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Physics of board breaking

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Many martial artists aspire to be able to break through ice, brick and/or wood blocks simply by hitting such object with their hand. But how hard or easy is this? In reality it is far easier than one may originally think. Consider a one inch thick piece of pine wood, because of Newton’s Third Law, for every action there is an equal and opposite reaction, if we consider the hand/forearm and board as the whole system, there was on the whole system no net force that affected the impact. Therefore, there is conservation of momentum, which implies mV = (m+M)U. When m= the mass of the hand/forearm, V= velocity of the hand/forearm right before impact, M=the mass of the board and U=the velocity of the board fragments and the hand/forearm after the impact. It is known that it would take approximately 5 joules of energy to break the board, and with the equation E = ½(mMV2)/(m+M) (When E=energy needed to break the board) we can substitute the mass of the hand/forearm (about 1.3 kilogram) and the mass of the board (about 0.5 kilogram) into the equation to determine that V, the velocity needed at impact to break the board is about 5 meters/sec, or about 11 miles/hour.

1 Comment

Excellent analysis that ties in directly to what we're learning in class.  Well stated!

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