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Holding the line

Question: How do welded railway lines, with no gaps, cope with expansion and
contraction in different temperatures?

Answer: Older rails buckled in hot weather once they had expanded to fill the
gaps between them. Welded rails are laid down under tension, by being stretched,
so a rise in temperatures causes only a drop in the tension. With no need for
gaps, passengers have a smoother ride. But if the temperature continues to rise
after the tension has dropped to zero, the rail will buckle, just like the old
type.

Barry Lord

Rochdale

Greater Manchester

Answer: In the US, welded railroad tracks accommodate linear expansion by
vertical movement. Watching a train pass by on a hot summer day here on the
prairie, you can see the track being pushed down against the ballast by the
locomotive, and rise again after the caboose (or guard’s van) has passed.

When the wooden sleepers are well secured to the rails, a railroad track
functions as a beam. Because the stiffness of a beam is proportional to the
square of its dimension in the direction of the force, the horizontal stiffness
of the track is approximately 60 times the vertical stiffness, so practically
all the expansion in length is accommodated by vertical waviness. In winter, the
waves disappear and the track becomes flat.

Ross Firestone

Winnetka, Illinois

Answer: Welded lines expand over their entire length, making them
considerably longer. This is why most trains are late.

Tony Holkham

Chichester, West Sussex

Keep off the grass

Question: Domestic cats bury their excreta, usually in next door’s garden and
often in well-cultivated soil containing seeds and bulbs. Most other animals
seem to be quite content to leave their droppings where they land,
undisturbed.

What evolutionary advantage has this extra effort conferred on cats?

Answer: Cats are fiercely territorial but very socially aware creatures. They
will leave their excreta uncovered to mark the boundaries of their territory,
even going so far as to position it on raised mounds so that the smell, and its
accompanying message, can waft as far afield as possible. However, they will
discreetly cover any waste products when on someone else’s patch, particularly
when, in the case of the domestic cat, that individual stands on two feet and
weights 20 times as much as a cat.

This extraordinarily hygienic behaviour is so deeply ingrained that a cat
will scrape furiously at any surface, however inappropriate, to cover its scent.
I am frequently awoken in the small hours by the sound of my cats scraping the
enamel sides of the bath after they have urinated down the plughole.

L. Parkin

Sale, Cheshire

Gas gassing

Question: Why does speaking through helium raise the frequency of the sounds
emitted, even when the final transmission to the hearer is through air?

Answer: Sound travels faster in helium than in air because helium atoms
(atomic mass 4) are lighter than nitrogen and oxygen molecules (molecular mass
14 and 16 respectively). In the voice, as in all wind instruments, the sound is
produced as a standing wave in a column of gas, normally air. A sound wave’s
frequency multiplied by its wavelength is equal to the speed of sound. The
wavelength is fixed by the shape of the mouth, nose and throat, so if the speed
of sound increases the frequency must do the same. Once sound leaves the mouth
its frequency is fixed, so the sound
arrives to you at the same pitch as it left the speaker. Imagine a roller
coaster ride. The car speeds up and slows down as it goes around the track, but
all cars follow exactly the same pattern. If one sets out every 30 seconds, they
will reach the end at the same rate, whatever happens in between.

In stringed instruments, the pitch depends on the length, thickness and
tension of the string, so the instrument is unaffected by the composition of the
air. Releasing helium in the middle of an orchestra would therefore create
havoc. The wind and brass would rise in pitch, while the pitch of the strings
and percussion would remain more or less the same.

In the Song of the White Horse by David Belford, the lead soprano is
required to breathe in helium to reach the extremely high top note.

Eoin McAuley

Dublin, Ireland

Not midsummer

Question: In the northern hemisphere the Sun reaches its highest point in the
sky on (or very close to) 21 June every year. Yet the warmest months tend to be
July or August. Why is that? Similarly, on 21 December the Sun reaches its
lowest point but the coldest months are usually Januaryand February. Can
somebody explain this?

Answer: The Earth has a certain heat capacity, which leads to a thermal time
lag. Therefore, when a hemisphere is experiencing its the longest day it is
still warming up, and will not reach its warmest until a few weeks later.
Similarly, on the shortest day it is still cooling down and will not reach its
coolest until a few weeks later.

Aidan Westwood and

Stephen Collins

University of Leeds

Answer: Although the northern hemisphere receives most sunlight at the end of
June, it is rather like heating up a room with a gas fire. Although the fire
gets hot very quickly, it takes a while for the air in the room to heat up. The
same applies to the atmosphere of the Earth. Likewise, the room does not get
cold the instant the fire is turned off— the air gradually cools in the
same way the air in the Earth’s atmosphere does.

Ian Hedley

Poole, Dorset

This week’s questions

Rotten hoarders: I have two red squirrels in my garden, which I feed. Any
nuts they leave in their feed box eventually go mouldy. Yet I recently saw one
of the squirrels burying its nuts in the lawn. Why don’t the buried nuts go
mouldy or rot?

J. Catlin

Seascale, Cumbria

Stumped: My four-year-old child was wondering if two parachutists would be
able to play baseball, cricket or even a simple game of catch while falling.
What would happen to the ball?

Anni Spring

St Paul, Minnesota

Berry worry: Why does one blackberry often ripen well before an adjacent
blackberry on the same bush?

Alan Slomson

University of Leeds