# Science: Archimedes strikes again; the ozone principle

### Technoquest

Questions and answers provided by Science Line's Dial-a-Scientist, on 0345 600444

Q Could you just explain Archimedes Principle again? I didn't understand it last time.

The important thing is not to confuse weight (or mass) and volume. When you immerse an object in any liquid, it must move its own volume in liquid out of the way. That displaced volume has a mass, which must be pushed up against the force of gravity. So there are opposing forces: gravity acting downwards on the object's mass, and pressure on the object from the displaced water volume. This second force acts upwards through the object's centre of gravity, and is called the buoyancy force. If the displaced water's weight is more than the weight of the completely immersed object, the object is pushed up until less of it is immersed - so less water is displaced. When the weight of the displaced water equals that of the whole object, it floats. However, if the displaced water weighs less than the object, the object sinks.

Q Why is there a greater loss of ozone over the South Pole compared to the North Pole?

The increased ozone loss over the South Pole is due to the atmospheric conditions over the region. The large land mass of Antarctica sets up a polar vortex, which leads to a trapping of the chlorofluorocarbon molecules (CFCs) in that region of the atmosphere. When the southern polar spring comes and the sun shines again, the solar radiation breaks down the CFCs, releasing radicals (highly reactive atoms and molecules) that destroy the ozone. Hence the greatest ozone loss occurs in only a few weeks around the begining of the polar spring. Over the North Pole, such a vortex cannot occur as the land surface is too small; so there is not such a build-up of CFCs. The northern regions still suffer an ozone loss, but it is less dramatic.

Q How is the skeleton held together?

The bones which make up the skeleton are held together by a support system consisting of skeletal muscles, ligaments, tendons, and cartilage. The skeletal muscles mean we can move whenever we feel like it. Ligaments join one bone to another, across a joint - without ligaments, joints would become dislocated very easily. Tendons join muscles to bones, across a joint. Some tendons are quite large and can be felt easily: the Achilles tendon at the back of the ankle and the two hamstring tendons at the back of the knee joint are good examples. Cartilage, or gristle, is a smooth, tough and flexible substance which is found in joints such as the knee. It covers the ends of bones at joints to allow smooth movement as the bones move against each other.

Q Is it possible to build a chess computer that knows every move so that it wins every time?

Computers have "solved" some simpler games so that they always either win or, at worst, draw. Chess is a much bigger problem. Clearly, at every step of a chess game, there are only a finite (but large) number of possible moves: 20 at the start, and so on. The number of possible sequences of moves to the end of a game is also finite - though of course even larger. So in theory one could "build a chess computer that knows all possible moves". However, the number of possible sequences of moves is astronomically large (400 just for the first two moves, after which it gets really hard), the ultimate speed of computers is limited (for example by the speed of light and therefore communication within the computer), so the shortest possible time needed just to compute all possible moves is many persons' lifetimes. And this does not even consider the time needed to evaluate those moves and to determine the "optimum". So the practical answer is no.

Q How does the technique of using planets to speed up satellites work? Doesn't this slow the planets down?

The planet's gravity is used as a "slingshot" to speed up the satellite on its journey. In the process, the planet does suffer a slight loss of its momentum. But the amount is negligible. For the Voyager flybys of Jupiter, for example, the planet's lost energy resulted in Jupiter's orbit being slowed down by about one foot per trillion years.

You can also visit the technoquest World Wide Web site at http://www.campus.bt.com/CampusWorld/pub/ ScienceNet

Questions for this column can be submitted by e-mail to sci.net@campus.bt.com

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