Human feet and legs store elastic energy when walking or running. They are not n

Question

Human feet and legs store elastic energy when walking or running. They are not nearly as efficient at doing so as kangaroo legs, but the effect is significant nonetheless. If not for the storage of elastic energy, a 70.0-kg man running at 4.00 m/s would lose about 100 J of mechanical energy each time he sets down a foot. Some of this energy is stored as elastic energy in the Achilles tendon and in the arch of the foot; the elastic energy is then converted back into the kinetic and gravitational potential energy of the leg, reducing the expenditure of metabolic energy. If the maximum tension in the Achilles tendon when the foot is set down is 4.38 kN and the tendon's spring constant is 321 kN/m, calculate how far the tendon stretches. Now calculate how much elastic energy is stored in it.

Explanation / Answer

0.5*70*4^2-100=0.5*321*x^2
tendon stretches is x=1.69
elastic energy is E=460

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