# Centrifuges

by Chris Woodford. Last updated: August 31, 2016.

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 Wikimedia Commons.

## 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're watching something turning round a curve and you're wondering about centrifugal force, you can quietly translate what you're seeing 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.

## Comparing centrifugal and centripetal force

A car's going around a bend in the road. How do we explain what's happening?

• Centrifugal force: Left to its own devices, the car would go in a circle (blue) but centrifugal force (purple) is constantly trying to push it outward. (The trouble with this is that it doesn't explain why the car is going in a circle.)
• Centripetal force: Left to its own devices, the car would go in a straight line (orange), but centripetal force pulls it inward (green), so it actually goes round in a circle (red).

You can also see now where these two funny words come from. Centrifugal (pronounced sen-tree-few-gul or sen-trif-ugul is related to the word "fugue", from the latin word for flight, so we're talking about something trying to get away from the center. Centripetal (pronounced sen-trip-itul or sen-tree-pee-tul) comes from the Latin "petere", meaning seek, so we're describing a force that makes things seek the center:

• Centrifugal = "center fleeing"
• Centripetal = "center seeking"

## 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 NASA on the Commons.

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.)

You might 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).

## How does a laboratory centrifuge work?

If you've studied biology or chemistry in school, you've probably used a small, laboratory centrifuge like this one. Although it's a fairly simple bit of equipment, it's cleverly and sturdily designed to survive the extremely high forces involved in spinning samples at speeds of 20,000 rpm or more.

Looking inside, from the front, we can see that the main parts are a powerful motor (orange, center); a spindle and flexible collar (blue); and multiple pairs of diametrically balanced pockets into which standard-sized test tubes can be inserted (colored red and green). You load up the machine and securely close the lid, which usually has a lock on it to prevent the centrifuge from being fired up when it's open. Then you switch on by turning a rotary dial on the outside of the case (not shown), which lets you adjust the speed up or down as necessary. Balance is absolutely critical in a high-speed centrifuge: if one of the pairs of pockets is empty and the other full, the difference in weight will make the machine vibrate dangerously, possibly smashing the sample tubes inside. But balancing test tubes exactly can be very difficult, so machines like this one have flexible rubber collars on their spindles that allow the loaded pockets to rotate about their true center of gravity, thus minimizing any unwanted vibration.

Artwork: Inside a typical high-speed laboratory centrifuge. From US Patent 2,699,289: High-speed centrifuge by Robert R. Allen, Custom Scientific Instruments Inc., patented January 11, 1955, courtesy of US Patent and Trademark Office.

And this is what it looks like in reality...

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.

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).

## 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

• 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.

• 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.

### Articles

• Why is the LHC tunnel so big? by John Butterworth. The Guardian, June 8, 2012. A particle physicist explains why centripetal force and synchrotron radiation are such a big problem at CERN.
• Accelerated Frames of Reference: Inertial Forces and Frames of Reference: The Centrifugal Force by David P. Stern, October 14, 2016. If you're a university student, you'll be able to appreciate this deeper-level explanation, and why it becomes appropriate to talk about centrifugal force if you're looking at things in a rotating (accelerating) frame of reference (instead of the usual inertial frame of reference). This kind of explanation isn't appropriate for school-level students, which is why I don't cover it above.

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