Please format all figures as inline Use Figure 1 descriptive title above each Also Tim see an interesting discussion of this expt in Feynman Physical Law Lecture 1 http www microsoft com presspass press 2009 jul09 07 14PhysicsLecturesPR mspx Gravitation in your Classroom Construction and Operation of an Inexpensive Cavendish Balance Demonstration Timothy W Hughes Jr Physics Department State University of New York Buffalo State College 1300 Elmwood Avenue Buffalo New York 14222 thughes williamsvillek12 org Abstract The Cavendish balance is an excellent appropriate tool in providing tangible evidence of gravitational force between two all ordinary room scale objects of mass HereMany articles have been written I review appropriate HS on the subject oliteraturef on design operation construction how it worksand making quantitative measurements of G with an inexpensive Cavendish balance and construction of this experiment Plug and play computer apparatus even exists for purchase of this experiment However few if any have had the goal of making this experiment easy and affordable Many problems also arise from this being a very sensitive experiment Here is where you will find how to tackle these obstacles in making a qualitative set up that will be indispensable in your physics classroom Tighten this last up to one sentence The info on commercial apparatus needs to be elsewhere Acknowledgements This paper addresses requirements for PHY690 Masters project at SUNY Buffalo State College under the guidance of Dr Dan MacIsaac I acknowledge the significant assistance of and the cooperation ofMr Timothy O Mara 1 Introduction In 1798 Henry Cavendish performed a powerful experiment that measured the universal gravitational constant G Carlson 1989 Actually he intended the experiment to measure the density of the earth and succeeded Clotfelter 1987 He never made any mention of a gravitational constant in his reports of on the torsion balance Clotfelter 1987 As Clotfelter 1987 reported a critical component of the unit of G is the newton and no unit of force was proposed until 1873 p 213 In this para you needlessly contradict yourself and are incomplete in the discussion of the history I would expand this one historical para to two and focus on getting it right Check the Wikipedia article for a little more guidance include a mention of previous researchwork done by Coulomb and you need to introduce the correct phrase torsion balance Then go into how a torsion balance works with a figure Comment on oscillating versus stationary balances Figure 1 Conceptual model of a Torsion Balance Is this figure yours You need full copyright to it Why the different font and weird fig 1 label Never the less it was discovered that the experiment is a reliable way to measure the gravitational constant and is described in most textbooks this way Of course in most high school classrooms it may seem a bit excessive and time consuming to challenge students to confirm this value through experimentation However the demonstration of the gravitational force between objects of mass other than the earth can be accomplished with a fair amount of effort and within a class period Moreover the experiment can be done rather cheaply and with items available at you local hardware store 2 Physics text books will tell you that everything that has mass has a gravitational attraction to every other thing that has mass Well this is hard to believe because the force is so small for ordinary objects that it is never noticed The only gravitational force most people recognize is the one between an object and the Earth But what if there was an instrument that was made so sensitive that it could detect these forces Better still what if this instrument could show directly that objects of relatively small mass were attracted to each other Too chatty in tone with rhetorical questions Please say what others have done then what you did what problems you found and then suggestions for others There is such an instrument and it is called the Cavendish balance To understand how this works one must know something about torsion It is the twisting of something by an applied torque A torque is a force applied about an axis perpendicular to it displacement from it Call it a twisting force Take a thread wire or any other long flexible object and if it is twisted it will tend to want to oppose this twisting force to restore its original shape The more you twist it the more strongly it opposes this force This is the characteristic of a torsion spring The strength of this spring depends on several factors one of which is length The longer the spring the weaker it is If we use a torsion spring to demonstrate the gravitational forces between two small objects we want the spring to be very weak thus making it very sensitive to extremely small forces If a mass is PAST TENSE hung at the end of a string it would be impossible to get the string to twist by approaching it with another mass because the gravitational force would always be acting upon the center of the hanging mass There would be no moment arm LAST IS UNCLEAR RESTATE OR EXPUNGE The mass must be offset from the axis of rotation for there to be any torque Since a string is not rigid we cannot expect to put a bend in the string at a right angle to achieve this The mass can be offset from the axis of 3 rotation if it is balanced by an object with an identical mass and separated with a rigid object The string hangs down attaches to the rigid object in the center and the masses balance on either end Now the string could conceivably be PAST TENSE THROUGHOUT PLEASE twisted by an attractive force on one or both of the two objects See figure 1 The mass on the other end does affect the experiment but placing another fixed mass next to it thus making the whole experiment symmetrical can offset this Since the Gravitational force is inversely proportional to the distance between the two objects the other two objects on the other side of the balance are sufficiently far away enough and at an angle such as to minimize the torque applied to not cause much error REALLY UNCLEAR AND POSSIBLY WRONG RETHINK AND RESTATE THIS LAST PARA PLEASE I think the size of the total torque is 2 Fgrav r due to two masses they BOTH COUNT Materials string a meter stick two 20 oz Soda bottles water three two gallon buckets CAVEAT plastic buckets lead to electrostatic forces 10 39X stronger than gravity use boxes of sand instead enough sand to
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