New Scientist - USA (2021-02-20)

20 February 2021 | New Scientist | 55

Tom Gauld

for New Scientist

Answers

into a glass, you may notice
that smaller bubbles rise in
straight lines, whereas larger
bubbles tend to follow a
zigzagging or spiralling path.
Leonardo da Vinci was among
the first to document this puzzling
phenomenon. Now known as
Leonardo’s paradox, it remained
unsolved for centuries until it was
tackled by the tools of modern
fluid mechanics in the past few
decades, and it is still an area of
active research.
The small bubbles are spherical
and generate small amounts of
spinning in the liquid that
surrounds them. This “vorticity”
can be shed behind them in a
symmetrical wake structure.
The large bubbles, however,
become spheroidal and generate
a greater amount of vorticity that
cannot efficiently be shed in a
symmetrical wake.
Instead, their wake forms a
more complex, non-symmetrical
structure, leading to a variety
of complex trajectories for the
bubbles, including zigzag,
spiral and even chaotic ones.

This leads to the path instability

of larger bubbles as they rise.

Groan up

Why do older people groan or say

“ohoo” when we sit down, stand

up or do pretty much any one-shot

physical action? Is it a cultural

convention or is there a physiological

reason? (continued)

Kate Wykes

Stoke on Trent, Staffordshire, UK

After coming down with a

pain and fatigue disorder in

my late 20s, I noticed that I was

making “ohoo”, “ooy” and “oof ”

sounds as I attempted physical

activities.

When exerting yourself

physically, it helps to brace and

stabilise the body. There is also an

emotional release: the sound is a

“huff ” that shrugs off some of the

mental distress from the pain,

enabling you to continue.

I also noticed that one of my

housemates at the time, who

started experiencing backache,

began making similar sounds to

myself, which I had never heard

them make before. So there are

possibly cultural and social causes

for this noise-making.

We are often told to hide our

pain and put up with it. When in a

situation or at an age where this is

more acceptable, perhaps we take

the opportunity to express how

we feel inside and stop with the

constant “I’m fine” charade.

Whatever your age, living

with pain or fatigue from day

to day, together with the mental

effects this can cause, is enough

to make you go “ohoo”.

Barrie Quilliam

Mobberley, Cheshire, UK

I am 85 years old and don’t

groan when I stand up, but

tend to fart. Is this caused

by something similar?  ❚

Quick quiz #89

Answers^

1 A shrewdness

2 13 years

3 Bog body

4 Eric Gutkind

5 B12^

Quick crossword

#76 Answers

ACROSS 1 Macrocephalous,

10 Rigid, 11 Down house,

12 Aileron, 13 Red heat,

14 Equal, 16 Melanotic,

19 Actinides, 20 Acute,

22 Aircrew, 25 Reticle,

27 Magnetite, 28 Prime,

29 Chlorpromazine

DOWN 2 Angel dust, 3 Radar,

4 Code-named, 5 Power,

6 Anhedonia, 7 Ovule, 8 Sceptic,

9 Triage, 15 Lunar year,

17  Laser beam, 18 T-junction,

19 Anaemic, 21 Eleven,

23  Rigel,  24 Whirr, 26 Topaz

#100 Late for

the gate^

Solution

You get to the gate faster by tying

your laces on the travelator or by

running on the carpet. Imagine

twins T1 and T2 walking side

by side on a carpet leading to the

travelator. T1 stops to tie their

laces just before the travelator,

while T2 stops just on the

travelator, a fraction of a second

later. T1 will not catch T2.

The situation with running is

harder to picture, but think about

the lead that T1 gains when

running on the carpet, which is

extended when they walk on the

travelator. T2 can never make up

all of the extra lead that T1 built.

Generally, if your new action (for

instance, tying laces) is slower

than your walking speed, do it on

the travelator; if it is faster than

walking, use the carpet.

“ Leonardo da Vinci

was one of the first

to notice the puzzling

phenomenon of the

zigzagging path of

large, rising bubbles”