Screwed-up
A cylindrical helix is just a spiral on a cylinder, like an ordinary spring or the thread on a bolt. There are two kinds, a right-handed helix and a left-handed helix and in the case of a bolt one has to be turned clockwise to screw it down and the other anti-clockwise. If I turn a left-handed helix over (top to bottom) does it become a right handed helix? Give as simple an explanation as you can.
We received two different explanations to this problem.
Danny Ng, 16, from Milliken Mills High School in Canada sent us
this solution:
"Turning a left-handed helix over does not make it become a right
handed helix.
When you observe a bolt at eye level, you see the parallel
threads.
There are two kinds of threads:
1) go "down-right", from upper-left down to lower-right
2) go "down-left", from upper-right down to lower-left.
If you draw a line that is "down-right" on a piece of paper(the
line goes from upper-left down to the lower-right) and turn the
paper upside down (just like turning a left-handed helix over) you
will notice the line is still "down-right" and it never goes
"down-left"."
Harry (Xiaotian) Li (Age: 15) from Riccarton High School,
Christchurch, New Zealand, sent us this solution:
"Consider, for example, the spiralling direction of a right-handed
one:
To screw down (from top) turn clockwise.
To screw up (from top) turn anti-clockwise.
After that we turn it over (top to bottom).
To screw it down, still turn clockwise.
To screw it up, still turn anti-clockwise.
And also, two kind of helix look different (opposite each other).
When you turn just one of them over they are still opposite, which
confirms that it has not changed.
Therefore, when we turn a left-handed helix over (top to bottom).
It does not become a right-handed helix."
Paul Marcinkowski, aged 14, from the Harwich School in GB convinced
himself by making a helix, and contributed:
"No because I have made a left handed helix and whatever way you
turn it up it is still a left handed helix."