Homework Help: Pendulum Minimum Speed

1. Nov 20, 2005

IB

A pendulum, with a 2.0 kg bob, is released from a height of 0.400 m and allowed to swing freely. Determine its minimum speed.
I got:
E_grav = E_kin
--> mgh = 1/2 mv^2
--> v = sqrt (2gh)
--> v = sqrt (2*9.8*.4)
--> v = 7.84 m/s
Is that right? It seems weird; so please tell me if i'm right or wrong. If wrong, please show me what I did wrong. Thanks a lot!

Last edited: Nov 20, 2005
2. Nov 20, 2005

Staff: Mentor

Your method is correct, but you forgot to take the square root. (I assume you were asked to find the maximum speed, not the minimum.)

3. Nov 20, 2005

IB

Ohh, I forgot that. sqrt (7.84) = 2.8
--> Isn't that the minimum speed?
if that's not the minimum speed, then...Hmm, how to find minimum speed..? Could the minimum speed of the pendulum be "0"? Or should we determine the minimum speed at exactly the highest point, when it gets its best gravitational energy?

Last edited: Nov 20, 2005
4. Nov 20, 2005

Staff: Mentor

Of course.
Assuming that its being "released" means that it starts at that point with zero speed, that's as high as it will get.

5. Nov 20, 2005

IB

Ahh, I see now. Thanks a lot, Doc Al. :)