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Cat napping

Question: Why, when I fall asleep for just a few minutes, do I feel worse
when I awake than I did before I dozed off?

Answer: Sleep has many different stages that run throughout a 90-minute
cycle. The reason you feel worse after only a brief nap is because when you are
asleep for only a short period you don’t have time to complete the full cycle,
which moves from very light sleep to deep sleep and then into dreaming—or
what is known as rapid eye movement—sleep.

It is necessary to have this full sleep cycle in order to refresh you, and if
your nap is too short for this your quality of sleep has probably been poor. So
you may feel even more tired because you missed out on the cyclical sleep that
you needed in order for you to feel refreshed.

Tom Mackay

Sleep Laboratory

Edinburgh Royal Infirmary

Which way is up?

Question: My whole class, including my mathematics teacher, is baffled. We
cannot work out how an aircraft can manage to 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 types of small aircraft seem able to maintain the upside down
position for long periods of flight. How do they do this?

Answer: Although the aerofoil shape of an aircraft’s wing produces some of
the lift in normal flight, the more important factor is the angle of
attack— the angle at which the air strikes the wing.

The wings of an aircraft are normally inclined to about 4° to the
horizontal when compared to the main body of the aircraft. This is known as the
chord angle of the wing.

So even when the fuselage is level, the angle of attack into the oncoming
wind is 4°. This produces lift in the same way that your hand experiences an
upward force when you hold it at about 45° to the horizontal in a
fast-moving stream of air. Your hand does not have an aerofoil shape but the
lift that you feel is caused by the angle of attack of your palm to the oncoming
wind.

It is this principle that allows an aircraft to fly upside down. The nose is
pointed further upwards than in standard flight because of the need to offset
the chord angle of the wing. But if the angle of attack is positive compared to
the relative airflow over the wing, then an upward force will still be produced.
It is this lifting force which overcomes the force produced by the shape of the
wing, and holds the aircraft in the air.

The bigger problem that pilots should be concerned about when flying their
aircraft upside down is the risk of the engine stopping, because both the oil
and fuel systems in most ordinary light aircraft are fed only by gravity. Flying
your aircraft upside down can easily cut off the fuel supply because the valve
that is feeding fuel to the engine suddenly finds itself at the top of the
tank.

Mark Mobley

Bristol

Clouding the issue

Question: Why do clouds darken to a very deep grey just before it is about to
rain or prior to a heavy thunderstorm?

Answer: Clouds darken from a pleasant fluffy white just before rain begins to
fall because they absorb more light.

Clouds normally appear white when the light which strikes them is scattered
by the small ice or water particles from which they are composed. However, as
the size of these ice and water particles increases—as it does just before
clouds begin to deposit rain—this scattering of light is increasingly
replaced by absorption.

As a result, much less light reaches the observer on the ground below and the
clouds look darker.

Keith Appleyard

Dundee

This week’s questions

Crawling along: How does the up-and-down kick of the legs that is used in the
front crawl (freestyle) stroke propel the swimmer?

When swimmers practise this technique using a float to support their face and
arms, their progress along the pool is very slow. Why not use the more powerful
breaststroke kick instead?

Adolph Smith

Santa Cruz, California

Shoal souls: How do large shoals of fish and flocks of birds change direction
very quickly without colliding with each other?

We humans have the greatest difficulty achieving even two-dimensional
travel without walking into each other, yet mass formations of fish and birds
seem able to manage it in three dimensions.

I have seen that very young fish are capable of this feat, so it would not
appear to be a trait that is learned. How do these animals communicate the order
to change direction and also the new direction that they are going to take?

Ray Wijewardena

Colombo, Sri Lanka

Rope trick: Under what circumstances are ropes and braids actually stronger
than the same number of individual fibres and why?

Patrick Andrews

Cambridge

Spearhead assault: What is the physics involved in achieving the maximum
range when throwing the javelin?

How is it affected by the angle at which it is thrown, and what forces act on
the javelin during its flight.

Alisa

Adelaide, South Australia

Canned eat: Fresh grapefruit are almost impossible to peel by hand without
splitting or tearing the fruit. So how are the perfect skinless segments found
in cans produced?

F. G. Grisley

Barry, Glamorgan

Deflation policy: Why do helium balloons deflate so quickly? When my children
bring balloons home from parties, the ones that are filled with helium are often
small and wizened by the following morning.

I realise that some of the size reduction is caused by deflation but
something else must be at work because standard air-filled balloons stay
inflated for much longer.

John Storr

Great Corby, Cumbria