A topic in Learners' conceptions and thinking
A common alternative conception concerns the belief that something moving in a circular arc is subject to a centrifugal force.
Read about common alternative conceptions
A playground roundabout (carousel) (Image by Bernhard Renner from Pixabay)
Centrifugal force – a fictitious force
Forces are sometimes introduced in introductory science as being 'pushes or pulls'. If you have ever sat on a children's roundabout that is spinning fast, you may have realised you need to hold on or you could be thrown off. There seems to be a force that is pushing you away from the spinning roundabout. Perhaps you have been sitting in the back set of a car that is travelling quickly around a roundabout, and found yourself slipping across the seat and pushed into the car door. Again, there seems to be a force pushing you outwards.
A force which pushes you away from the centre of the circle is known as a centrifugal force. The term is likely to be familiar from that common piece of scientific apparatus: the centrifuge. A centrifuge is used to separate different substances in a mixture by density. For example, there are many reactions in chemistry which give rise to precipitates, but initially give the reaction mixture a 'cloudy' appearance. So, if lead nitrate solution is mixed with potassium iodide solution then the mixture appears bright yellow and is opaque (you cannot see through the test-tube or other reaction vessel). The yellow is tiny crystals of lead iodide formed (and suspended) in the mixture. But very slowly the yellow crystals will settle to the bottom of the tube – this is the 'precipitate' – leaving a clear fluid above (potassium nitrate solution 1) as they are denser than the liquor. If the tube is placed in a centrifuge, and spun very quickly, the settling process is accelerated (as if the tube is in a much stronger gravitational field).
…but there are no centrifugal forces
I have given a little detail there, because if scientists use centrifuges – and they clearly work – it may be surprising that centrifugal forces are imaginary. Indeed centrifugal forces are sometimes described as fictitious.
To appreciate this means thinking about the concept of inertia, and Newton's first law which tells us that any object remains in its current state of motion (has a constant velocity, which might be zero if it is not moving) unless acted upon by a net (unbalanced or resultant) force.
Read about Newton's first law of motion
When you are sitting on the roundabout, at any moment your velocity is directed along a direction tangential to the circular motion. Your momentum will cause you to slide in a straight line, off the roundabout, unless there is a centripetal force to cause you to change direction and carry on going round. When you hold tight to the handrail this provides that centripetal force.
In the centrifuge, there is a centripetal force acting on the reaction mixture causing the tube and its contexts to spin round rather than move off in a straight line. (Maybe they should be called 'centripetes'?) Very powerful centrifuges have very strong outer walls – because if a sample tube breaks, any glass and contents no longer experiencing the centripetal force will 'fly off' at a tangent to the intended notion (as Newton's first law suggest) at considerable speed.
Laboratory centrifuges are designed with strong outer walls to contain ejected materials when there is a breakage (Image by scotth23 from Pixabay)
Why do people get so confused and think there is a centrifugal force?
Historically, people used to think that circular motion was natural for heavenly bodies.So the motion of the moon around the earth or of mercury around the sun, was considered 'natural'. Although scientists have long explained that is not so, it is still common for people to think that orbital motion does not need any overall force – with centripetal force (from gravity) balanced by centrifugal force. Actually a centripetal force is need to keep forcing the orbiting body to change direction as it orbits.
Read about conceptions of orbital motion
I think psychology is involved as well. When you are sitting on a playground roundabout or driving around a roadway roundabout you are focused on the intended motion in an arc, and anything that seems to stop you moving along that circular path (even if it is your own momentum) seems like a force pushing you outwards.
nb. This figure is set up as a kind of concept cartoon – a focus for classroom discussion to elicit aspects of learners' thinking about science topics
Centrifugal force in science sources
It is not just learners who may hold misconceptions about this topic. Centrifugal forces are sometimes referred to in works about science.
The 'birth' of the solar system
Here, the author of a popular science book (Dr Natalie Starkey) seems to hold the alternative conception that there was a centrifugal force acting when the sun first formed from a cloud of space dust:
"The dust and gas in the solar nebula surrounding the infant Sun continue to fall, under gravity, inwards to the centre and the whole system continues to spin. These processes can't continue forever, though, otherwise the nebula would just get smaller and smaller and eventually disappear in a black hole. The reason this doesn't happen is centrifugal force. This is a clever bit of physics that acts to balance the inward pull of gravity – stopping the gas and dust falling inwards – but also slowing down the spinning. … However the centrifugal force only acts in the plane of rotation. This is exactly [sic] why the rather irregularly shaped cloud starts to flatten out to form a disc, because matter outside the plane of rotation continues to fall in."
