Library › 11. Physics in karate › Force
Nothing moves by itself. No opponent falls, no board breaks, no fist flies – without force. Force accelerates, brakes, redirects, deforms. In 1687 Isaac Newton cast this into three laws in the Philosophiae Naturalis Principia Mathematica . They hold in the laboratory – and in the Dōjō.
Newton’s three laws
| Law | Statement | Connection to karate |
|---|---|---|
| 1. Law of inertia | A body remains at rest or in uniform motion as long as no force acts on it. | A stable stance (Sanchin-Dachi, Shiko-Dachi) uses the inertia of one’s own mass; an opponent in motion is hard to stop abruptly but easy to redirect. |
| 2. Law of acceleration | Force is mass times acceleration: F = m · a | More force comes from more mass involved (body commitment, hips) or from stronger acceleration (fast, relaxed movement all the way to the target). |
| 3. Law of action and reaction | When one body exerts a force on a second, an equal and opposite force acts back (“actio = reactio”). | On impact, the same force acts back on one’s own fist, wrist and arm. Joints that are not stably aligned give way – force is lost, injuries loom. |
Force in training
- Mass: A punch from the shoulder alone? Weak. It moves only a few kilograms. When the movement is initiated from the legs and hips and transmitted through a stable trunk, a larger part of the body mass joins in (“effective mass”).
- Acceleration: Tense muscles brake the movement. That is why in Gōjū-Ryū the acceleration is as loose as possible during the movement, with tension only at the point of impact (Kime, see Body tension).
- Counterforce: Through the feet, the floor supplies the counterforce that drives the body forward. A firm stance is therefore a prerequisite for an effective technique.
Worked example
If an effective mass of 2 kg is accelerated at 300 m/s², the force is F = 2 kg · 300 m/s² = 600 N. That corresponds roughly to the weight of a 60 kg mass.
Arm alone or the whole body? The calculation is under Momentum and impulse.
All calculated values: simplified examples, not measurements. The body is not a rigid block – real values depend heavily on technique, target and measuring method.
Sources
- Isaac Newton: Philosophiae Naturalis Principia Mathematica. 3. Aufl., London 1726.
- Jerry B. Marion, Stephen T. Thornton: Classical Dynamics of Particles and Systems. Harcourt, 1995, ISBN 0-03-097302-3.
- Jearl Walker (Hrsg.), David Halliday, Robert Resnick: Halliday Physik. Wiley-VCH, Weinheim (verschiedene Auflagen).
← 11. Physics in karate · Table of contents · Force and pressure →