Solving Pendulum Problem (b): Kinetic & Potential Energy

[jack1234]

Hi, for this question
http://tinyurl.com/2qgco5
I have problem for question b

My attempts for question (b)
The potential energy of the pendulum when it is released from the horizontal position
=The kinetic energy of pendulum at lowest point
=mgL

In order to make it swing in a complete circle, it needs to reach the height 2(L-d)
hence mgL=mg(2(L-d)); and I get d=L/2

This is wrong, what it wants is d=3/5L, may I know what is my mistake, and what is the correct approach?

 

[Doc Al]

In order to make it swing in a complete circle, it needs to reach the height 2(L-d)
hence mgL=mg(2(L-d)); and I get d=L/2

This assumes that the sphere can make a complete revolution about the peg with a speed of zero at the top. This is incorrect--figure out the minimum speed needed at the top. Hint: Consider Newton's 2nd law and circular motion.

 

[jack1234]

Thanks!

 

[jack1234]

I have another question here
https://www.physicsforums.com/showthread.php?p=1526743#post1526743

This question solution seems not to put the kinetic energy at the highest point into consideration, what is the reason?

 

[Doc Al]

See my response in the other thread.