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## Re: [Fhsst-physics] False Information in an example?

**From**: |
Markus Oldenburg |

**Subject**: |
Re: [Fhsst-physics] False Information in an example? |

**Date**: |
Sat, 21 Oct 2006 11:42:40 +0200 |

**User-agent**: |
Thunderbird 2.0a1 (Windows/20060724) |

Hello Lucas,

`thanks for coming back to us with your findings. It is great to hear
``that you can make good use of the book 'even in Switzerland'. ;-)
`

`I checked the worked example 75 and you are right. There is an error
``with the sin and the cos. Unfortunately there is another subsequent
``error, which renders the whole solution wrong (numerically). We will
``change the book accordingly.
`
For now here is how it is supposed to be:

`[Remark: All the text stays the same (except for the last sentence),
``just the equations change.]
`
First change: Step 3
F_E = T * cos (60 degrees) F_g = T * sin (60 degrees)
This is the error you spotted.

`Explanation: T is the 60 degree projection on F_E, therefore it has to
``be the cos. On F_g instead T is the 30 degree (90 degrees - 60 degrees)
``projection, which leaves us with cos (30 degrees) = sin (60 degrees).
`
Second problem: Step 3, second equation:

`This equation is mathematically correct, but the result is wrong. It
``should be T=115.5 N (not 1155 N).
`

`Subsequently, step 3, third equation (the cos (60 degrees) was and is
``correct here, the typo happened earlier as mentioned above):
``Putting the correct result for T from the last equation here gives us:
``F_E = 115.5 N * cos (60 degrees) = 57.75 N.
`
Finally, step 4:

`|Q_x| is numerically wrong, due to the error introduced earlier. The
``correct value is:
``|Q_x| = sqrt( F_E * r^2 / k) = sqrt ( (57.75 N) * (0.5 m)^2) / ( 8.99 *
``10^9 N * m^2 / C^2) )
` = 5.67 * 10^(-5) C
"This the charge on X is -5.67 * 10^(-5) C."

`Thanks again for finding this inconsistency. Please let me know if
``everything is clear to you now.
``What physics test are you learning for? Where in Switzerland are you?
``You can also contact me in German (not in French, yet ;-) ) if this is
``easier for you.
`
Greetings from Geneva,
Markus
address@hidden
Lucas Vogelsang wrote:

Hi,

`I have found your book via Wikibooks. I am currently learning for a
``physicstest and think that this book is quite helpfull for me, even
``though I am from Switzerland.
`
However one thing confused me:
The worked example 75 seems to be wrong.
It first says:
F_{E} = T*sin(60) and F_{g} = T*sin(60)
next time they refer to F_{E} as F_{E} = T* cos(60) = ...
So what is correct?

`I have found this in a pdf. It is on the pdf page 230 or the page 222
``according to the documents page numbers.
`
regards,
Lucas
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