Icy shake
Question: To supplement our meagre bush diet here in Tanzania, we
periodically fetch a Thermos flask of strawberry milkshake from Mwanza, the
nearest large town. On filling the flask in town, the milkshake is definitely
fluid. Yet, when we open the flask again several hours later after a hot ride in
the bus, the top layer of the shake is frozen. Any explanations?
Answer: The milkshake is composed of small ice particles that are suspended
in a sugar, fat and protein solution (the sweetened milk). This fluid has a
lower melting point than the ice because of the solutes it contains, so the ice
particles are free to creep gradually upwards through the slushy mixture,
probably helped by the vibration while travelling along bumpy roads. Once all
the crystals have got to the top they will amalgamate to form the icy plug that
your reader describes.
Guy King
Advertisement
London
All together now
Question: A double kayak travels faster when its occupants synchronise their
strokes, paddling together on alternate sides of the craft, rather than each of
them sticking to one side. Why is this?
Answer: Kayak paddlers aren鈥檛 able to stroke on opposite sides while
alternating because their paddles would clash and even if this weren鈥檛 the case,
it would cause balance and timing problems.
A kayak has two-bladed paddles. One stroke is made on one side of the boat
with one end of the paddle and the next stroke on the other side with the other
end. The three phases of a paddle stroke are catch (putting the blade in the
water), pull and recovery.
The scenario that your questioner describes, where each paddler sticks to the
same side as his or her partner, is the situation in a 鈥渟eated鈥� Canadian canoe.
These have single-bladed paddles and paddlers usually stick to the same side as
their partner. When in the 鈥渒neeling鈥� position they only paddle on the one
side.
The advantage of a kayak over a canoe in terms of stroke is mostly to do with
the amount of time the paddler spends in the recovery phase. When the paddle
exits on the left side of the kayak, the right blade is very close to being
ready for the next stroke because the body is in pretty much the correct
position. Essentially every stroke in a kayak gets you ready for the stroke on
the other side of the boat. More time is spent applying power and propelling the
boat and less is spent slowing down.
Canoe paddlers spend a higher percentage of their time in recovery, so the
boat spends more time slowing down. After a full stroke, the paddle is not
immediately ready for the next stage as in a kayak. Paddlers in racing Canadian
canoes increase the biomechanical advantage by kneeling, but even so the canoes
are still slower than racing kayaks in general.
Damien Elsworth
Essendon Canoe Club
Ascot Vale, Victoria
Answer: Completing one stroke on one side of the kayak positions the opposite
end of the paddle over the start position on the other side. Lowering that end
into the water consumes less time and energy than would swivelling the opposite
end back to the start of another cycle. This means that the paddle spends more
time in the water producing forward momentum.
This side-to-side action is similar to other physical activities such as
walking and running. It evenly distributes effort between the paddlers鈥� left and
right limbs and torso, and helps to delay fatigue.
The key to the entire operation seems to be symmetry, co-ordination and
economy of effort.
Andrew Stone
Heston, Middlesex
Rubber reversal
Question: On a recent trip to Malaysia I visited a commercial rubber
plantation and was surprised to see that the colour of the raw product was a
whitish yellow. Why, then, are car tyres almost universally black?
Answer: Tyres are black because they contain carbon black. Carbon black is
added to tyres as a reinforcing agent that keeps them from wearing out too
quickly. Added carbon will extend the lifetime of a rubber tyre from 8000
kilometres to 100,000 kilometres.
Carbon black has been produced for more than 2000 years and has been used as
a colorant for most of that time. A common modern method of production involves
the controlled burning of a hydrocarbon, such as acetylene. Carbon black is
produced in a variety of grades that differ in particle size, and in the size of
aggregates of the single particles. Carbon particles and aggregates are used to
coat the individual polymer strands of both synthetic and natural rubber.
It is believed that this helps the individual polymer strands to orient,
leading to a stronger product, although the exact nature of the carbon-rubber
and carbon-carbon interactions are still not completely understood and are a
topic of significant industrial research.
Which variety of carbon black is used in a particular tyre depends on the
desired result. The amount and size of the particles added can lead to rubber
that is very soft or that is very hard, which can affect the ride of the tyre
and its wear. Softer tyres grip better in wet weather but wear out sooner than
harder ones. Modern tyres are given a particular set of properties by adding
carbon black particles of six or more different sizes and aggregation
parameters.
Mark Dubin
Boulder, Colorado
Answer: Not all latex is white. The tree species Sebertia accuminata
occurs on ultramafic rock in New Caledonia and has bright green sap due to the
very high levels of nickel, up to 26 per cent, in the soil.
Tim Bromilow
Ayr
This week鈥檚 question
Energy gap: The oil refinery near my home sports a constantly burning flame.
Other refineries around the world do the same. Why is this energy apparently
wasted and why can鈥檛 it be used as a power source?
N. M. MacNaughtan
Edinburgh