## SCIENCE FOR GRADE 1-8 Understanding spinning disks

A disc on a string...

When you pull on the twisted string, you put in WORK or ENERGY and as a result the disc will spin in one direction and then in the other direction. Once you get it started it will spin very fast and…as long as you put energy in it will keep going!

When discs or wheels, like a bicycle wheel, spin fast they ROTATE around their CENTRE OF GRAVITY. They get a mind of their own. That is why you can stay on the bike, and why the whirling twirler works its magic!

What in the world is a centre of gravity.

It is the point about which the disc will BALANCE. Put a ball on the floor and put a dinner plate on top, you can balance the plate if you put its centre of gravity precisely on top of the ball. When it balances you have found the centre of gravity. For a disc shaped object it is the centre. For other objects it might be in an other place on its body. Give it a try and experiment.

The spinning disk is a spinning top on a string. It acts like a GYROSCOPE as it spins on its own around its centre of gravity while the flexible string allows it to do that , AND… the string puts in the energy needed to keep the disks spinniing. Well….this is getting difficult, but read on!

It is YOU who puts in the energy by pulling on the string. Your muscles do the work.

But… WHERE DO THEY GET THEIR ENERGY FROM?

When you eat your sandwich or supper, you put energy in your body and that is stored for use later… when you play with your whirling twirler, for example, you use up some of the energy!

The yo-yo works on the same principle, you put some energy in when it is at its lowest point, but otherwise it is the POTENTIAL ENERGY and the KINETIC ENERGY that are exchanged, potential energy is stored in the yo-yo at its highest point, then at the lowest point it is all changed into kinetic energy of the spinning discs. This kinetic energy allows the yo-yo to climb up the string to get its potential energy back! If you don’t tug it a little it will slow down because of FRICTION, give that a try and look what happens…

Have fun.