# Physics Problem, Week 2

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#### Saiph

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<p>Alrighty, even though I have yet to post "Physics Solution, Week 1" as my scanner is misbehaving, I will post this weeks problem...only 3 days late.&nbsp; A detailed solution and a nice handwritten copy of my work will be posted as soon as possible.</p><p>Feel free to post your answers as you get them, and if you wish to post your reasoning or work please do so after, say, wednesday. </p><p>----------------</p><p>This weeks problem is an extension of last weeks problem.&nbsp; So we shall use the same initial setup.&nbsp; I.e. a 12kg block, on a wooden board, coefficient of friction is 0.2 static, and 0.1 kinetic.&nbsp; However!&nbsp; there is a a twist.</p><p>a) At what angle must we tilt the board, for the block to begin sliding due only to gravity?</p><p>b) The board is only 15m long, How fast is the block moving at the bottom of the board&nbsp;</p><p>c) Assuming there is a smooth transition between the tilted board, and the level surface, how far will the block travel (assume the surface has a kinetic friction coefficient of 0.8).</p><p>d) How much energy is wasted via friction?&nbsp;</p><p>You may assume the acceleration of gravity = 10 m/s^2, and you may use either newtonian forces, or energy theorems to provide your answers.&nbsp;</p><p>--------------------&nbsp;</p><p>Preview for next week:&nbsp; The loop-da-loop! </p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>...d) How much energy is wasted via friction?&nbsp;...Posted by Saiph</DIV><br /><br />Geez, physicists !!!</p><p>What is this ? A philosophical question ?</p><p>Do you want to know how much energy is dissipated by friction at the bottom of the board ?&nbsp; At the end of the run (where the answer is all of it)?&nbsp; Or what ?&nbsp; The amount wasted rather depends on your objective doesn't it ?&nbsp; If I rub my hands together all of the energy is turned into heat by friction, but that is why I was rubbing them -- to warm them up -- so none of it is wasted.</p><p>An engineer,&nbsp;a physicist and amathematician are traveling&nbsp;by car and pass a rancher's pasture.&nbsp; The&nbsp;e ngineer&nbsp;says, "I see a gray horse."&nbsp; The mathematician says, "I see a horse that is gray on at least one side."&nbsp; The physicist says, " Consider a spherical horse."</p> <div class="Discussion_UserSignature"> </div>

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#### Saiph

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What...they're not spherical? <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### Mee_n_Mac

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<p>May we make the definition of the boards length, 15m, to be 15m from the leading edge of the block to the edge of the smooth transition area ? Just saves any arguments as to how far the block has to move before it encounters said transition area. </p><p>I'll (try to) start any discussion with my answer for a.&nbsp; Again I'll leave out any derivation for the moment so lurkers may check their results vs mine and ... well .... discuss.&nbsp; I note that the answer doesn't depend on the acceleration of gravity nor the mass of the block, just the static coefficient of friction.</p><p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p><p>Spherical horses ...&nbsp;indeed.&nbsp; <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-laughing.gif" border="0" alt="Laughing" title="Laughing" /></p><p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p><p>Oh yeah, the answer .... when the angle =&nbsp;arctan(0.2) the block will be at the point of sliding.&nbsp; About 11.31 degrees.</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> <p>-----------------------------------------------------</p><p><font color="#ff0000">Ask not what your Forum Software can do do on you,</font></p><p><font color="#ff0000">Ask it to, please for the love of all that's Holy, <strong>STOP</strong> !</font></p> </div>

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#### Saiph

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<p>Of course you can define the length of the board that way. </p><p>&nbsp;</p><p>Sorry for the delay, real life reared it's ugly head (and a really good book).&nbsp;</p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>... (and a really good book).&nbsp; <br />Posted by Saiph</DIV></p><p>Once upon a time there was a spherical cow. ......<img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-wink.gif" border="0" alt="Wink" title="Wink" /><br /></p> <div class="Discussion_UserSignature"> </div>

