Centrifuges
by Chris Woodford. Last updated: September 20, 2012.
If you need to wash and dry a pair of jeans in a hurry, you'll be awfully glad you have a
centrifuge. That's what your clothes washer
becomes when it spins wet laundry at high speed to remove the water. A centrifuge is
simply a machine that spins around to make a large and useful force.
Small centrifuges are used in scientific laboratories (for example,
to separate blood products). You can find much bigger ones in
aerospace-labs, where they're used for testing
astronauts, pilots, and their equipment to absolute breaking point.
Let's take a closer look!
Photo: The medium-sized space-station centrifuge used by NASA. This one has a radius of 1.8m (6 ft). Photo by courtesy of NASA Ames Research Center and Great Images in NASA.
What is a centrifuge?
Photo: A clothes washer drum is a type of centrifuge. During the wash
cycle, the paddles agitate the clothes in the soapy water. When it comes to the spin, holes
in the drum let the water out.
Hold something heavy in one hand and whirl your arm around your head. Feel a force that
seems to be pulling your shoulder out of its socket? That's the
principle of the centrifuge at work—and you can look at it from two
different angles. In popular books and magazines, people talk about
something called centrifugal force: the force that seems to
make things shoot outward when they go round in a circle. So, when a
bus goes around a bend at high speed, you'll read that it's
centrifugal force trying to tip the thing over. When your clothes
are spinning in the drum, it's centrifugal force that throws the
water out through the little holes so your washing ends up much
drier.
Or is it?
Centrifugal force or centripetal force?
Science teachers will tell you this is wrong: there is actually no such thing as
centrifugal force. We can understand what's really happening by
considering Isaac Newton's famous laws of motion. When a car begins
to enter a bend, its natural tendency is to keep going in a straight
line—and it will do so unless a force acts on it. When it follows
the bend, it does so because there's a force (called centripetal
force) constantly tugging it inward from its straight line
course. What we see as the centrifugal force is really the car's
tendency to go straight, if left to its own devices.
Anytime you hear people talking about centrifugal force, you can quietly
correct them (in your own mind, to be polite!) and translate what
they're saying into centripetal force. So "centrifugal force
gets your washing dry because it makes the water fly out" becomes
"Centripetal force between your clothes and the inside of the drum
pushes them around in a circle. There's nothing to give the water
the same kind of push because it can slip straight through
the drum holes. The clothes experience centripetal force, the water doesn't. The clothes
go round in a circle, the water goes in a straight line—straight through the holes.
And that's what gets your washing dry."
Confused? Don't be! It's this simple: if something is moving in a circle,
there must be a force acting on it somewhere to make it turn, otherwise it would go
in a straight line. So look at the situation carefully and figure out where
the inward pushing or pulling force is coming from. That's the
centripetal force. If some part of the object is flying outward,
that's not because there's centrifugal force: it's because there's no
centripetal force to make it go in a circle.
How big is centripetal force?
Why is there always a centripetal force when things move in circles? Think back to
the laws of motion. If something is following a curved path, its
velocity is changing all the time because its direction is changing
all the time. If the velocity is changing, it's accelerating (even if
its speed is constant). If it's accelerating, a force must be
acting—sometimes a very big force. All these statements follow
directly from the laws of motion.
Photo: Now that's what I call a centrifuge! You can see how big it is from the
little man standing in the bottom, center, wearing a red safety hat. This is the launch-phase simulator that NASA
uses to see if pieces of spacecraft (and even whole satellites) can withstand extreme forces. The huge
spinning arms are powered by two 1250-horsepower motors and produce
forces up to 30g (30 times the force of gravity or 15 times the force you feel on a typical
roller coaster). Photo by courtesy of NASA Goddard Space Flight Center and Great Images in NASA.
Spin things at high speeds and you build up big forces very quickly. You'll have seen
that very clearly if you've ever washed something like a heavy pair
of curtains in a clothes washing machine and they've bunched up on one side
of the drum. When it comes to the final spin, the machine will be
banging and clattering about and doing its level best to escape from
your kitchen (probably damaging itself in the process). When things
spin at high speed, the forces involved can be huge. That's why it's
best to balance your washing machine with a mixed load (never, ever
wash one item by itself) and why you need to balance laboratory
centrifuges very carefully as well.
How big are centripetal forces, exactly? The
laws of physics tell us that the centripetal force needed to make an
object go round in a circle is given by this little equation:
F =(mv2)/r
Here, m is the mass of the object, v is the velocity, and r
is the radius of the circle. So the bigger the mass of the object and
the faster it goes, the more force is needed to keep it turning.
