Physics Lecture Week 1

Saiph · May 2, 2008

Okay, so some of you may have noticed my physics pop quiz and thought I'm nuts, nobody's gonna work problems on their own!  Don't bring such things to our quiet amateur forum!Some of you may be interested in the material, but not know about the science behind it, or how to even begin solving such problems.  This lecture is for you.With every pop quiz there is a load of concepts integrated into the question.  And a lot of material if you wish to dig deeper than the strict "how do I solve it" mentality engineers have

So I hope to expound upon the material in the question, and hopefully explain how one solves such problems.Things taken for granted in this particular problem are balancing of forces, an understanding of the "normal force", decomposition of vectors (well, sorta, more on that next week) and probably a few things I'm over looking due to familiarity with the subject.So, on to the actual "lecture" and feel free to ask ?'s and discuss!  ---------------------- Part 1, Types of Friction, and coefficients ------------------------------This week's problem focuses primarily upon friction, and how it interacts with the motion of objects.  To understand how to solve this problem, one must examine the various components of the frictional force that motion across a surface.  Nobody knows, precisely, what causes friction, though there are a couple of general ideas about it.  Part of the problem is that there are actually two kinds of friction!  There is "static" friction and "kinetic" friction.  These vary in strength depending on the materials involved but always (as far as I know) the static friction is stronger than the kinetic friction.  Have you ever noticed that it's harder to start moving an object, than it is to keep pushing it once it's moving?  You'll push and pull and tug on that old refrigerator until it finally budges, and then once it's moving you have a much easier time of it.  Of course, at some point you stop pushing again, and once again, it's really hard to start sliding.  This is due to the differences in the two types of friction.  Friction in general is believed to be a combination of the two surfaces catching and snagging against eachother, even on an atomic scale.  The rougher the surfaces, the more they catch, the stronger the friction.  Unfortunately there are exceptions to this rule.  For instance if you take two highly polished and clean steel plates, and place them together, they can actually fuse together far stronger than if they were rough pieces of metal.  This is because there are two other mechanisms for friction.  One is sorta like a suction cup action, where small cracks and fissures in the surface of a material, even a smooth material, shift and flex when the objects are brought into contact.  This flexing can create a sort of suction seal that needs to be broken before the object moves.  Then there is of cold welding or bonding.  This is the main cause of the two steel plates sticking together.  If you bring the material close enough the molecules between the two objects will actually form weak bonds, melding the two materials together.  Of these three basic mechanisms for friction, two of them work best when the object is stationary, allowing them time to adhere (though all three work all the time).  And as a stationary has 3 strong frictional mechanisms at work, the resulting force due to friction, for a stationary object, is much higher than if the object is moving, where only the "catch and snag" mechanism works efficiently.  The strength of these two types of friction are contained and communicated by their "coefficients."  These are constants, reffered to with the greek letter mu (&micro

and are usually determined empirically.  The larger the coefficient, the stronger the frictional force.  Typically this value is less than one, however if you use adhesives, the value can exceed 1.  To calculate the frictional force then, you need to know the strength of each type of friction, and correctly use the right type!  If the object is sliding past the surface, you use kinetic friction.  If the object is stationary on the surface, you use the "static" friction.  There are cases where this can be hard to determine.  Take, for instance, a walking man.  The man is moving, so you would think to use the kinetic friction coefficient.  But you'd be mistaken.  The key is where the man touches the ground.  Usually a walking mans feet do not slide across the surface.  They stick firm, an the man then sorta falls past his feet.  Since his feet are not moving across the surface themselves you have to use the static friction coefficient. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"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" -- The Tenth Doctor, "Blink" </div>

