About Kinetic energy of circular motion

1. Dec 3, 2007

jack1234

Last edited: Dec 3, 2007
2. Dec 3, 2007

G01

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.

3. Dec 3, 2007

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?

4. Dec 3, 2007

G01

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.

5. Dec 3, 2007

jack1234

Hi, for your reference, this is the question for pendulum bob that I asked before:

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

6. Dec 3, 2007

Staff: Mentor

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.

7. Dec 3, 2007

jack1234

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