About Kinetic energy of circular motion

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Homework Help Overview

The discussion revolves around the kinetic energy of objects in circular motion, specifically comparing two scenarios involving a pendulum bob and a rigid rod. Participants are exploring the assumptions regarding kinetic energy at the highest point of a circular path.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants question why the solution for one problem assumes negligible kinetic energy at the top of the circle while another problem does not. There is a focus on the conditions under which these assumptions are valid, particularly regarding the tension in the string and the speed of the bob.

Discussion Status

The discussion is ongoing, with participants seeking clarification on the differing assumptions between the two questions. Some guidance has been offered regarding the necessity of tension in the string for the pendulum bob to maintain motion, but no consensus has been reached.

Contextual Notes

Participants are referencing specific questions and solutions from external links, which may limit the context available to others not privy to those resources. There is also mention of the pendulum bob's motion concerning a peg, which adds complexity to the assumptions being discussed.

jack1234
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jack1234 said:
This is the question,
http://tinyurl.com/ywxlyu
And this is the solution:
http://tinyurl.com/yt4qn2

May I know why the solution assume it does not need kinetic energy at the highest point to go through the complete circle?

The bob is assumed to "barely make it around the circle." This tells us that the bob's speed is almost zero at the top of the circle. Since the speed is so small up there, the kinetic energy would be very, very close to zero. This means that the approximation of kinetic energy = 0 at the top of the loop is justified.
 
Hi, thanks, but I have updated my question for including a question B for compare.
May I know why in question B, we cannot assume this?
 
jack1234 said:
Hi, thanks, but I have updated my question for including a question B for compare.
May I know why in question B, we cannot assume this?

I am not sure what you are asking. Where in the solution to this second problem would you like to make that assumption? The pendulum bob is not going around the top of the loop.
 
jack1234 said:
May I know why the solution for question A assume it does not need kinetic energy at the highest point to go through the complete circle? But question B assume it needed?(mg=mv^2/r)
For a bob attached to a flexible string to make it around the loop, the string must have some tension in it at all points. Not so for the rigid rod. In order for the string to have some slight non-zero tension at the top of the loop, a minimum speed is required.
 
I see, a simple keyword make so much different! Thanks a lot :)
 

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