DrRocket · May 2, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Okay, so ...static friction coefficient. Posted by Saiph</DIV>Nice lecture.  Excellent.I would add that your example of the static coefficient of friction applying to a walking man is spot on.  Folks might  be surprised to know that it is also the static coefficient that applies to the interface between a rolling tire and the pavement, until you start to skid.  This is because a tire, at the point of contact has zero velocity with respect to the pavement.  The rotational speed backward exactly compensates for the speed of the car in the forward direction.And to amplify your point that the precise atomic mechanisms of friction are not totally understood, it may interest some to know that Richard Feynman once decided to try to develop a fundamental theory of friction.  Now, Feynman was such a good physicist that he managed to develop a good explanation for quantum electrodynamics (not exactly and easy subject).  But he got nowhere in his search for a fundamental theory of friction.  <div class="Discussion_UserSignature"> </div>

Saiph · May 3, 2008

Okay, so I’ve discussed the cause(s) of friction, and why there are two different types.  But what determines the strength of the friction?  The answer is twofold.  First, is the materials in question, and their specific properties.  Are you looking at rubber on paper?  Or metal on concrete? Sheep on various surfaces? Etc…  Then you have to consider things like is it rough, smooth, how it’s textured, yada yada yada…   Only serious nuts actually try to figure these things out from scratch.  Most of the time the answer is determined empirically.  You drag the object and measure the forces required to start it moving, and to keep it moving.  This is all wrapped up in the coefficients I discussed in my last post.   The other key factor is pressure, namely how hard the two objects are pressed together.  This can be as simple as the weight of the object (on a level surface), to how hard you press the objects together (like sand paper and wood).  It can also get a bit more complicated, say if you’re concerned about the object on a tilted surface (a.k.a. ramp).  If the two surfaces are pressed together harder, then all three sources of friction I mentioned above work better.  Things deform and snag more often, the molecules bond together better, and more miniscule stress fractures arise to help “suction cup” the surfaces together.   This force pressing the two objects together is characterized as the “Normal Force.”  This normal force is actually the force exerted by the surface, on the object.  It’s related to gravity, and to other forces bringing the objects together, but it is explicitly independent as it can be proportional to combinations, or only portions, of many of these other forces.  This Normal Force is the force the surface exerts to prevent the object from passing through it.  For example it’s the force your chair is exerting upon your derrier keeping you from falling to the floor.   The reason it’s called the “Normal” force is because of the way the force is oriented compared to the surface creating it. It is always, always, always, 90 degrees to the surface (perpendicular, orthogonal or, gasp! NORMAL to the surface).  On a flat level table, the normal force is pointed straight up. If you tilt the table, the normal force is also tilted (by the exact same amount!)   The magnitude (read “strength” for those not steeped in physics lingo) of this force is exactly equal to, and opposite of any forces pressing straight down into the surface (i.e. also NORMAL to the surface, but pointed towards the surface).  Take our chair, derrier example.  The chair pushes up against your body with the exact same force that your body pushes down upon it.  They completely balance out and so you don’t move at all.  If they didn’t balance out you’d have one of two consequences:   A)    The chair does not push back hard enough, meaning the “downward” force of gravity isn’t completely nullified, and so you accelerate downwards…through the chair.  I.e. you broke the chair!   B)  The chair pushes back harder than you are pressed down into it, causing you to accelerate upwards, and be launched out of the chair.  Now, this doesn’t happen unless somebody spring loaded your chair as a nasty practical joke, so we can rule this one out.   Now, if the surface is tilted, you have to do a bit of math on the forces involved to figure out how much of the forces exerted upon the object are directed straight into the surface below.  Basically you have to break apart each force into separate components, a “vertical” and “horizontal” component.  The “vertical” components are directed perpendicular to the surface…which is why I put “s around the word vertical.  Afterall, in physics any way I want to call up, is up.  It’s only required that I be consistent.  These “vertical” portions of the force push the object straight into the surface, acting only to increase the friction.  They do nothing to move the object.    The “horizontal” portions are aligned parallel to the surface and do nothing to add the friction.  These horizontal portions act only to move the object!   If that last paragraph confused you a bit, you’re not alone.  Thousands of first years physics students struggle with the concept every semester.  I’ll also work to clear this up next week.   Luckily, this weeks problem is on a flat and level surface, and the force being applied is directly down (gravity) or directly across (pulling/pushing to move it).  As no force is at an angle, we don’t have to hurt our brains.   So, the normal force in todays problem is just like our chair-derrier example.  Gravity acts upon the block down towards the surface.  The surface exerts the Normal force, straight up and directly countering gravity.  So the normal force = gravity in this case.  To determine to the force of gravity…I’ll leave up to you for now.     -------------------------------- Jack Harvey, John Culvenor, Warren Payne, Steve Cowley, Michael Lawrance, David Stuart, and Robyn Williams of Australia, for their irresistible report "An Analysis of the Forces Required to Drag Sheep over Various Surfaces." [PUBLISHED IN: Applied Ergonomics, vol. 33, no. 6, November 2002, pp. 523-31.] WHO ATTENDED THE IG NOBEL CEREMONY: John Culvenor. ---------------------------- <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"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" -- The Tenth Doctor, "Blink" </div>

