태권도 물리: Lesson 1 – energy and force

First, we start with $E=\frac{1}{2}mv^2+mg_y$

• Let m = mass of the fist and arm of a 45 kg (100 pounds) martial artist, approximately 10 kg (22 pounds).
• Let v = velocity of the fist in the horizontal, or x direction only. Assume 5 m/s, for discussion sake (to be refined in a later blog).

Second, plug and chug: $E_{arm}=\frac{1}{2}(10)(5)^2-(5)(9.8)=125J-49J=76J$

Next, let us find the actual force: $F_{arm}=(10)(9.8)=980N$

This is about 220 pounds of force, if the entire arm is treated as a solid body. This means that a well-trained martial artists can translate 2.2X of his/her mass into a punch! Later we will try to verify that 5 m/s is a good assumption on velocity.

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