Final answer:
John's mechanical energy when standing still on the diving board with 200 Joules of potential energy is also 200 Joules since he has no kinetic energy at that moment.
Step-by-step explanation:
The question is asking for John's mechanical energy when he is standing on a diving board with a given amount of potential energy. In physical terms, mechanical energy is the sum of potential energy and kinetic energy (KE + PE). Since John is standing still on the diving board, his kinetic energy is zero, and thus, his mechanical energy is entirely due to his potential energy. Given that John has 200 Joules of potential energy, his mechanical energy is also 200 Joules, assuming no kinetic energy is present, and that no non-conservative forces like air resistance or friction are doing work on the system.