The boy plays "solitary seesaw" as shown. We can understand this by imagining that he has an invisible partner who is actually at the
1) fulcrum.
2) right end of the board.
3) rotational inertia of the seesaw as a whole.
4) seesaw's center of gravity.
Since the seesaw is not rotating, we can say that
5) the net torque on the seesaw is zero.
6) potential energy and kinetic energy are equal to each other.
7) the boy's and the seesaw's centers of gravity must be equal distances from the fulcrum.
8) the boy and the seesaw must have the same weight.
Answer: 4, 5
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