 PM   #12
Dave de Vos
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Dojo: Shoryukai, Breda (aikikai) & Aiki-Budocentrum Breda (yoseikan)
Location: Baarle-Nassau
Join Date: Nov 2010
Posts: 348
Re: Restart on Jin/kokyu and "Spiraling"

This is what I know about forces:

A force has
  • a direction,
  • a magnitude
  • a point of application (which associates a work line with the force)

External forces on a human body involved in some interaction with another human body include are gravity, friction of the ground, the surface normal force of the ground and one or more forces applied by the other human body.

These forces have different directions, magnitudes and points of application. Even when the net force vector on the body is zero, so that the center of gravity remains stationary, the forces would still put stress on the body, compressing or stretching it.

When work lines of opposing forces don't coincide (like the friction of the ground and a horizontal push from the other human), the forces would apply a torque to the body, which could result in torsion and/or rotation of the body, even when its center of gravity remains stationary.

In short:
A force is linear (it has a work line). When it's work line does not go through the center of gravity of the body that the force applies to, it also generates a torque around its center of gravity.

Then spiralling. What does that mean? In my understanding it describes some motion trajectory.

The shape of a trajectory can be very different from the direction of forces. For instance, the moon orbits the earth in a circle by gravity pulling it straight to earth and with mechanisms like joints you can get all kinds of motion trajectories, even though forces are always linear.

So I don't understand how linearity of forces and spiralling motion would contradict each other.

Last edited by Dave de Vos : 08-08-2011 at 03:19 PM.
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