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Author Topic: MASS/MATHS Question for geeks?  (Read 6108 times)

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amazonian

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MASS/MATHS Question for geeks?
« on: 20 January 2018, 15:39:35 »
Can any of you guys answer this?

A cylinder or disc (say the rough shape of a small roll of cellotape) has a diameter of 500mm.
It is 51mm long and it has a bore of 203mm.

The question is,  if the diameter of 500mm was reduced by 18mm,  what percentage of mass would be lost?

I have tried twice and been wrong both times, and cant think what I am missing (apart from numerous brain cells)

 :) ;)
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Doctor Gollum

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Re: MASS/MATHS Question for geeks?
« Reply #1 on: 20 January 2018, 16:02:58 »
AreaT equals 2(3.142x250)-2(3.142x101.5)
Volume equals AreaT x 51.

Are you losing 18mm from the outside diameter or is the bore increasing by 18mm?

Your answer will depend on which... ;)

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amazonian

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Re: MASS/MATHS Question for geeks?
« Reply #2 on: 20 January 2018, 16:29:42 »
Yes, losing 18mm from the outside diameter.
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Doctor Gollum

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Re: MASS/MATHS Question for geeks?
« Reply #3 on: 20 January 2018, 18:25:31 »
OK, new equation is:

AreaT2 equals 2(3.142x241) - 2(3.142x101.5)

New VolumeT2 equals AreaT 2 x 51.

The difference is obviously VolumeT - VolumeT2 :y
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mantaray

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Re: MASS/MATHS Question for geeks?
« Reply #4 on: 20 January 2018, 20:37:55 »
Don't forget, the area of a circle is π r².
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Doctor Gollum

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Re: MASS/MATHS Question for geeks?
« Reply #5 on: 20 January 2018, 21:37:07 »
Rookie mistake :-[

2piR is the circumference  ::)

Adjust the math accordingly, but the order of the workings is sound ;)
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Re: MASS/MATHS Question for geeks?
« Reply #6 on: 21 January 2018, 13:30:16 »
Quote from: Doctor Gollum on 20 January 2018, 21:37:07
Rookie mistake :-[

2piR is the circumference  ::)

Adjust the math accordingly, but the order of the workings is sound ;)
Maths , were English not American old chap  :y
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Doctor Gollum

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Re: MASS/MATHS Question for geeks?
« Reply #7 on: 21 January 2018, 19:17:13 »
Quote from: redelitev6 on 21 January 2018, 13:30:16
Quote from: Doctor Gollum on 20 January 2018, 21:37:07
Rookie mistake :-[

2piR is the circumference  ::)

Adjust the math accordingly, but the order of the workings is sound ;)
Maths , were English not American old chap  :y
Who was?
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amazonian

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Re: MASS/MATHS Question for geeks?
« Reply #8 on: 21 January 2018, 22:09:51 »

 Ha ha!  Touché .

 :) :)
 
 




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Doctor Gollum

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Re: MASS/MATHS Question for geeks?
« Reply #9 on: 22 January 2018, 00:31:18 »
Quote from: Doctor Gollum on 20 January 2018, 16:02:58
AreaT equals 3.142(250x250)-3.142(101.5x101.5)
VolumeT equals AreaT x 51.

AreaT2 equals 3.142(241x241) - 3.142(101.5x101.5)

New VolumeT2 equals AreaT 2 x 51.

The difference is obviously VolumeT - VolumeT2 :y

In answer to the original question...  ;)
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Re: MASS/MATHS Question for geeks?
« Reply #10 on: 22 January 2018, 00:33:22 »
Quote from: amazonian on 21 January 2018, 22:09:51

 Ha ha!  Touché .

 :) :)
 
Leave the door open wide enough and someone will eventually walk through it :P
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amazonian

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Re: MASS/MATHS Question for geeks?
« Reply #11 on: 22 January 2018, 22:16:07 »
Dr Gollum
                                                                                                                                                          It is so long since my school maths lessons I cannot even remember what to do with figures in brackets etc.

One of my grandsons lives in Finland and his maths book shows a completely different method to my English grandsons, and both are different to my own vague memories.

I made the answer 6.1%,  my son says it is 8.4%.  I don't know what you made it for the above stated reason,  but thank you very much for taking the trouble to reply.

 :y :)

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Andy H

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Re: MASS/MATHS Question for geeks?
« Reply #12 on: 22 January 2018, 22:51:54 »
Quote from: amazonian on 20 January 2018, 15:39:35
Can any of you guys answer this?

A cylinder or disc (say the rough shape of a small roll of cellotape) has a diameter of 500mm.
It is 51mm long and it has a bore of 203mm.

The question is,  if the diameter of 500mm was reduced by 18mm,  what percentage of mass would be lost?

I have tried twice and been wrong both times, and cant think what I am missing (apart from numerous brain cells)

 :) ;)
Ignore the length - it is common to both cases.

Using area = pi.r2

case 1 = pi x 2502 - pi x 101.52
case 2 = pi x 2412 - pi x 101.52

percentage loss = [case 1 - case 2] / case 1 x 100%

I make it 8.46% (rounded to 2 decimal places)
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LC0112G

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Re: MASS/MATHS Question for geeks?
« Reply #13 on: 23 January 2018, 10:15:37 »
A roll of cellotape has a plastic & glue outer and a cardboard centre. Reducing the diameter by removing some/all of the tape won't alter the mass in the way  these sums predict due to the differences in density of the two materials.

So, is the disk a constant density?  ::)
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