Board Breaking: Why Strike Center for Good Break?

[makamo66]

I have read that a practitioner of a martial art striking a target of wood or concrete needs to strike as much as possible to the center of a target in order to get good break. What is the physical reason or formula behind this? I'm thinking maybe you don't want to create a torque.

 

[jrmichler]

Short answer: Because the bending moment is highest when hitting the center.

Longer answer: Search simply supported beam moment. The bending stress, which is the stress that breaks the wood/brick, is proportional to the bending moment.

 

[makamo66]

Thanks

 

[A.T.]

I have read that a practitioner of a martial art striking a target of wood or concrete needs to strike as much as possible to the center of a target in order to get good break.

This probably assumes the board is fixed/supported at the ends. Striking where the board is supported would just compress the board, but it is much easier to break it by bending (see post by @jrmichler)

I'm thinking maybe you don't want to create a torque.

For a fixed object, you want to maximize the torques around the support locations. For a free floating object you want avoid the object rotating away from the impact.

 

[Herman Trivilino]

I'm thinking maybe you don't want to create a torque.

But you do. You want to create the maximum torque, so striking it in the center gives you the largest lever arm and thus the larger torque.

 

[makamo66]

I don't think that's correct. If a force acts on a line through the axis of rotation, it exerts no torque.

 

[phinds]

I don't think that's correct. If a force acts on a line through the axis of rotation, it exerts no torque.

The other answers are correct and you are misunderstanding what the strike looks like, or what the forces look like, one or the other.

Support a board on each end and then strike downward in the middle. That creates max torque because it is removed from the point of rotation by the max amount possible in that configuration.

 

[makamo66]

Torque is the ability to rotate. Striking at the center causes no rotation, so no torque. The moment of inertia is zero because the perpendicular distance of the body from the rotation axis is zero.

 

[phinds]

Torque is the ability to rotate. Striking at the center causes no rotation, so no torque. The moment of inertia is zero because the perpendicular distance of the body from the rotation axis is zero.

Well, I guess you should recommend to martial arts experts that they start hitting the board at the point of its support instead of in the middle. Of course, you might not want to hang around after, because once they break their hand, they might decide to break you next.

 

[Hornbein]

Torque is the ability to rotate. Striking at the center causes no rotation, so no torque. The moment of inertia is zero because the perpendicular distance of the body from the rotation axis is zero.

In the case of the supported board the center is rotating around both points of support. The signs of the two torques are opposite but there's no rule against that.

The same thing happens with the unsupported board but the two ends of the board are the centers of rotation due to the inertia and bendability of the board.

 

[hutchphd]

Torque is the ability to rotate.

No. Torque is moment of force (force acting through a perpendicular distance).

The moment of inertia is zero because the perpendicular distance of the body from the rotation axis is zero.

Perhaps it is easier if you think that it is the torque from each support (caused admittedly by your pushing the board ) that causes the bending of the board about the hand contact point.

 

[Ibix]

Maybe I got confused because the bases of support are at the edges and not at the center.

Imagine sawing the board in half then joining the halves with a hinge, then punching it at the hinge. Each half of the board clearly rotates in that case, and you can trace the torque on each half to the strike of the fist and the reaction force from one support. The same remains true if you use a slightly stiffer hinge, and the original board is identical to the case where the stiffness of the hinge is the same as the board itself. There's still a torque on each half of the board, although it will bend rather than rotating rigidly.

Martial artists may consider sawing the board in half to be cheating.