Does Zero Force Really Hurt When Breaking Boards in Taekwondo?

  • Thread starter Deep Thought
  • Start date
  • Tags
    Force Zero
In summary: The skill is in both being able to hit the board as fast as possible and in pulling the blow immediately AFTER the impact so that any residual forward movement is stopped before your hand either hits the floor or you pull a muscle.
  • #1
Deep Thought
2
0
I think I understand this concept but wanted to get other opinions. My son and I are Taekwondo Black Belts and have to break boards occasionally. My son frequently has trouble breaking his and this got me thinking about the physics aspect of board breaking.
I know that the force applied to the board in a strike is F=ma where the body is the mass and the acceleration is the velocity change of the punch. (This example assumes the punches are parallel to the ground). If he tries to strike the board and reaches constant velocity before impact then the force should be zero since acceleration is zero. Additionally, the force could be negative if he pulls the punch so that it is slowing (decelerating) before impact. This just seemed counter-intuitive that the force is 0 since it certainly doesn't feel like a zero force impact.
After thinking about this, my conclusion is that when the force = 0, then the board exerts the same force as the punch resulting in a net of 0. If the force is negative then the energy of the punch (which is not the same as force) is transmitted back to hand by the board (which is why it hurts). Only by creating a positive force that exceeds the breaking strength of the board through punch acceleration can the board be broken.
Do I have this analysis correct?

Thanks!
Deep Thought
 
Physics news on Phys.org
  • #2
Deep Thought said:
If he tries to strike the board and reaches constant velocity before impact then the force should be zero since acceleration is zero. Additionally, the force could be negative if he pulls the punch so that it is slowing (decelerating) before impact. This just seemed counter-intuitive that the force is 0 since it certainly doesn't feel like a zero force impact.
The fact that the striking hand may have a constant velocity before contact is made does not imply that the force of impact is zero! The acceleration that matters is the one caused by the collision of hand with board, not the acceleration of the hand caused by your muscles prior to the strike making contact.

After thinking about this, my conclusion is that when the force = 0, then the board exerts the same force as the punch resulting in a net of 0.
You will always be exerting a "positive" force on the board. And, from Newton's 3rd law, the board will always hit back with the same force that you hit it--but since the forces act on different objects (your hand versus the board), they do not result in a net force of zero on either object.
If the force is negative then the energy of the punch (which is not the same as force) is transmitted back to hand by the board (which is why it hurts).
It's certainly true that if your strike is off (too slow, wrong hand formation, poor body mechanics, wrong focus) you will not transmit enough energy to break the board or, alternatively, not create enough force upon impact to break the board. When it doesn't break, some of that energy gets absorbed by you. Not fun.
Only by creating a positive force that exceeds the breaking strength of the board through punch acceleration can the board be broken.
Do I have this analysis correct?
Your analysis is off, but nonetheless you generally want the strike to be as fast as possible and to be timed so that contact is made when the striking hand still has some "juice" left in the strike. (How's that for physics terminology? I'm a TKD blackbelt also, so I've been there; however, I no longer practice that art.)
 
  • #3
The acceleration in "F=ma" is one thing that confuses people the first time.
The slowing down to a stop in an impact is also 'acceleration', so in a car crash if you go from 55mph to zero in 0.1s that provides a lot of 'F'.

Breaking boards require that you hit the board with as much speed as possible, the deceleration time will depend on only on the size and material of the board. So the faster you are going to start with the more 'acceleration' there is when you hit the wood.
The skill is in both being able to hit the board as fast as possible and in pulling the blow immediately AFTER the impact so that any residual forward movement is stopped before your hand either hits the floor or you pull a muscle.

(I'm only a karate white belt so can probably break matchsticks)
 
  • #4
mgb_phys said:
The skill is in both being able to hit the board as fast as possible and in pulling the blow immediately AFTER the impact so that any residual forward movement is stopped before your hand either hits the floor or you pull a muscle.

Well, I may not know the Physics as well, but I do know board breaking. :smile:

The most critical parts of the breaking art is the speed and size of the impact area. The goal is to concentrate the force in as small an impact area as possible. I've never really had an issue with pulling the punch back as I find that my hand rarely travels farther than the length of my arm and forward motion is usually stopped by the hapless fellow student holding my boards. :smile:

Thanks for the answers!
 
  • #5
The area and force thing deal with pressure, with deals with (correct me if I'm wrong here, I haven't studied Rhealogy very well, I'm only about to enter Freshmen year of college) Tensile points and pressure.

[tex]P = \frac{F}{A}[/tex]

And since the force is constant (when it hits the board), it's a question of how much area. When dividing by smaller numbers, pressure goes up. When you reach the right pressure, the Tensile point is reached and the board breaks.

The force, however, is not determined by:

[tex]\Sigma F = m a[/tex]

It is determined by how much force your muscles can give due to the electric interactions within them.
 
  • #6
GoldPheonix said:
The force, however, is not determined by:

[tex]\Sigma F = m a[/tex]

It is determined by how much force your muscles can give due to the electric interactions within them.
That's true in a static force but not in a kinetic Karate punch.
The reason you can easily break boards and rocks with a hand is that you hit them with high speed. The punch is slowed to an almost stop in a very short time giving high accelaration and so a high force.
 

1. What is "Zero Force Still Hurts"?

Zero Force Still Hurts is a phenomenon that occurs when an object experiences no external forces, yet still experiences pain or discomfort.

2. How does "Zero Force Still Hurts" happen?

There are a few different explanations for this phenomenon. One theory is that the object may be experiencing internal forces, such as tension or compression, that are causing the pain. Another explanation is that the object may have previously experienced a force and is still in a state of discomfort, even after the force is removed.

3. Can humans experience "Zero Force Still Hurts"?

Yes, humans can experience this phenomenon. For example, someone may experience pain in a previously injured body part, even if there is no external force currently acting on it.

4. Is "Zero Force Still Hurts" a common occurrence?

It is not a common occurrence, but it has been observed in various situations. For instance, in physics experiments involving objects suspended by strings, the strings may experience tension even when no external forces are acting on them.

5. How is "Zero Force Still Hurts" relevant to science?

Understanding this phenomenon can help scientists better understand the complexities of forces and how they interact with objects. It can also have practical applications, such as in the design and testing of materials and structures that need to withstand internal forces to prevent pain or discomfort.

Similar threads

Replies
7
Views
697
Replies
39
Views
3K
  • Classical Physics
Replies
9
Views
1K
Replies
15
Views
1K
Replies
15
Views
1K
  • Introductory Physics Homework Help
Replies
9
Views
1K
Replies
3
Views
381
  • Introductory Physics Homework Help
Replies
4
Views
2K
  • Other Physics Topics
Replies
22
Views
2K
Back
Top