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free body diagrams
free body diagrams  
A rock attached to a string swings in a vertical circle. Which free body diagram could correctly describe the force(s) on the rock when the string is in one possible horizontal position?

When the rock in in the horizontal position there will be two forces acting on the rock; the force of gravity Fg and the force of tension in the string Ft. The force of gravity, of course, is always straight down while the force of tension is always directed along the length of the string and must, in this case, be directed horizontally. The answer must, therefore, be C since there is no normal force because the rock is NOT in contact with any surface that could exert such a force.


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James J. Kovalcin


I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 34 years. I am constantly updating my skills and have a particular interest in modern physics topics.

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