Silent flight
Question: Last week, on a very dark evening, I was walking down a country
lane and was startled by a low-flying owl. Although it passed within a few
metres of my head, I didn鈥檛 hear it at all, and became aware of it only when it
flew into vision. How can an owl fly so silently?
Answer: Owls have a number of features which aid silent flight. First, they
have a relatively small wing loading, that is a large wing area relative to
their body weight. This means that, compared to other birds, they put less
effort into each downstroke to keep them aloft. This ensures a quieter wing beat
and also gives them their distinctively buoyant flight.
Secondly, the owl鈥檚 flight feathers鈥攑articularly the outer
ones鈥攈ave a stiff comb-like fringe along their leading edge. This keeps
the flow of air over the wing regular and cuts down the amount of noise.
Similarly, the trailing edges of the flight feathers have a soft fringe, which
reduces the noise-causing air turbulence created at the back of the wing when
the airflows over and under the wing meet.
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The upper surfaces of owls鈥� wing feathers also have a downy covering made up
of elongated barbules, the interlocking components that hold the feather
together. This makes the wings very soft to the touch and probably cuts down
noise from feathers moving against one another during flight.
Interestingly, the fish-eating owls of South-East Asia and Africa do not have
these feather modifications, and have a much noisier flight. Presumably, in
their case, noisy flight is less likely to frighten their prey.
Silent flight not only makes it easier for owls to steal up on their prey
(and startle late-night walkers) but also ensures that they are not deafened by
their own flight.
Rob Robinson
British Trust for Ornithology
Norfolk
Big hitters
Question: In cricket and, I have heard, baseball, a batsman who wants to
strike a boundary or a home run will select a weighty bat. But it is harder for
a batsman to swing a heavy bat, so the bat鈥檚 speed when it strikes the ball will
be much lower than if a light bat was used. Can somebody explain this paradox to
me?
Answer: Clearly to hit a boundary or a home run, the ball must be given the
maximum possible speed at the time of impact.
However, this is a question about momentum rather than just speed. Momentum
is defined as the velocity of an object multiplied by its mass. In any impact,
such as that between a bat and a ball, momentum will be conserved. In the case
of the heavy bat, the speed is slower, but this is compensated for by the
greater mass.
But why is this better than using a faster, lighter bat? The reason is that
although a light bat has the potential to be swung very fast, there is a limit
to how fast a particular person can swing a bat. This means that while a heavier
bat will be swung slower, the difference is relatively small. The small velocity
difference is more than made up for by the mass difference, and so the heavier
bat will ultimately have a larger momentum on contact with the ball.
Because the ball has the same mass in both the light and heavy bat scenarios,
and receives a greater momentum from the heavier bat, it will travel at a
greater velocity, increasing the chances of it reaching the boundary.
Abbas Kaveh Barkhordar
London
Diet cheats
Question: Have you ever stood on your bathroom scales and leaned forward?
It鈥檚 great for dieters because you seem to lose a few pounds.
On the other hand, don鈥檛 swing your arms upwards because that way you will
seem to gain weight. As all your weight is still carried by the scales
irrespective of your position, why does this happen?
Answer: There are two reasons why a difference in weight might occur.
First, weighing machines with a square platform have only one weight
detector, usually some form of a spring. The weight from above must be
communicated to this spring equally, no matter where the load is on the plate.
This is achieved by a system of levers that support the plate at four
symmetrically located points. The levers also reduce the force applied to the
spring so that it does not need to be too large. If some of the levers are not
quite the right length, then a load placed at one side of the scales might
result in an incorrect force being applied to the spring.
Early weighing machines such as beam scales avoided these problems by using
plates suspended below a single support, so no matter where the load was placed,
the plate swung until the centre of gravity was vertically below the support.
Until quite recently high-precision balances still used the beam scale
principle.
The second possible reason for error is that the bathroom scales are sitting
on a soft carpet. As you move from end to end the scales tip. The scales measure
the force being applied perpendicularly to the plate, and this decreases as you
tip the scales away from the horizontal.
The answer to the second part of the question, regarding swinging your arms,
is that this is a dynamic effect. As you accelerate your arms upwards your body
will exert a greater force on the scale, proportional to the acceleration of the
arms and their mass, and equal and opposite (that is downwards) to the force
necessary to produce the acceleration. As soon as the arms stop accelerating,
the force disappears.
Eric McCrossan
Trading Standards Office
Lisburn, County Antrim
This week鈥檚 questions
Goggle-eyed: It has always intrigued me that I can see clearly underwater if
I wear goggles or a mask, yet if I don鈥檛 everything is blurred. What is it about
my eyes or the water that causes this effect?
Michael Slater
Bristol
Gurgle time: Does liquid pouring from an inverted bottle flow faster at the
beginning and end of its expulsion or when it reaches the 鈥済lug-glug鈥� point
somewhere in the middle? And, whatever the case, what is the explanation for the
different velocities?
Randy Baron
Basle, Switzerland
All together now: 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?
Cliff Clokie
Auckland, New Zealand