# 17.29 Centrifugal force

In post 17.13, we met the idea of centripetal force – the force that is needed to pull an object to keep it moving round a circle. In post 17.28, we saw how the force exerted by the earth’s gravitational field provides the centripetal force to keep a satellite in orbit.

It’s important not to confuse the ideas of centripetal and centrifugal force.

What is centrifugal force? To answer this question, let’s think about the fairground ride shown in the picture above. Initially, the circular base of the cylinder is horizontal and people stand with their backs against the wall. The cylinder starts to rotate around its axis. Gradually, the speed of rotation increases. The cylinder continues to rotate about its axis and, at the same time, it tilts – as shown in the picture.

The people at the top of the cylinder don’t fall. They feel a force pushing them against the wall. This force is called the centrifugal force. You can also experience a centrifugal force when you are in a vehicle that goes rapidly around a corner.

Centrifugal force is used in a device called a centrifuge that can be used, for example, to separate blood cells from the surrounding liquid (plasma). Glass tubes, containing blood, are placed into the holes in the white circles in the picture. The lid is closed and the black block rotates around the central metal axis. The tubes move into a horizontal position and the centrifugal force pushes the blood cells to the bottom of the tube.

So, centrifugal force is something that we can experience for ourselves and it can be used to separate solid particles from liquids.

Despite what I have written so far, centrifugal force is sometimes described as being a “pseudo-force”, “fictional” or “not real”. The worst comment I have found about centrifugal force states that “[The belief in centrifugal force] is often fervently adhered to despite the clear presentation by a textbook or teacher of an inward force requirement” (http://www.physicsclassroom.com/class/circles/Lesson-1/The-Forbidden-F-Word)
The person who wrote this appears not to understand how science works and to be confusing centripetal and centrifugal forces! The purpose of science is to explain what we observe. If we detect a force (on the fairground ride or in a cornering car), we should try and explain it. If, in the early twentieth century, people studying the motion of electrons had believed that Newton’s laws of motion always work (which is what their teachers and textbooks would have told them), they wouldn’t have discovered quantum mechanics (see post 16.2). As I have written before, “I believe we should try to understand the reasons for things and not hide behind a belief in authority. If we simply believe what others tell us we risk being victims of their lies, propaganda, mistakes and misunderstandings” (post 17.9). The “inward force requirement” in the quotation refers to centripetal force – not to centrifugal force.

I think the problem arises because you can explain the effects of centrifugal force simply by using Newton’s first law of motion (post 16.2). So, a person on the fairground ride tends to continue in a straight line and so moves towards the wall. However, this doesn’t mean that the person on the fairground ride doesn’t experience a force.

The clearest statement about the two forces that I have found is quoted next. “Centrifugal force is every bit as real as centripetal force. It’s just in a different frame. “Centripetal” means “center-seeking”, and “centrifugal” means “outwards-seeking” or, more literally, “center-fleeing”. You’d think these are opposites, but they are in fact the same thing! It just depends on your point of view.” (http://blogs.discovermagazine.com/badastronomy/2006/08/30/when-i-say-centrifugal-i-mean-centrifugal/#.WU6L4). This is explained much more mathematically (although with an amusing cartoon that makes the point very clearly) at http://www.damtp.cam.ac.uk/user/tong/relativity/six.pdf. This explanation involves the vector product or cross product of vectors – a subject for a later post. (Although appendix 2 of post 17.13 describes the scalar product or dot product of vectors.)

The important point about centrifugal force is that you experience it only if you are the rotating object or inside the rotating object. In post 16.4, we saw that the motion of an object depends on how the observer is moving. The motion of the observer defines a frame of reference that we use to describe the motion. If two frames of reference move at a constant velocity, they are called inertial frames of reference and observations about acceleration will be the same in both – the Galilean principle of relativity (post 16.12). A rotating object can be used to define a rotating frame of reference (post 16.9). Because a rotating object is always accelerating (post 17.4), its velocity is not constant and it defines a non-inertial frame of reference – now the Galilean principle of relativity does not apply. So, accelerations and, therefore, forces (post 16.13) are not the same in different non-inertial frames of reference.

The person throwing the hammer in post 17.13 is pulling on the cable to keep the hammer moving in a circle. Now put yourself in the place of the hammer and suppose that you can see only the person pulling the cable and nothing else. You see him moving in a circle around you – just as we see the sun moving round the earth (post 16.9). Because he is moving in a circle, he is accelerating (post 17.4) in your frame of reference. He has mass, so to keep him accelerating, there must be a force (post 16.13) acting towards you. This force is equal but opposite to the force he believes he is applying to you and is called the centrifugal force. The force you experience depends whether you are moving in the circular path or are at the centre of the circle.

If you’re not convinced, partly fill a bucket with water and tie a rope to its handle. Now use the rope to swing the bucket around your head – like the man throwing the hammer in post 17.13. You will need to exert a centripetal force, that acts towards you, on the bucket. But you won’t get wet because the centrifugal force pushes the water into the bucket.

Related posts

Follow-up posts

21.13 Centrifugation