[78-L] how long is it?

[Malcolm]

That was quick!
An approximation is all that was needed.
Thank you, Per.
Malcolm

*******

On 9/25/2016 10:25 AM, Per Ahlin wrote:
> This is an approximate solution.
>
> A recording of exactly 3 min at exactly 78 rpm
> will consist of 3 x 78 = 234 revolutions.
> The circumference of a circle of radius r is
> 2 x pi x r. Assume that the radius varies from
> 5 to 1 inches. The average radius is 3 inches.
> The total length is approximately:
> 234 x 2 x pi x 3 = 4411 inches.
>
> Per Ahlin
>
>
>> I just got an interesting question from a friend who has nothing to do
>> with 78s! He asked: "If a record didn't go round  and round but a 3
>> minute (groove) went straight,  how (long would the groove be)?"
>>
>> To which I replied: "There's got to be a math formula for this - how
>> does one figure out the length of a decreasing spiral over 2 radial
>> dimensions? First, you'd have to know the pitch of the groove - that's
>> how many lines per inch along a radius the groove makes. Secondly,
>> you'd have to measure the inside and outside radii of the spiral. I
>> think that's all that is needed to plug into a set formula. Someone in
>> 78-land will know this. I'll ask."
>>
>> Anyone?
>>
>> Malcolm
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