You may also like

problem icon

Just Rolling Round

P is a point on the circumference of a circle radius r which rolls, without slipping, inside a circle of radius 2r. What is the locus of P?

problem icon

Coke Machine

The coke machine in college takes 50 pence pieces. It also takes a certain foreign coin of traditional design...

problem icon

Rotating Triangle

What happens to the perimeter of triangle ABC as the two smaller circles change size and roll around inside the bigger circle?

Contact

Stage: 4 Challenge Level: Challenge Level:2 Challenge Level:2

When the disc is in the corner there is one quarter of the circumference (that is a length $2\pi/4 = \pi/2$) which does not come in contact with the edge of the tray. As the disc rolls round one circuit:
an arc of length $2$ units comes into contact,
then an arc of length $\pi/2$ gets missed,
then an arc of length $1$ unit comes into contact,
then an arc of length $\pi/2$ gets missed,
then an arc of length $2$ units comes into contact,
then an arc of length $\pi/2$ gets missed,
then an arc of length $1$ unit comes into contact.

contact

Some points never touch, other points touch only once, other points touch twice.

As an extension to this problem, can you now work out how much of the circumference of the disc never touches the edge of the tray?

The article 'A Rolling Disc' discusses variations of this problem.