## Saturday, January 6, 2018

### Physics of karate

Even though I have never done karate myself, in the past I have seen many karate videos of people breaking concrete bricks and wooden boards with their bare hands. Looking at a basic principle of physics helps to understand how people can pull off these impressive moves.
Since F=ma and Momentum = mass x velocity, force and mass are positively related. When trying to break a board, force needs to be transferred as fast as possible and the person also has to use as much of his or her body mass in the motion as possible.
In addition, the extension of a person’s arm is crucial to breaking the board. Since momentum and velocity are also positively related, the extension point at which the person’s hand hits the board must maximize velocity. Therefore, his or her arm must not be fully extended. Rather, it should be at a point of extension in which the hand has a positive or zero acceleration.
Finally, the person must attempt to exert as much force per square inch on the board as possible in order to break it. That is why hitting the board with the side of the hand is much more effective than hitting it with the palm.
By using the side of the hand, learning to efficiently channel body mass, and finding the optimal point of arm extension, one can plausibly break a board with lots of practice.
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## Friday, January 5, 2018

### Ice vs. Field Hockey

Having played field hockey for much of my life, I never really thought about why field hockey sticks are so stiff compared to ice hockey sticks and what implication this has on shooting in both sports. The fundamentals of physics suggest that the ball velocity of a field hockey shot is created slightly differently than the puck velocity of an ice hockey shot.
Since field hockey sticks are so hard and stiff, the stick is minimally flexed before it makes contact with the ball. When hit, a field hockey ball gains its velocity predominantly by the kinetic energy created as the player accelerates the stick. A large force is exerted on the ball and momentum is transferred from the stick to the ball.
However, in ice hockey, the puck velocity on a shot is created through kinetic energy and elastic potential energy. While the puck gains velocity through kinetic energy in a similar mechanism as a field hockey ball, a puck gains additional velocity through the release of potential energy that is stored when the hockey stick is flexed against the ice. The momentum created by the velocity of the moving hockey stick and the release of the stick flex is transferred from the stick to the puck and determines the velocity of an ice hockey shot.
Another reason why ice hockey is a much faster paced sport is because of the friction involved. Field hockey is commonly played on grass turf, which, compared to ice, is much rougher. Assuming hockey pucks and field hockey balls are made of the same type of plastic, the coefficient of friction for the between the turf and ball is much greater than that between the ice and puck. Thus, if one was the apply the same amount of force to both the puck and ball, the puck would travel much farther. This is why the field hockey turf is watered at the beginning of the game and a halftime, as water helps to reduce the friction between the ball and turf.