Brain Teasers
Gramophone Record
Mad Ade has found an old style gramophone type record player, along with several 12 inch gramophone records to go with it, including the all time classic, "Stig O'Tonedeaf Hums the theme from M.A.S.H !". Each disc has a 4 inch diameter middle section and a quarter inch outer border. Ignoring the fact that the discs have only a single spiralling groove, but measuring from the outer edge inwards, Mad Ade determines that there are 100 separate grooves per inch on the remaining playing surface. How far does the needle travel during one complete play of one side of the record?
Answer
The needle only travels 3.75 inches, as it really only moves from the outer edge of the record to the middle piece.Hide Answer Show Answer
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Comments
Of course most people will realize there are really two answers to this question, since distance is a relative measure. Someone who takes a winding road by car to arrive in London travels 50 kilometers according to the odometer, but 25 kilometers as the crow flies, and less if you consider the curvature of the earth.
True, but the car did actually travel the full 50km, where as the needle does not really travel anywhere else other than the distance between the outer and inner part of the record, it is the record that does all the moving.
Fun - but it really should be under "Trick," right? Of course, in this
case, that might give the whole show away immediately.
case, that might give the whole show away immediately.
Why should it be under trick Dewtell? It is a straight forward question with a straight forward answer. No trickery. I only asked how far the needle traveled, where is the trick in that?
answer still isn't right. there is a hole in the middle so it's not 4 inch diameter - .25 for the outer section.
It realy is 4 inch diameter -.25 outer and then - at least .25 for the middle and the small buffer area around the hole.
It realy is 4 inch diameter -.25 outer and then - at least .25 for the middle and the small buffer area around the hole.
when i first read that I was like, "what?" and then when i read the answer i got it...kind of good teaser though
does the needle actually move? i've never seen a gramophone, even on tv, so i didn't know that...
i just said that, because the gramophone is assumed to be stationary, the needle didn't move at all.
i just said that, because the gramophone is assumed to be stationary, the needle didn't move at all.
Jun 11, 2009
The needle only travels 1.75" to the middle.
If the record has a 4" diameter then it has a 2" radius and 1/4" is the outer edge. It doesn't travel to the other side of the disc.
If the record has a 4" diameter then it has a 2" radius and 1/4" is the outer edge. It doesn't travel to the other side of the disc.
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