西雅圖‧高雄 大城市‧小角落 我的心世界

The first scenario we will look at is a boat sitting on a still lake with no net movement.

Peaceful Floating Boat

Free Body Diagram

This first diagram shows the forces acting on a boat when it is sitting on a perfectly smooth body of water. In this ideal situation, the only forces acting on the boat are gravity and the buoyant force of the water. These forces perfectly balance each other, so the net force in this situation is zero. This shows that the boat is at rest with no movement.

Energy Cubes

Before we look at the energy cubes, here are some Keys for how to read the energy cubes:

K: Kinetic Energy

C: Chemical Energy

P: Potential Energy

T: Thermal Energy

W: Wave Energy

Friendly warning: These keys also apply to the later scenarios! 

At the beginning...

Hours later...

: Wait... aren't they the same pictures?

: Nope, they're not the same, but yes... there is no change in energy, theoretically!

The energy representation is simple for this first scenario. When a boat is sitting on still water with no movement, it does not experience any transfer of energy. It stays in one place, which is shown by the same amount of potential energy that the boat starts and ends with.

Floating Boat with Waves

Free Body Diagram

The second force diagram shows what happens when a boat is sitting still but in a slightly more complicated situation when waves are creating some movement. In this case, the waves act with a force on the boat, pushing it. However, since the boat follows the wave’s motion, the boat moves in a circular pattern. The boat returns back to where it started, so there is no net movement. This is shown in the diagram because all of the forces cancel each other out, so there is no net force. This demonstrates that the boat is still at rest.

Energy Cubes

In the second scenario, the energy is transferred in a few ways. The boat starts with a certain amount of potential energy because of its position. The waves start with wave energy, so when the waves start to push on the boat, some wave energy is turned into kinetic as the boat moves in a circular pattern. Since the boat then changes height, or position, because of this circular motion, the potential energy in this situation becomes smaller and then larger.

The second scenario addressed brings up a few questions. Since energy is always conserved, never created nor destroyed, it is interesting that the wave energy just seems to appear. This is not shown in the diagrams, but it comes from the transfer of wind energy as wind blows over the surface of the water. Again, the energy from the wind had to come from somewhere: this energy is brought about by changes in atmospheric pressure. This can be traced even further, but proves the point we are trying to make. The energy always comes from somewhere. Even in this simple scenario, energy is conserved.

Other Scenarios - Propeller Boat (  please click me!)