# Introducing circular motion

##### Demonstration

Use these to introduce the concept that an object travelling in a circle will fly off tangentially rather than radially. Any or all of these demonstrations can be used depending on time and space constraints.

Part 1 is best done outdoors.

#### Apparatus and materials

Sparkler

Battery-powered drill

Rubber bung on the end of a string (about 1m)

30 cm piece of plastic waste pipe (10 cm diameter minimum, 20 cm better) with a notch about 4 cm x 2 cm cut in one end

Eye protection

#### Health & Safety and Technical notes

Eye protection should be worn in parts 1 and 2.

The third demonstration requires the construction of a very simple piece of apparatus. Take a hollow tube of diameter at least 10 cm, but no larger than the size of an OHP. A piece of plastic drainpipe or waste pipe is suitable. In one end cut a small notch, large enough for a marble to pass through.

The third demonstration requires the construction of a very simple piece of apparatus. Take a hollow tube of diameter at least 10 cm, but no larger than the size of an OHP. A piece of plastic drainpipe or waste pipe is suitable. In one end cut a small notch, large enough for a marble to pass through.

Hollow plastic pipe with notch cut in one end to allow the marble to escape. Placed like this on an OHP the path of the marble is projected as a shadow.

#### Procedure

Part 1:

a Bend the handle of a sparkler at a 90 degree angle. Mount it in a drill so that it makes a circle in a vertical plane in front of you. When lit, the sparks fly off tangentially, not radially.

b Light the sparkler and spin it.

Part 2: Before you begin this demonstration, you may need to clear away any objects that could be damaged in the direction of where you will release the bung.

Take a student who is sceptical about the ideas of tangential motion. Make them stand in front of you. Put an apple on their head. Tell them you are going to swing the bung around in a horizontal circle and let it go when it points from you to them. If the student is right, the bung will fly outwards and hit the apple.

Swing the bung around your head, counting down 3,2,1... each time the bung is in line between you and the student. Release on the next swing and the bung passes harmlessly off at 90 degrees.

Part 3: Place the drainpipe on an OHP and put a marble inside. With the notch at the top the marble can be flicked round to describe circular motion. With the notch at the bottom the marble will go round the circle and exit tangentially through the notch - it is slow and very clear.

#### Teaching notes

1 One of the hardest ideas in circular motion, because it is so counter-intuitive, is that objects move off tangentially when the centripetal force is removed, not radially.

Students expect objects to fly off radially because they think of centrifugal forces. Convincing them with demonstrations they will remember will help give them something concrete to go back to. Each of these experiments is memorable and clear.

2 My favourite of these demonstrations for sheer wow factor is Part 1, putting the sparkler in a drill bit. Parts 2 and 3 also work well in the classroom and provide good discussion points. Part 3 however, is the most elegant for being slow and simple. The use of a shadow projection on an OHP removes all extraneous detail. In explaining the motion you can point out that the wall provides an inward force to push the marble round in a circle. And then, when the wall is removed...presto, it goes in a straight line.

This experiment was submitted by Ken Zetie, Head of Physics at St Paul's School in West London. He is on the editorial board of Physics Education and regularly contributes to Physics Review.