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The reason why ships float although they often are made of a material that naturally sinks is mainly due to their shape. When a ball of steel, for example, is dropped in the water, it sinks, because all the force of gravity acting on it focused on one point in the water, so no amount of water pressure can counteract the ball's gravity. But when that same volume of steel is flattened out and shaped into a boat, the amount of gravity acting on it is spread out over a larger surface area, so the water pressure is able to counteract it.
This week, to show our understanding of buoyancy, density, and particles, we created boats using aluminum foil, modelling clay, and/or Play-doh, all of which are denser than water. The first challenge was to create a boat that floated, but then we were given the additional challenge of making it carry as much weight as possible without sinking. While building my boat, I decided that aluminum foil was the best material, higher sides made the boat stay afloat longer, and larger boats held more weight.
When testing and redesigning my boat I made a number of different changes, including making the shape more elongated, the bottom flatter (it distributed the weight better), the sides higher, and I added a couple extra layers to hold extra air and prevent leaks in the main body of the boat. The design that worked best was my final product. It was the largest yet so that the gravity acting on it was spread over a greater surface area, and higher sides let it sink lower in the water before filling with water. The boat design I tried that worked least well was one of my earliest designs, one that had a more traditional boat bottom, because it forced all the gravity acting on it to focus on less surface area. This made the design sink faster.
During class, we learned about volume, mass, density, and buoyancy through many different types of exploration. After all that we learned, I think it's important to remember these three central points:
- Materials that would usually sink in water can be made to float by shaping the material, so that the amount of gravity acting on the material is spread out over a greater surface area.
- Where something floats in the water is determined by how far down in the water there is enough water pressure to counteract the gravity acting on the object.
- The volume and mass of an amount of water displaced by an object is equal to the volume and mass of the object itself.
These three points can be used to determine how an object floats, why an object floats, and how to make an object float or sink.
