When does a pendulum have the greatest mechanical energy

A swinging  pendulum has its greatest kinetic energy and least  potential energy in the vertical position in which its speed is greatest and height least.

It has its least kinetic energy and  greatest potential energy at the extremities of its swing, in which its speed is zero and its height greatest. As the pendulum moves, energy is continuously passing back and forth between the two forms.

Neglecting friction at the pivot and air resistance, the mechanical energy (sum of kinetic and potential energies) of the pendulum is constant.

the correct answer is: " ventricles " .

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