About Kinetic energy of circular motion

[jack1234]

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

Question B,
Question
http://tinyurl.com/2qgco5
Solution
http://tinyurl.com/yrl64q

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)

 

[G01]

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.

 

[jack1234]

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?

 

[G01]

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]

Hi, for your reference, this is the question for pendulum bob that I asked before:
https://www.physicsforums.com/showthread.php?t=201906

The pendulum is going around to the top of the loop with respect the "peg".

 

[Doc Al]

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.

 

[jack1234]

I see, a simple keyword make so much different! Thanks a lot :)