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# Math: 2 Coins Problem

DUCM900

User ID: 32573380
Italy
01/19/2013 11:00 AM
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Math: 2 Coins Problem
Listen to that guy:

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Last Edited by IWASTHERE on 01/19/2013 11:28 AM
Anonymous Coward
User ID: 32358435
United States
01/19/2013 11:03 AM
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Re: Math: 2 Coins Problem
3.14159265 turns.
mirabe

User ID: 19542799
Australia
01/19/2013 11:11 AM
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Re: Math: 2 Coins Problem
half a full turn
DUCM900 (OP)

User ID: 32573380
Italy
01/19/2013 11:17 AM
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Re: Math: 2 Coins Problem

How PI or 3.14 was discovered and which tools were used 2.000 year ago?

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Last Edited by IWASTHERE on 01/19/2013 11:22 AM
Anonymous Coward
User ID: 32358435
United States
01/19/2013 11:20 AM
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Re: Math: 2 Coins Problem

How PI or 3.14 was discovered and which tools were used?

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Quoting: DUCM900

Pothagarus knew that you need 3.1 of a circles radius to make the area of a circle. Honestly he just drew a circle and got a piece of string the length of the radius and measured it out he knew it was close to 3.1 and decades later we realized that it was 3.14159265...
Anonymous Coward
User ID: 32390079
United States
01/19/2013 11:23 AM
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Re: Math: 2 Coins Problem

How PI or 3.14 was discovered and which tools were used?

.
Quoting: DUCM900

Pothagarus knew that you need 3.1 of a circles radius to make the area of a circle. Honestly he just drew a circle and got a piece of string the length of the radius and measured it out he knew it was close to 3.1 and decades later we realized that it was 3.14159265...
Quoting: Anonymous Coward 32358435

cough~Pythagoras~cough
Anonymous Coward
User ID: 32358435
United States
01/19/2013 11:25 AM
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Re: Math: 2 Coins Problem

How PI or 3.14 was discovered and which tools were used?

.
Quoting: DUCM900

Pothagarus knew that you need 3.1 of a circles radius to make the area of a circle. Honestly he just drew a circle and got a piece of string the length of the radius and measured it out he knew it was close to 3.1 and decades later we realized that it was 3.14159265...
Quoting: Anonymous Coward 32358435

cough~Pythagoras~cough
Quoting: Anonymous Coward 32390079

Fuck spelling fuck 12
DUCM900 (OP)

User ID: 32573380
Italy
01/19/2013 11:49 AM
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Re: Math: 2 Coins Problem

Now, can someone reply with math formula what that guy explains graphically in the above video?

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Anonymous Coward
User ID: 1018225
United States
01/19/2013 12:29 PM
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Re: Math: 2 Coins Problem

Now, can someone reply with math formula what that guy explains graphically in the above video?

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Quoting: DUCM900

1 full revolution of the top coin = 1 full revolution of the top coin.

To compare the rotation of the two coins if they were gears with equal teeth and radius it would take 1 full revolution of the system to return to start position.

In his example the top coin is still making one full revolution, however because its origin is moving in a circle itself instead of stationary the rotation of the bottom coin is transferred to the top coin, so the entire system is only making 1/2 of a revolution to return the top coin to its original rotation.

x=360/2
x is the target angle in degrees.
360 represents the degrees in a circle.
the total is divided by 2 because a circle rolling around a stationary circle has its rotation multiplied by two if they are equally sized.

x=360/2
DUCM900 (OP)

User ID: 32598679
Italy
01/19/2013 06:51 PM
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Re: Math: 2 Coins Problem
Hi there, do you mean that the exernal gear do twice revolution?

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Anonymous Coward
User ID: 27112816
United States
01/19/2013 07:34 PM
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Re: Math: 2 Coins Problem
Lets pretend that the radius of each coin is 1 to make it simple like a unit circle.

So the length of the perimeter of the center coin is 2*PI and the length of the revolving coin is the same of course).

Moving clockwise:

START: at the top it is 0*PI Rads
at the right side it is PI/2 Rads
1/2 WAY AROUND: at the bottom it is PI Rads
at the left side it is 3/2*PI Rads
FINISH: at the top it is 2*PI Rads

So when it sits on top of the center coin, the contact point is directly under the 500 on the rotating coin, when it reaches the bottom of the center coin the contact point reaches PI rads on the rotating coin (the top of the rotating coin above the 500). Then it travels PI rads around the center coin once again reaching the contact point where it is exactly below the 500 on the rotating coin once again.

Make sense?