Force required to hold a pendulum at an angle

AI Thread Summary
The discussion centers on calculating the minimum horizontal force required to hold a pendulum at a 60-degree angle from the vertical. Two methods were explored: one using standard mechanics, which resulted in F = mg√(3), and another applying the Law of Conservation of Energy, yielding F = mg/√(3). The latter approach was deemed incorrect by participants, who emphasized that conservation of energy is not applicable in this scenario. The correct solution is confirmed to be F = mg√(3), indicating a misunderstanding in the application of energy principles. The conversation highlights the importance of using appropriate mechanics for pendulum problems.
kabirtomer
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What is the minimum horizontal force "F" required to hold a simple pendulum (mass "m") with string of length "l" at an angle of 60 degrees with the vertical. [ given answer = mg/√(3) , veracity uncertain ]This at first seemed simple enough, standard mechanics apply, leading to :
F/mg = tan(θ) where "θ" is the angle of the string from the vertical

here:
F/mg = tan(60)
F = mg√(3)

My second attempt, using the Law of Conservation of Energy, lead to this :
Taking "mean" position of pendulum as the Origin and Assuming that bob is brought up -
Final ( KE + PE ) (of bob) = Initial ( KE + PE ) + Work done on bob
mgl(1-cos(60)) = 0 + Work done by Tension + Work done by F
mgl(1-cos(60)) = 0 + 0 + F (l sin(60))
mgl/2 = Fl √(3)/2
F = mg/√(3)
huh.
Why would they give different solutions?
I am assuming I did something wrong.
I apologize if it is hard to understand, I have trouble conveying it without a diagram.
 
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kabirtomer said:
What is the minimum horizontal force "F" required to hold a simple pendulum (mass "m") with string of length "l" at an angle of 60 degrees with the vertical. [ given answer = mg/√(3) , veracity uncertain ]This at first seemed simple enough, standard mechanics apply, leading to :
F/mg = tan(θ) where "θ" is the angle of the string from the vertical

here:
F/mg = tan(60)
F = mg√(3)


That is the correct solution. It is not a process, no sense to apply conservation of energy.
 
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