When you are lying on your board in still water waiting for the next wave, there are two important forces at work: gravity and buoyancy.

This force pulls on every atom in you and in your board—yet it behaves as if it were acting just at your center of mass. For most people, their center of mass is conveniently located behind their navel in the middle of their body when they’re standing up straight. Your center of mass is your balance point.. Place a support under your center of mass and you can rest in balance. As the surfer paddles out, she is floating almost level with the surface of the water. At rest, the gravitational force and the buoyant force are equal and opposite. The net force on the surfer plus the board is zero.

Move back on a surfboard, behind its center of mass, and the nose of the board tilts up until the buoyant force aligns with gravity once again. As this surfer moves to the right, the tail of the board pushes hard on the water it’s moving through, bringing him to a stop. If the downward force of gravity and the upward force of buoyancy are in line, they add to zero and things are stable. The Name’s Calvin, Dang it!

First the surfer moves back. When the surfer moves back on the board, gravity and buoyancy move out of alignment and create a torque—a twisting force—on the surfboard. The board rotates until the forces are realigned. Then the board responds by rotating. The surfboard rotates until the buoyancy force through the center of mass of the displaced water is aligned with the gravity force on the surfer. As the board rotates, the center of buoyancy—the center of mass of the displaced water— moves toward the back of the board. When buoyancy and gravity are again in alignment, there’s no longer any torque.

You are looking only on the surface of things. If anyone is confident that he belongs to Christ, he should consider again that we belong to Christ just as much as he. 2 Corinthians 10:7