Mike’s and my challenge is to determine how much weight Kate needs to wear in order to move around at will in the ocean. If she gets in just as she is or, even worse, with a dive suit, she will float to the surface of the water and will have a tough time staying down for a dive. If we put too much weight on her, she’ll sink to the bottom, which isn’t so good for looking around. We need to determine the amount of weight to give her such that she can move around comfortably without floating to the top or sinking to the bottom.

When Mike and I were first given this challenge, we talked about the procedure. We knew what to do, based on readings and classes we’ve had in the past. But why does this work? It sounds a bit like science lab, doesn’t it—you know what to do but not why. What do you get when you submerge someone in water and collect the water that runs out? Mass? Weight? Volume? A heck of a lot of water? In these situations, my strategy is to place the challenge in my subconscious, roll it over and over, and wait for something to pop out. In the meantime, I have a cup of tea and eat chocolate biscuits.

Now, it seems obvious. Stick with me and see what you think. When we dunked Kate, good sport that she is, in a bathtub full to the brim, the water had to “make room” for her. In this case, the water “got out of the way” by spilling out of the tub through a hole, where we collected it. This is the amount of water that “got out of the way” for Kate. Or, said differently, the amount of space Kate took up, which is her volume.

Now, if Kate were made of sea water, we wouldn’t have to do this at all. There would be no point in the challenge. Sea water can move around in sea water. But Kate, and especially Kate in her dive suit, is less dense than sea water. Said another way, if we put Kate in her dive suit on a balance and put sea water on the other end of the balance—equivalent to the amount of space Kate’s body and dive suit take up, the sea water will weigh more. It has more mass. By putting just enough lead weights on Kate to make the balance balance, Kate will be the same density of water, be neutrally buoyant, and be able to swim around without issue.

This one is conceptually so hard. Perhaps one just needs to try it! Fill a bucket to the top with water. Put the bucket in a larger pan. Push a toy into the water and submerge it. Collect the water that runs over. Build a balance and see if the amount of water that has run over is heavier or lighter than the toy you put in the water. Unless it is a metal truck or something, the amount of water will probably be heavier, because water is pretty dense. To make the toy not sink or float, you’d need to add enough additional weight to the toy to make the balance balance.

Thus, Mike and my antics with a lot of wood, rope, buckets, and Kate.

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Content last updated: 26/01/2005