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
The coke machine in college takes 50 pence pieces. It also takes a certain foreign coin of traditional design...
What happens to the perimeter of triangle ABC as the two smaller
circles change size and roll around inside the bigger circle?
Jack from Wolgarston High School gave
this a lot of thought, here are some more ideas about the bike
We know that there's an important difference between a back
wheel and a front wheel : the front wheel can point in any
direction, just turn the handle bar, but the back wheel always
points exactly forward towards the front wheel, or more exactly,
towards the point where the front wheel has contact with the
So we could test to see if a track could have been made by that
back wheel :
From the track being tested draw out a line that seems to be the
wheel's direction at that point, and then continue that direction
line until it hits the other track - that's where the front wheel
would have needed to be.
Do that for lots of points along the 'tested back wheel track',
and if that was a genuine back wheel track the distance along any
of those direction lines to the front wheel contact point would be
a constant length.
That's because, although the front wheel can turn the contact
point with the ground is the same however you angle the handle
If the line first tested is unsuccessful the other track ought
to be the back wheel, but it's a good idea to do the test and check
The straight line from a point on the back wheel track which
shows the direction of the back wheel at
that point is called the 'tangent'.