What is the Minimum Speed of a Pendulum Released from a Height of 0.400 m?

[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!

 

[Doc Al]

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.)

 

[IB]

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.)

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?

 

[Doc Al]

Could the minimum speed of the pendulum be "0"?

Of course.

Or should we determine the minimum speed at exactly the highest point, when it gets its best gravitational energy?

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

 

[IB]

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