Why is mg=mv^2/r for centripetal motion instead of mg+T=mv^2/r?

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    Rope Tension
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Homework Help Overview

The discussion revolves around the dynamics of centripetal motion, specifically questioning the relationship between gravitational force, tension, and centripetal acceleration. Participants are examining the forces acting on an object in circular motion and the conditions under which certain equations apply.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are exploring why the equation mg=mv^2/r is used instead of mg+T=mv^2/r, with some questioning the role of tension and gravity in the context of centripetal motion.

Discussion Status

There is an ongoing exploration of the forces involved in the scenario, with some participants suggesting that tension may not contribute to the net force at certain points in the motion. Clarifications about the conditions under which tension is relevant are being discussed.

Contextual Notes

Participants are considering the specific case of a pendulum completing a full circle, where the presence of tension may vary depending on the position in the motion. There is a focus on understanding the forces at play, particularly at the top of the circular path.

jack1234
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Hi, for this question
http://tinyurl.com/2qgco5
this is the solution for (b)
http://tinyurl.com/yrl64q

May I know why for centripetal motion, it is mg=mv^2/r, but not mg+T=mv^2/r (T is the tension of the rope)?
 
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Because the rope is acting perpendicular to the motion and is prescisely enough to offset the force of gravity in that direction.
 
Sorry, I am not very clear for the explanation.
I assume "the rope is acting perpendicular to the motion" is the downward force for the tension of rope, visually, it is two forces acting on the object, one is the tension of rope, one is the gravitational force, I am not sure what is the incorrectness of my thinking...
 
jack1234 said:
Sorry, I am not very clear for the explanation.
I assume "the rope is acting perpendicular to the motion" is the downward force for the tension of rope, visually, it is two forces acting on the object, one is the tension of rope, one is the gravitational force, I am not sure what is the incorrectness of my thinking...
Don't forget that you are looking for just enough energy to have the pendulum complete a full circle around the peg...in which case there is no tension at the top of its circle, and the only force acting on the pendulum is gravity at that particular point.
 

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