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        riddles >> microsoft >> Spinning record disc
        
(Message started by: Nick on Feb 13^th, 2003, 12:25am)

Title: Spinning record disc
Post by Nick on Feb 13^th, 2003, 12:25am

Dear all,
Another classic.

Imagine a disk spinning like a record player turn table. Half of the disk is black and the other is white. Assume you have an unlimited number of color sensors. How many sensors would you have to place around the disk to determine the direction the disk is spinning? Where would they be placed?

Nick

Title: Re: Spinning record disc
Post by aero_guy on Feb 16^th, 2003, 7:55am

Nick, every riddle you have put up is on the site somewhere. We are always looking for new riddles, but it would help if you could check to see if it is already there. I think William has a search function on the site.

Title: Re: Spinning record disc
Post by Eleanor Pickron on Jul 12^th, 2003, 10:55pm

This is interesting; there are two common and an unlimited number of cases that this can be divided into:

1. The black and white fields are concentric circles. No sensors will work in this case to determine direction.

2. The black and white fields are half circles, in which case 2 sensors placed at any interval, even right next to each other, will work.

Other:

Half can mean several spots of color, which would also render any sensors useless.

Title: Re: Spinning record disc
Post by asharma on Nov 26^th, 2003, 12:13am

What exactly is the assumption here. Whether the disk is initially stopped and we can put the sensors or the disk is spinning and now we have to put the sensors and check the direction.
And the answer posted by "Eleanor Pickron", the meaning of "right next to each other" is not clear, whether this means that put the 2 sensors one at white part and other at black or anywhere.

Title: Re: Spinning record disc
Post by John_Gaughan on Dec 12^th, 2003, 7:56pm

on 07/12/03 at 22:55:04, Eleanor Pickron wrote:

2. The black and white fields are half circles, in which case 2 sensors placed at any interval, even right next to each other, will work.

As long as the sensors are not directly across from each other it will work -- in that case the color fields will change at the exact same time, which is ambiguous.

Title: Re: Spinning record disc
Post by grimbal on Apr 30^th, 2004, 2:19pm

Suppose the disk is black on one face and white on the other. Suppose it is spinning horizontally around his "natural" axis. Have the sensor dangle down over the disk until it touches it. But not near the center. The direction the sensor moves from the vertical indicates the direction of rotation.

OK, I am not serious, but I like to be creative. And, what the hell, it works.

Title: Re: Spinning record disc
Post by coolnfundu on Nov 5^th, 2004, 1:25am

::)one ...

Title: Re: Spinning record disc
Post by Perez on May 5^th, 2005, 1:11pm

Just 2 sensors may not work if they lie on the radii of the circular disc as they will change colour at the same time. So the positioning of sensors is crucial.

Title: Re: Spinning record disc
Post by Icarus on May 5^th, 2005, 6:28pm

Note that in the question, the placement of sensors is part of the answer required.

Two sensors, placed very close to each other near the circumference of the disc are sufficient to determine direction unless the disc is spinning too fast for the response times of the sensors.

Title: Re: Spinning record disc
Post by viks on Apr 12^th, 2008, 12:24am

Two sensors sould suffic. Place the two senors adjacent to each other on the circumfference of the disk and say they are S1 and S2.

The output of the sensors with disk rotationg in clockwise direction will be

S1 |------______------_______ (reference sensor)

S2 |_------______------______ (anti-cloclwise)

S2 |-----______------_______ (cloclwise)

From the output of this graph (the shift in the direction of graph of S2) we can make in which direction the disk is rotating