54 | New Scientist | 1 June 2019
Speed limits
The most powerful cars intended
for road use tend to have a top
speed that ranges between 300
and 350 kilometres per hour. Is this
due to some physical limitation or
just practicality of design because
you will never be able to reach
these speeds let alone go faster
on public roads?Gordon Drennan
Adelaide, South Australia
The amount of power it takes a car
to go faster goes up exponentially.
To double a car’s top speed, its
engine must be eight times as
powerful. Aerodynamic lift
over the body also increases
exponentially. At the same time,
the traction of the tyres has to be
better to deliver the higher power
required to the road.
Such cars consequently cost a
lot more, and as a result, there are
far fewer people who can afford to
pay for them and who see any
point in having such a car.
The fastest road-legal car
currently on the market is
the Koenigsegg Jesko, which is
expected to have a top speed of
483 kilometres per hour – and
would cost around A$6.5 million
here in Australia, if it weren’t
already sold out. But there is no
fundamental reason why a road
car couldn’t go faster.Mike Clarke
Castle Hedingham, Essex, UK
It is both difficult and expensive
to design tyres capable of speeds
over 300 kilometres per hour.
Even tyres designed for these
speeds need to be in “as new”
condition to operate safely. After
only a few weeks of use at normal
speeds, they probably wouldn’t
be safe to use at high speeds.
A further limitation is the
driver. At 300 kilometres per
hour, you are travelling more than
80 metres per second, but human
reaction times vary between about
0.7 and 3 seconds. That means
anything 100 metres ahead of you
on the road, such as debris, would
be almost impossible to react to.Stephen Johnson
Eugene, Oregon, US
A powerful car whose top speed
isn’t restricted by an onboard
computer eventually reaches a
speed where the vehicle can’t
compress any more wind in front
of itself. This compression goes up
with the square of speed, so that
each doubling of speed requires
four times as much energy to
overcome this effect. In addition,
there is drag at the rear of the car
from the induced vacuum.
A final limit of sorts on speed
is the strength of the suspension
system of the car. To prevent high-
speed cars from trying to fly as
the wind speed under the vehicle
increases, spoilers are employed
at the rear of the car and air dams
at the front. These increase
downward force to the point
where the effective weight of a
moving car on the road can more
than double. In a tunnel of the
right shape, it would be possible
to drive such a car upside down,
as it would effectively stick to the
road with a force greater than
its weight.Toast, not toast
Heating bread in a toaster
and a microwave oven creates
very different results. What do
microwaves do to bread?Isabella Van Damme
Arborfield, Berkshire, UK
The results are different because
microwaves heat bread from the
inside, while a toaster heats from
the outside. In a toaster, the
bread’s surface quickly reaches
temperatures at which the
Maillard reaction causes
browning and a toasted
flavour. Excessive heating can
lead to carbonisation or burning.
In contrast, heating a slice of
bread in a microwave will dry it
out without achieving the surface
temperatures required for the
browning reactions. Excessive
microwave heating can “burn”dry food on the inside without
showing any signs on the outside.Parvez M. Ashraf
Dhaka, Bangladesh
As well as heating only the
surface of the bread, using a
toaster causes the bread to lose
moisture, making it dry and crispy.
In microwave heating,
the starch of the bread is
re-gelatinised. That is to say,
the starch molecules bind to water
molecules, which sort of dissolve
or plasticise the starch, making the
texture of the bread softer overall.Maria Tsikkinis
London, UK
One of the properties of
microwaves is that they are
absorbed by water. When bread
is irradiated with microwaves,
the water in the bread heats up
and conducts the heat to the
surrounding food.
When the water molecules
absorb enough energy, they
evaporate, which is why the bread
is steaming when you remove it
from the microwave and why the
slice, soft when you take it out,
quickly becomes rock hard once
it cools down.
A toaster, on the other hand,
uses infrared radiation. These
are heat waves and their energy
is absorbed by the whole of the
bread, rather than just the water.Theo Megarrity,
Brisbane, Australia
“Toasting” toast in the microwave
is like trying to start a fire with a
pot of boiling water. Microwaves
cook through radiation (emitting
waves), and toasters cook through
proximity to hot elements, which
results in the crispy delight that
is toast.This week’s questions
When a Tyrannosaurus rex or other carnivorous dinosaur
is depicted on screen, it roars like a carnivorous mammal.
But birds developed from dinosaurs, so could they have
screeched or called like the modern cassowary, or made
no noise at all? Andrew Johnson, Banbury, Oxfordshire, UKWhen I drain a can of chickpeas, the liquid forms a soapy
foam. What causes this, and if it is related to soap, could
it be used as an eco-friendly replacement? John Munroe,
Edinburgh, UKWant to send us a question or answer?
Email us at [email protected]
Questions should be about everyday science phenomena
Full terms and conditions at newscientist.com/lw-termsThe back pages Almost the last word
UNIVERSAL
PI
CTURESDid this T. r e x sound more
like a screeching bird than
a roaring lion?