Natalie Starkey (Catching Stardust. Comets, asteroids and the birth of the solar system)
Dr Starkey is correct that had nothing stopped the collapse of the gas cloud it would have eventually led to a black hole singularity. But she is incorrect in suggesting why. For one thing, our sun is not massive enough to form a black hole as its gravity is not strong enough to overcome the repulsive forces that operate when the fundamental particles in its materials are squashed together. So that would have stopped the collpase. This fate can however ultimately happen to much heavier stars – when they are no longer radiating as visible stars.
In any case, the contracting ball of gas and dust became very hot (there had been an enormous amount of potential energy in the vastly spread cloud, which meant the converging particles were moving very fast) and nuclear fusion began producing a great deal more heat. The pressure in the centre of this huge nuclear 'fireball' (the sun) is large enough to balance the gravitational attraction and stop further collapse while the nuclear reactions continue. 2 This may be considered 'a clever bit of physics' but has nothing to do with the fictitious centrifugal force! But even without repelling particles and nuclear reactions, we do not need centrifugal forces to explain why orbiting bodies such as the earth do not fall into the sun.
A centripetal force is needed to make something veer from a straight line trajectory and orbit another body. A gravitational force on an object orbiting a central body is directed toward the centre of mass of that body. When the gravitational force matches the required centripetal force the object will have a constant orbit and not move any closer the central body. What is 'balanced' (better, matched) is the gravitational force acting and the centripetal force needed for circular motion. But these are the same force, not opposing forces.(A bit like seeing the money left in your pay packet as being your holiday spending money – it is the same money that has a source and a use.) {analogy}
(As angular momentum is conserved in physical processes, I am not sure why a supposed outwards centrifugal force that {supposedly} balanced an inwards centripetal force {and so supposedly led to no overall force acting} would change the rate of spinning. Generally in science, a lack of cause means no effect.)
… flatten out to form a disc…
To make something move in a circle, there needs to be a force directed to the centre of that circle. Centripetal force acting on a particle moving in a circle is directed towards the centre of that circle. For a particle in the plane of the 'disc' the gravitational force is directly towards the centre of mass which is also the centre of the disc.
For a particle in the cloud 'above' the main disc,3 the gravitational force will still be towards the centre of the disc, but this is now below the circle it is orbiting in. In physics we often deal with such situations by resolving the force into two perpendicular forces to help us see what will happen. (This means we do a kind of thought experiment where we replace the force acting by two other forces that would collectively have exactly the same effect. That might seem to complicates matter further! But choosing the directions of the 'component' forces carefully, means it actually helps us analyse the situation.)
So, if we resolve the gravitational force into a component that is towards the axis of rotation of the cloud, the other component will be towards the plane of the disc. We can then consider the inwards force to provide the centripetal force that causes the particle to orbit, and the downwards force to accelerate the particle down towards the disc. Really there is one force whose action will combine these effects.4 The overall effect is that the particle will spiral down to become part of the main disc. Again, there is no need to invoke a non-existent centrifugal force.
Notes:
1 This would be the case if precisely the right amount of lead nitrate was used to react fully with the potassium iodide. Otherwise the liquor would also include either some lead ions or some iodide ions.
2 This is a simplification. A star evolves as it reacts its material through successive nuclear reactions and so there will be changes in temperature and pressure and its size according to which state it is in. For a star such as our sun halfway through its initial reaction of hydrogen such changes are only very minor and it is effectively in a stable state. Later it will experience quite substantial changes – but they are billions of years away.
The sun is itself rotating – so some of the gravitational force acting on the material of the sun itself is providing the centripetal force maintaining rotation: the rest is balanced by the forces due to the internal pressure.
3 There is of course no absolute distinction between above and below the disc, and a choice of an 'upwards' reference direction in space is arbitrary. Any alien cartographers visiting our solar system to carry out a survey are just as likely to put Antarctica at the top of 'planet 3 surface features map' as at the bottom.
If we use gravity as the indicator of downwards, as we do at all points on the earth's surface, then an observer not in the plane of the disc will be above it – and another observer on the opposite side of the the disc will also be (from her perspective) above it – just as people in the North and Southern hemispheres all consider themselves to be standing on the earth, not under it.
4 Strictly forces always act on two bodies mutually (this is Newton's second law of motion). The gravitational force is between the particle and the cloud. (Actually, we could strictly consider this as many separate forces between the particle we have highlighted and each other particle in the cloud – luckily we can consider the cloud as one body for present purposes.) Being pedantic, the disk also moves up towards the highlighted particle, BUT the cloud as a whole is so much more massive than the particle we are considering that this effect would be absolutely minuscule compared with the movement of the focal particle.
Read about Newton's second law
