### What that shows:

The advertise momentum to organize is prove in a many dramatic method by breaking number of boards v the punch of your fist. You need not be a karate professional to show how the pressure of a fine executed hammer-fist to win will easily break a ridge of 5 to eight boards. The advertise is offered byimpulse = F∆t = ∆mvThe point of the show is: the greater the speed, the smaller sized ∆t will be and also thus the higher the force.

You are watching: Why is a force that is applied for a short time more effective in karate?

Images by Jeffrey Pike

### How it works:

A detailed summary of the physics that karate has showed up in a clinical American article. 1 We will certainly only current the salient functions here pertinent to the impulse momentum theorem. Very first we calculation the inert of the fist. Its efficient mass is better than just the hand—after all, you"re swinging the entirety arm to which her fist is attached. Relaxing the forearm top top a scale says an efficient mass somewhere between 2 and also 4 kg. Next we usage a photogate to measure the time it take away the to mark fist to pass through it, developing a velocity 2 of 10 m/sec and also a momentum of 30 kg m/s.Wood is a reasonably elastic material and a 30 cm lengthy board frequently requires around a centimeter that deflection prior to breaking; 500 Newtons will certainly deflect that to that extent. If us assume that a 2 cm thick board actually stops the swing of the fist, the activity takes ar over a street of 2 centimeter + 1 centimeter = 3 cm and the time the this communication will be the distance split by the average velocity that the fist = (vi + vf)/2 = 7.5 m/sec. Hence Δt = 0.03 m ÷ 7.5 m/sec = 4 millisec. In this time the momentum of the fist would change by 45 kg m/s. Thus

That"s indistinguishable to 2250 lbs! Of course the board breaks with this kind of force and does not prevent the fist.The boards are stacked on top of one another and also kept slightly different with pencils placed between them; rupture can proceed successively with the boards through each rupture including a smaller force than if a solitary thick plank were used. This likewise has the result of the momentum of the downward-moving broken pieces of the peak board help to break the board beneath it, and also so on under the stack. Hence the peak force to break, say, eight boards is less than eight time the force needed to rest one board.

### Setting the up:

The boards we usage are reduced from a so referred to as 1×12 (nominally 3/4" by 11.5") of dry white pine. They"re reduced 6" vast with the serial parallel come the width. (All the references point out that the karate blow need to be parallel to the grain. If that"s how the karate experts do it, it"s good enough because that us; ignore the wise male that constantly says, "oh, that"s straightforward if you have actually the grain running that way...ha, ha." nevertheless of the means the grain is oriented, a ridge of 5 to eight boards look at quite exceptional indeed.) different the 2 lecture benches about 10" apart and also stack the boards straddling the two benches. 2 sturdy Harvard chairs work-related well too. The lecturer need to do this, and slowly build up the stack (and the tension) come heighten the anticipation in the audience.