Force required to hold a pendulum at an angle

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SUMMARY

The minimum horizontal force "F" required to hold a simple pendulum with mass "m" and string length "l" at an angle of 60 degrees from the vertical is definitively calculated as F = mg/√(3). This conclusion is derived from applying standard mechanics, specifically the relationship F/mg = tan(θ) where θ is the angle of the string. The alternative method using the Law of Conservation of Energy is unnecessary and leads to confusion, as it does not align with the straightforward mechanics approach.

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  • Understanding of basic mechanics, specifically forces and angles.
  • Familiarity with the concept of pendulum motion.
  • Knowledge of trigonometric functions, particularly tangent.
  • Ability to apply the Law of Conservation of Energy in physics.
NEXT STEPS
  • Study the derivation of forces in pendulum systems using trigonometric identities.
  • Explore the implications of the Law of Conservation of Energy in different mechanical systems.
  • Learn about the dynamics of pendulum motion under various forces and angles.
  • Investigate the effects of varying mass and string length on pendulum behavior.
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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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