One Last Problem, also related to energy

  • Thread starter Antepolleo
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Ok, this is the last time I'll bug you for a while, I promise. Actually I have two questions here.

I cannot for the life of me figure out why these answers are wrong. Here's the first question:

A child's pogo stick stores energy in a spring with a force constant of 2.40 104 N/m. At position A (xA = -0.140 m), the spring compression is a maximum and the child is momentarily at rest. At position B (xB = 0), the spring is relaxed and the child is moving upward. At position C , the child is again momentarily at rest at the top of the jump.

This problem has 5 parts, and I've managed to successfully obtain the answer to four of those parts. Here's the trouble:

(d) Determine the value of x for which the kinetic energy of the system is a maximum.

Now some information is imparted in an eariler question here:

(a) Calculate the total energy of the child-stick-Earth system if both gravitational and elastic potential energies are zero for x = 0.

I got the answer to that just fine. I noted that all associated potential energies are 0 for x = 0. Since this problem never mentions nonconservative forces, wouldn't it be safe to assume, due to the concept of conservation of energy, that the kinetic energy of the system is at a maximum when x = 0? I tried that and it did not work.
 

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