There is a great deal of confusion about the terms centripetal and centrifugal when things are whizzing around in circles. So let's get that straight from the start. You will need a soft toy and a ribbon or piece of rope. Of wombats A wombat is a tubby mammal which lives in Australia. They are about the size of a medium dog, but very rotund with short stubbly legs. They are marsupials so they carry their young in a pouch. They look very cuddly and are popular as soft toys. Spinning live wombats around your head to do physics experiments is frowned upon. And they are very heavy. Hence this task will be based on a toy wombat who kindly volunteered for the role. You will need a volunteer soft toy.

Image courtesy Damian Kelly

The volunteer, Wombatius softfella

What is the difference between centrifugal and centripetal forces? A lot of people get centripetal and centrifugal forces mixed up. They are forces to do with things spinning in circles. 'Centripetal' comes from the Latin for 'centre seeking' so the direction of centripetal force is towards the centre of the circle. 'Centrifugal' has two Latin roots in it. 'Centri-' is for centre. The Latin root 'fugere' means 'to flee'. So, 'centrifugal' means 'to flee from the centre'. So let's find out which force is acting when you spin Wombatius around your head. You will have to substitute your volunteer's name wherever 'Wombatius' appears. Attach Wombatius to a rope or ribbon ready to spin him.

Think about what you can feel As you spin your volunteer, concentrate on the direction of the force in action. A force is a push or a pull. Wombatius is flying around in the circle with gravity acting down and only one other force acting on him - the force in the rope. What direction is that force on him? You can feel the tension in the rope on your hand pulling out away from you. That force is balanced by the force on Wombatius - back towards you. Try pushing on him and he won't go around in circles. The only way you can get him to stay in a circle is to keep pulling on that rope. The force on Wombatius is towards the centre - it is a centripetal force!

So why does intuition tell you the force is outward? Because you instinctively know what will happen if you let go. Your brain is saying that Wombatius is pulling out - you can feel the pull. But that is the force on you - not the force on Wombatius. As he is the one who is in circular motion, we need to consider the forces on him. If you let Wombatius go he will fly out. Why? It's all to do with newton's First Law of Motion:

1. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

What this law says is that a body at rest will remain at rest. A body in motion will continue in a straight line unless a force is acting upon it. This tendency to keep moving unless something stops you is known as your inertia. At any instant in the spinning of Wombatius, he is going in a straight line tangential to the circle, as shown in the diagram below. You keep pulling him in and changing his direction.

When you took away the force acting on Wombatius. You stopped the pulling on him towards the centre. So he kept going in a  straight line. He wasn't flying out as much as he was no longer being pulled in.

Now let's use Wombatius' frame of reference

If we use his frame of reference, that means we think of everything as if Wombatius is still - he is 'at rest'. Everything else is moving. So you are the one in circular motion around Wombatius. What does he feel?

Here we need Newton's Third Law:

Wombatius can feel the pull of the rope toward you. He is at rest - it is his frame of reference after all! So there must be an equal an opposite reaction force acting in the opposite direction to the direction that you are pulling on him so all the forces are balanced. There must be an outward force.

That is the centrifugal force.

Virtual forces exist when when a body is accelerating. It is called a non-inertial frame of reference. We  usually think in terms of the Earth's frame of reference - as if the Earth is always at rest. That is the most natural for us.

So let's stick to the Earth's frame of reference. Let's think of ourselves as still when we are spinning Wombatius, and then all will be simple. In the inertial frame of reference of the stationary Earth, there will be only one force causing Wombatius' circular motion - an inward force. It's centripetal!

Your task is to get the concept of a centripetal force across to other people by having them spin your version of Wombatius.
Ask people if the force is inward or outward in circular motion.
Select someone who answers that it is 'out'.
Then give them a go of your spinning volunteer.
Can you convince them that the force is inward?
How hard was it to explain?

All experiments supervised by the the RSPC(T)W - the Royal Society for the Prevention of Cruelty To (Toy) Wombats.