This problem clearly got a lot of you
thinking! Several of you sent in the correct answer, including
Gemma, Rachel, David, Alex, Charlie, Robert, Joel, Jamie, Carys,
Soph, Bex, Rhi, Joe, Veronica, Harriet and Elspeth, all from
Cowbridge Comprehensive School.
As Azeem of Mason Middle School states:
You have to have a triangle with 40 radius. Also, you must place
the red dot at the bottom left corner of the triangle.
Some of you worked it out using some
systematic thought and trial and error.
Sathya and Michael made a good effort at
explaining how they worked this out .
Sathya from Scots College, New Zealand
considered whether or not the dot could be placed in the centre of
I first checked whether it could have been the centre of the
shapes. This is impossible as it always results in a linear
Sathya then went on to consider how many sides
the polygon might have:
Then I checked how many segments in the line (changes in the shape
of the line) and found there was 1 for every side of the shape i.e.
for a pentagon 5, square 4 etc. Therefore, the graph shown had 3
segments, so I worked out it was a triangle (obviously!).
Michael from St John Payne School went a
little further in exploring where the dot might be placed:
First I knew it had to be on the vertex because there was a part of
the graph that was flat.
The only point at which the dot isn't travelling anywhere when the
polygon is rolling is on a vertex, because then the dot is always
in contact with the floor.
He then looked at the units on the graph to
consider which vertex the point would be on:
This told me the time period for one turn about a vertex was
roughly 2.5 units as this was the length of time for which the
gradient was zero.
This meant that the polygon had gone through 2 cycles before
it was going to pivot on the vertex that held the dot (because
there was roughly 5 units of time before the gradient was zero and
5/2.5 = 2) .
This meant that the red dot had to be 2 vertices after the
bottom right vertex.
This gave enough grounds for trial and error to find the right
polygon fairly quickly.
Thank you to Ken Nisbet, the class
teacher of 4YP at Madras College in St Andrews, Scotland, for
sending us this message and the very impressive work carried out by
some of his students:
Hi to 'The Nrich Team'
My 'top' S4 set were this month inspired by your various
'Rolling Polygon' problems.
We decided to investigate the total distance travelled by the
dot, when it was placed at one of the vertices, from 'take-off' to
'landing' when each polygon had side length 1 unit.
The class divided into groups of 2 or 3 students. Exact values
were the order of the day. The Equilateral triangle and Square were
fine though much discussion was needed regarding the final form of
the answers. The pentagon proved a lot more challenging with the
Golden ratio eventually surfacing.
The overall pattern in the answers were quite remarkable with
a very curious sequence emerging:
1, 2, 5, 12, ...
The groups spent over a week working on this investigation
with excitement mounting as the sequence developed. Predictions
were made at the stage when the hexagon revealed the number 12 as
the 4th term. For the Septagon exact values were not possible so
conjectures for the 5th term of the sequence were tested using very
accurate calculator work (Sine Rule & Cosine Rule etc).
What was a complete surprise was that a non-integral value for
the 5th term emerged. There are not many situations, at this level
of work, where approximate values can be harnessed to disprove an
integer sequence conjecture. "This must be wrong" was the general
feeling and all the sums were checked again and again... but
eventually calculations by three separate independent groups
confirmed the result.
I feel that the challenge and sheer range of technique
required for this investigation has benefited my students
immensely. They are aged 14 to 15 and produced work of impressive
depth and quality. Thanks for the stimulation that your questions
have provided ... keep up the great work. I have attached
the write-up produced by David, Nicholas
and Robert as it was a superb exposition.... I'm sure you will
All the best Ken Nisbet
We do agree! Many thanks.