THERMO Spoken Here! ~ J. Pohl © ~ 2017 (C5375-155)

Skate Board Trick

A standard feat of skate board competitions is "get air" which means simply to "jump" in which the skater starts by tipping into the ramp (1), gaining momentum with the fall then "getting air" or height on the opposite side (2). Skate boarders know "Air is not for free."

What must a skate boarder "pay" to attain "one meter of air"?
♦  The system is the person (60 kg) and the board (2 kg). The event starts with zero velocity and ends with zero velocity. We write the energy equation; it will help us think about the event. Wind drag and rolling friction, the only boundary forces; we assume are negligibly small. Heats are inconsequential.

(1) 1

The initial and final kinetic energies are zero. Let's take the datum for potential energy at the initial level of the center of mass. Rewrite the equation.

(2) 2

An answer is immediate but for the term, ΔU of the board. For the event, the temperature of the board increases but very little. Good boards act like springs. Skater-applied force will distort the board, elastically. Knowing the board will spring back to its original shape, the agile skater put energy into the board while falling then retrieve it to advantage in feat. Skate boarders employ their skate boards in a similar manner, as pole-vaulter's do their poles and as bungee jumpers use bungee cords. Among equal competitors, he with the better skate board (pole or bungee cord) wins the competition.

In the equation, assuming the board is no help whatsoever, the internal energy change of the skate boarder is the cost of the jump. This energy is at least:

(3) 3

This discussion raises the question of "performance enhancing equipment" such as the golf clubs prolifically advertised on TV in the wee early AM. Do they work? - of course! But don't buy the equipment until they explain the system and its internal energy changes.

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