Pool pull
Question: The density of saltwater is higher than that of freshwater, so
swimming in saltwater should be marginally faster than in freshwater because
swimmers are carried slightly higher in the water and suffer less friction.
Given the split-second timing of world records, are the water densities of
the various competition pools measured and controlled in the same way that wind
speeds for athletic track events are kept within limits?
Answer: It is a misconception that the higher you are in the water, the
faster you can swim. On the contrary, the highest speeds are reached by swimming
completely submerged, because this allows for more efficient transfer of
momentum to the water (which creates forward thrust according to Newton鈥檚 third
law), and because less energy is wasted splashing water.
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Hans Starnberg
Department of Physics
Gothenburg University, Sweden
Answer: Turbulence at the surface of the water increases drag and slows
swimmers down. Trained swimmers know that swimming underwater is faster than
swimming at the surface.
In all the different types of competition stroke, from Olympic level down to
club level and at all distances, the number of underwater strokes is strictly
limited (especially at the turn) for this very reason. Competitors are
disqualified and records annulled if they break the rules.
Kevin Dixon-Jackson
Eccles, Lancashire
The rules on turning vary depending on the stroke. In freestyle, backstroke
and butterfly, competitors may travel no more than 15 metres underwater after a
turn. The rules for breaststroke are more complicated, but in effect swimmers
are allowed only one full kick and one full arm pull before surfacing.
The density of pool water in competitions is not regulated, but the
temperature of the pool must be kept at 26 掳C, although a variance of 1
degree above and below this temperature is permissible in international
肠辞尘辫别迟颈迟颈辞苍鈥抬诲
Unblemished reputation
Question: What makes stainless steel stainless?
Answer: Metals such as chromium react extremely quickly with oxygen in the
air. Because the sizes of atomic chromium and its oxide are similar, they sit
comfortably next to each other and the oxide forms a stable layer only a few
atoms thick over the whole surface.
If the oxide layer is broken or the metal is cut or scratched, more oxide
rapidly forms and covers the exposed surface. This prevents any further
corrosion, which is why chromium retains its shine.
If a high proportion of chromium is added to steel to make stainless steel,
much the same effect is produced. The oxide layer prevents only oxidising
reactions, so attack from other forms of corrosive is still possible, though
limited, which means that stainless steel is not truly stainless. However, for
the purposes of general household use it appears so, retaining its sheen long
after other metals have become dull.
The reason that rust occurs so readily on iron is that atomic iron is much
smaller than its oxide, so they cannot pack together neatly. Instead the oxide
sits loosely on the surface of the metal. This is why rust flakes off rather
than forming a stable protective layer.
Joe Geesin
Heathfield, East Sussex
Answer: Stainless steel is stainless because the extra elements such as
vanadium and chromium that are added to the iron and carbon that make up
ordinary steel react with oxygen to form an airtight coating. This protects the
steel and ensures that its surface remains distinctly shiny.
Ordinary steel rusts because the molecules of iron oxide do not pack neatly
together to form a coating. This can be seen when rusty reinforcing rods in
buildings swell and split the concrete.
But stainless steel can also corrode. It needs oxygen to reach its surface to
repair the coating if it becomes damaged. In water this does not happen, and a
yacht keel that is held in place by stainless steel bolts will drop off the hull
in just a few months as the corrosion can continue unabated when immersed in the
sea.
Ralph Houston
Nijmegen, The Netherlands
Answer: Stainless steel was accidentally discovered by Harry Brearley in
1913, while he was working for Thomas Fearth and Sons in Sheffield, Yorkshire.
While experimenting with different alloys for rifle barrels, he created some
alloys that were resistant to corrosion and hit upon the idea of using them to
make cutlery.
As with all great discoveries, his ideas were initially opposed. He had to
fight quite a battle to get people to realise the benefits of stainless steel
and for manufacturers to be prepared to produce it commercially. Eventually, of
course, thanks to his work, Sheffield became the steel capital of the world.
Harini Nagendra
San Diego, California
This week鈥檚 questions
By the board: When skateboarders jump over an obstacle, the board also goes
over the obstacle. I have watched carefully, and the boarders do not lift the
board by placing a foot under it. So how do they lift the board? It鈥檚 very
spectacular because sometimes they can leap over jumps that are knee height.
Ian Atkinson
Bailly, France
Effortless ascent: When you are in a lift that is travelling either down or
up at speed and you see a fly buzzing away in front of your nose, how come the
fly is not affected by the speed of the ascent or descent? How does it manage to
remain hovering in the same place?
Khaled Mazhar
G卯za, Egypt
Mind the gaps: When I stand on a railway platform behind a knot of people
trying to get through the door of a crowded commuter train, do I maximise my
chances of getting a seat by approaching from behind the centre of the group, or
along the sides of the carriage?
Are there any specific mathematical rules governing such crowd behaviour?
Darragh Owens
Dublin
Which way is up? My whole class, including my mathematics teacher, is
baffled. We cannot work out how an aircraft can fly upside down without crashing
into the ground.
We understand that the wings are designed to provide uplift when the plane is
flying horizontally. However, when the plane flies on its back as some smaller
jets often do, surely the uplift is working in reverse and forcing the plane
back down towards the ground.
Yet most small aircraft seem able to maintain the upside down position for
long periods of flight. How do they do this?
Nik Yusoff
London