54 | New Scientist | 8 June 2019
Green hands
Which is better for the environment:
using a heated hand dryer for
30 seconds, or using two
disposable paper hand towels?Sandy Henderson
Dunblane, Stirling, UK
A typically cited figure for the
carbon “intensity” of UK
electricity generation is
0.2 kilograms of CO 2 per kilowatt-
hour. Allowing for around 5 per
cent transmission losses, this
means that a 1.2-kilowatt hand
dryer is responsible for 2.1 grams of
CO 2 when it is run for 30 seconds.
Paper hand towels weigh
around 2 grams per sheet,
and paper requires about
2.9 megawatt-hours of energy
per tonne in its manufacture.
Assuming that this comes from
standard UK electricity (which
is, admittedly, quite a bold
assumption), two paper towels
are responsible for about
2.3 grams of CO 2.
That probably isn’t the whole
story, however. Other factors are
the manufacture of the heater on
one side and paper dispenser on
the other, and the transport and
distribution costs of both. On
balance, it comes down to how
much the dryer is used and
where it gets its electricity from.
For a building with low footfall
and few occupants who are out
for most of the day, the paper
towels might well have the
edge over a hand dryer.Jay Pasachoff
Pasadena, California, US
The environmental impact is
important, of course, and the
electricity needed for a single
hand drying could use fewer
resources than the production
of two paper towels.
But we must also consider the
health consequences of blowing
bacteria and viruses around the
room, as well as the effect on
hearing if you hold your hands
close enough to an accelerated
blower to make the resulting noise
very loud.Car crash
If a car moving at 100 kilometres
per hour hits a concrete wall,
all of its kinetic energy must be
turned into other forms of energy.
What are those forms, and can
anyone describe the overall
energy distribution an instant
after the car hits the wall?Michael Paine
Sydney, Australia
As a consulting mechanical
engineer, I assessed more than
400 crash tests for the Australasian
New Car Assessment Program
from 1996 to 2017. It is always
interesting to answer questions
like this. The physics of a car crash
are very helpful when explaining
Newton’s laws to students.
The front of a car is designed
to crush in a fairly predictable way
during a crash. A lot of the kinetic
energy is converted to heat during
the metal deformation. However,
there is always some springiness
in the car’s structure, and so there
will be a degree of rebound.
At the peak of the crash, whenthe velocity is effectively zero
(and thus its kinetic energy is too),
the car acts like a compressed
spring, storing potential energy.
On rebounding, the car regains
a smaller amount of kinetic
energy – this time travelling
backwards. So the overall process
is that kinetic energy becomes
heat and spring potential energy,
which results in heat, smaller
kinetic energy and, hopefully,
surviving occupants.
A small amount of energy is
also dispersed as frictional heat, as
the locked tyres and the underside
of the deformed car drag along the
road. Things get more complicated
when two cars collide head-on,
but that is another story.William Cummings
Falkirk, UK
Kinetic energy is calculated as half
of the object’s mass multiplied bythe velocity squared. My people
carrier weighs about 2 tonnes.
Travelling at 100 kilometres
per hour, it has approximately
770,000 joules of kinetic energy.
If this vehicle collided with a
concrete wall, it would deform,
transferring some of its kinetic
energy into the concrete
molecules. Depending on the
relative mass and anchorage of
the wall, this may cause it to shift.
It would certainly make some
of the concrete molecules lose
adhesion, forming cracks and
dust. The rest of the energy would
be dissipated as heat and sound.Peter Jacobsen
Davis, California, US
Firstly, thanks for not calling this
event an “accident”, a word that
implies the event was unavoidable
and impossible to predict. The BMJ
has a great article explaining why
it banned the word back in 2001.
In this car crash, there are
three collisions. First, the car hits
the concrete wall, then the bodies
of the vehicle occupants hit seat
belts and airbags, and, finally, their
heart hit their ribcages. The goal
of both vehicle and road designers
is to provide enough energy
absorption to keep that final
collision mild enough that the
aorta doesn’t tear from the heart
and cause internal bleeding.
A modern car with unibody
construction crushes on impact,
absorbing a lot of energy. An older
vehicle with a body-on-chassis
has less ability to absorb energy.
In a modern car, a person wearing
a seat belt will probably survive
a crash into a wall at speeds of
up to 70 kilometres per hour.
Preventing head-on crashes
into stationary objects is why
modern roads have side barriers. ❚This week’s questions
When we feel itchy, a scratch brings instant relief. But when
the itch is caused by a mosquito bite, scratching increases the
irritation. Why the difference? Klaus Ehrmann, Queenscliff,
New South Wales, AustraliaSome hospital wards remove flowers at night, as they
supposedly emit an amount of carbon dioxide sufficient to
affect patient health. Is this true? Stewart Katz, London, UKWant to send us a question or answer?
Email us at [email protected]
Questions should be about everyday science phenomena
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