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#### Mee_n_Mac

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d) All of it ! <div class="Discussion_UserSignature"> <p>-----------------------------------------------------</p><p><font color="#ff0000">Ask not what your Forum Software can do do on you,</font></p><p><font color="#ff0000">Ask it to, please for the love of all that's Holy, <strong>STOP</strong> !</font></p> </div>

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#### Saiph

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<p>Sorry for the delay in posting guys, my computer is currently terminally ill.&nbsp; I'm pretty sure my 8 year old, 20gb hard drive has finally bit the dust...or it could be my ram's gone bad (again, my mobo seems to eat the stuff for breakfast).</p><p>I'll update the physics problem threads as soon as thats up, otherwise I'm stealing time from work during breaks for my posting...assuming it doesn't get insanely busy agian.</p><p>&nbsp;</p><p>btw, mee_n_mac has posted the correct answers for part a and d (with his tongue only slightly in his cheek on d )&nbsp;</p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### Mee_n_Mac

##### Guest
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Sorry for the delay in posting guys, my computer is currently terminally ill.&nbsp; I'm pretty sure my 8 year old, 20gb hard drive has finally bit the dust...or it could be my ram's gone bad (again, my mobo seems to eat the stuff for breakfast).I'll update the physics problem threads as soon as thats up, otherwise I'm stealing time from work during breaks for my posting...assuming it doesn't get insanely busy agian.&nbsp;btw, mee_n_mac has posted the correct answers for part a and d (with his tongue only slightly in his cheek on d )&nbsp; <br />Posted by <strong>Saiph</strong></DIV><br /></p><p>Slightly .... ?&nbsp; &nbsp;<img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-wink.gif" border="0" alt="Wink" title="Wink" /></p><p>&nbsp;</p><p>If I can avoid death-by-wife and have time after getting the new PWC, I'll see if I can write something up for b & c this weekend. I'm going to have to scratch my head a bit to see how handle the transistion w/o going beyond what I think is the scope of the lesson.</p> <div class="Discussion_UserSignature"> <p>-----------------------------------------------------</p><p><font color="#ff0000">Ask not what your Forum Software can do do on you,</font></p><p><font color="#ff0000">Ask it to, please for the love of all that's Holy, <strong>STOP</strong> !</font></p> </div>

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#### DrRocket

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<p>This is largely an experiment to see if I can successfully post a scanned version of a solution worked out by hand.&nbsp; If it works this is my attempt at a solution to the week 2 problem.</p><p><br /><img src="http://sitelife.space.com/ver1.0/Content/images/store/14/13/8e32c238-1a9d-4dc1-9d92-7c7223444ba1.Medium.gif" alt="" /><br /><br /><img src="http://sitelife.space.com/ver1.0/Content/images/store/15/5/9ffa348c-2e1c-4903-b5f2-40731848a0c7.Medium.gif" alt="" /><br /></p> <div class="Discussion_UserSignature"> </div>

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#### Saiph

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<p>my intentions were to make the transition a simple discontinuity.&nbsp; I.e. at x distance (i.e. 15m) you now use a different set of equations.&nbsp; Don't over-complicate it, I certainly won't.</p><p>&nbsp;</p><p>DrRocket:&nbsp; Posted fine, the images are bit small, but legible...&nbsp;</p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### DrRocket

##### Guest
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>my intentions were to make the transition a simple discontinuity.&nbsp; I.e. at x distance (i.e. 15m) you now use a different set of equations.&nbsp; Don't over-complicate it, I certainly won't.&nbsp;DrRocket:&nbsp; Posted fine, the images are bit small, but legible...&nbsp; <br />Posted by Saiph</DIV></p><p>If I click on the image to get the large picture and then magnify that the size seems about like the original, but there is a little fuzz in the image. Not too bad.&nbsp; If you can read the small figures, then I would be more than willing to trade eyes with you.<br /></p> <div class="Discussion_UserSignature"> </div>

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