About the equipment:
The seesaw is built for 2 to 4 riders to share, with 2 seats on each side. Ride the seesaw up and down, and notice that no matter where you stop the seesaw, the blue liquid in the tube stops with its surface parallel to the floor. And the surface always stops at the same height. Notice too that although there is a constantly changing angle between the seesaw and the floor, your seats stay parallel to the floor.
Laws of physics:
Parallel motion — The equipment is built to keep you sitting parallel to the floor as you ride.
Communicating vessels — Communicating vessels are containers that are joined so that liquid can flow freely between them. According to physics, the liquid in communicating vessels moves to an even level. Notice that when you stop the seesaw, the liquid levels off against itself in the sections of the tube.
Elastic energy — The energy in the springs is elastic energy: energy that’s bottled up inside and can escape by becoming a different energy form. When the spring returns to relaxed shape, it releases the energy that squeezing or stretching put into it. The released energy helps push the seesaw.
The planet and the universe:
The earth’s gravity is responsible for the law governing communicating vessels. Gravity pulls the liquid downward everywhere until it can’t go down any further, and in that way the level of the liquid becomes equal. The same law works on the oceans that connect with one another. Their level tends toward being equal, while the Dead Sea, for example — which does not connect with them — is lower than they are. The moon’s gravity, pulling against the earth’s gravity, is responsible for the tides in the ocean.
Alternative energy:
As you ride the seesaw upward, kinetic energy (which is the energy of motion) turns into gravitational potential energy (which is the energy bottled up inside an object because of gravity — and the higher the object, the greater the energy). After the seesaw has hit its highest point, the energy turns back into kinetic energy as you come down. And as you come down, your partners going up on the other side of the seesaw are taking their turn at changing their kinetic energy into gravitational potential energy.