Let's try out some real numbers. Say you have a
1000kg car going round a bend of 50m radius at 40mph (roughly 20m/s).
The force between the tires and the road is 1000 x 20 x 20 / 50 =
8000 newtons or roughly 10 times a typical person's weight. Where
does the centripetal force come from when a car goes round a bend?
There's only one place it can come from: the friction between the
tires and the road. Ten times a person's weight is quite a lot of
force for four little rubber tires to provide,
especially when you consider that only a tiny patch of each tire
is ever touching the road surface. Now make the bend
twice as tight and you'll see the force is doubled to 16,000 newtons
(because r is halved). If the car doubles its speed (v is doubled),
the force is quadrupled to 32,000 newtons (40 times a person's
weight—or a force equal to 10 people weighing on each tire). That's why you're much more likely to skid at high speeds: friction between your car tires and the road can't provide enough centripetal force to keep
you going in a circle, so you whiz off at a tangent: you bow to your natural
tendency and keep going in a straight line.
What are centrifuges used for?
Now forget all about the spinning: if you need a big force quite quickly, a centrifuge is a really
handy way to generate one. Because you're spinning on the spot, you
don't need lots of space. That makes centrifuges perfect for use in
scientific laboratories. One of the most common uses for centrifuges
is in separating mixtures of things. A washing machine is a mixture
of clothes and water and the spinning drum separates those very
efficiently. Laboratory centrifuges are used to separate things like
blood, which consists of red blood cells suspended in plasma (a
yellowish fluid). Put some blood in a test-tube and spin it at high
speed and you separate out these two components very quickly, with
the plasma at the top of the tube and the red blood cells at the
bottom. (They travel to the bottom because they're heavier, so need
more centripetal force to push them round in a circle. The force
comes from the bottom of the tube pushing inward against the blood
cells clumped there.)
Photo: Left: Loading up a laboratory centrifuge with samples in test tubes. Note that you can do lots of test tubes at the same time, but you have to make sure the machine is balanced with matching numbers of tubes on
each side of the spinning arms. Photo by Scott H. Spitzer courtesy of US Air Force and Defense Imagery.
Right: This is what samples look like when they've been centrifuged. The
lighter components (almost transparent here and quite hard to make out) are at the top of the tube, while the heavier ones (darker here) are at the bottom. Photo by Jim Yost Photography courtesy of US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).
You may have seen astronauts and pilots whizzing round in centrifuges too. There's no
special reason why they spin round in a circle; that's pretty much
irrelevant. What's being tested is their ability to withstand very
high forces—and the easiest way to experience that is at the end of
a centrifuge beam. You might feel a bit dizzy at the end but, hey ho,
this is rocket science after all!
Photo: Students try out a bicycle-powered centrifuge at the Museum of Science and Industry (MOSI) in Tampa, Florida. Note how three people sit at equal angles (sixty degrees apart) to make the centrifuge balance properly as it spins. Of course, if one person is much heavier than another, there's going to be a problem! Photo courtesy of NASA Kennedy Space Center (NASA-KSC).
Find out more
On this website
You might like these other articles on our site covering related topics:
Other websites
- Centripetal force: A good summary of the basic math and equations from the Hyperphysics project.
- 20g centrifuge @ NASA Ames: An interesting set of photos of people training on a NASA centrifuge from Alexander van Dijk.
Books
For older readers
- Newtonian Mechanics by AP French. W.W. Norton, 1971. This is one of the all-time classic student texts on force and motion—the book I learned from as a physics undergraduate. Still in print after over 40 years!
- The Feynman Lectures on Physics by Richard P. Feynman. Pearson/Addison Wesley, 2006. (Three volumes). Another classic text, this comes from one of the greatest scientific communicators of all-time and has been in print since the 1960s. It's still superb today.
For younger readers
- Can you feel the force? by Richard Hammond. Dorling Kindersley, 2006/2010. A fine introduction to forces and physics from one of the UK's better known "car-geeks." I was one of the contributors to this book, which won the Royal Society Junior Prize for Science Books, 2007.
- Eyewitness: Force and Motion by Peter Lafferty. Dorling Kindersley, 2000. One of the classic DK Eyewitness books, with quite an emphasis on history as well as a simple explanation of the theory. Most suitable for ages 9-12.
- DK Online Science Encyclopedia by Various contributors. Dorling Kindersley, 2006. This book contains a basic, 40-page (or so) section on forces and motion, written by me, and covering such things as laws of motion, simple machines, pressure, gravity, and so on. The rest of the book is a colorful and useful guide to virtually the whole of science, and a good overview for curious children from ages 9-12.