DrRocket · May 3, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>... Jack Harvey, John Culvenor, Warren Payne, Steve Cowley, Michael Lawrance, David Stuart, and Robyn Williams of Australia, for their irresistible report "An Analysis of the Forces Required to Drag Sheep over Various Surfaces." [PUBLISHED IN: Applied Ergonomics, vol. 33, no. 6, November 2002, pp. 523-31.] WHO ATTENDED THE IG NOBEL CEREMONY: John Culvenor. ---------------------------- Posted by Saiph</DIV>Nice "lecture" again.  The sheep example, with reference, is a really nice touch. However did you find this ?   It is hysterical, the more so because the authors are absolutely serious. <div class="Discussion_UserSignature"> </div>

Saiph · May 3, 2008

I've actually posted links to the site before, but nobody seems to peruse it.  The site is for the Ig Nobel, a prize commitee akin to the Nobel prize...  Here's the wiki for it: http://en.wikipedia.org/wiki/Ig_Nobel_PrizeThe prizes are handed out for improbable and quirky science, by actual Nobel Laureats!  the site is..or was, seems to be having problems atm: http://www.improbable.com/ <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"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" -- The Tenth Doctor, "Blink" </div>

Mee_n_Mac · May 6, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>  Luckily, this weeks problem is on a flat and level surface, and the force being applied is directly down (gravity) or directly across (pulling/pushing to move it).  Posted by Saiph</DIV> Are the "students" going to be given an explanation as to how the lecture specifically applies to the given Week1 problem(s) ? <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-laughing.gif" border="0" alt="Laughing" title="Laughing" />  <div class="Discussion_UserSignature"> -----------------------------------------------------Ask not what your Forum Software can do do on you,Ask it to, please for the love of all that's Holy, STOP ! </div>

Saiph · May 6, 2008

I'm going to leave the problem specific information in the solution thread.  which should be hammered out here in the next day or so.If, of course, anybody has questions as to how this relates to the problem, feel free to ask!  I like questions.  Much more entertaining than slapping up text pre-emptively.  <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"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" -- The Tenth Doctor, "Blink" </div>

Mee_n_Mac · May 6, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>I'm going to leave the problem specific information in the solution thread.  which should be hammered out here in the next day or so.If, of course, anybody has questions as to how this relates to the problem, feel free to ask!  I like questions.  Much more entertaining than slapping up text pre-emptively.  Posted by Saiph</DIV> Sounds good. Thx for your efforts. When "we" get around to relativity I gots some questions .... <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-surprised.gif" border="0" alt="Surprised" title="Surprised" /> <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-wink.gif" border="0" alt="Wink" title="Wink" /> <div class="Discussion_UserSignature"> -----------------------------------------------------Ask not what your Forum Software can do do on you,Ask it to, please for the love of all that's Holy, STOP ! </div>

Saiph · May 6, 2008

...relativity... right... you really think I'll go there? ....well, you'd be right.  If you have any pet topics, feel free to throw them out there.  I'm starting simple for others, and for myself (i'm outa practice), but I can put relativity on my short list.  <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"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" -- The Tenth Doctor, "Blink" </div>

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