# I think my answer should be right, but it isn't energy problem

## Homework Statement

A light rigid rod of length L has a mass m attached to its end, forming a simple pendulum. It is inverted and then released (I don't know what that's supposed to mean). What are:

a) the speed v at its lowest point
b) the tension T in the suspension at that instant

## The Attempt at a Solution

at the beginning, when it is dropped down, we should have K = 0 and U = mgL

at the bottom when it is finished dropping, we should have K = 1/2mv^2 U = 0

1/2 mv^2 = mgl

v = sqrt(2gL)

but the book says 2sqrt(gl)...

what did I do wrong?

thanks

I'm guessing "inverted" means the mass is a height L above the pivot point, so it's height above its lowest point is actually 2L rather than L

WOW. Thanks so much, I would never have known...

b) the tension T in the suspension at that instant..

Shouldn't it just be the negative weight? what answer book says 5mg.. :S

Thanks

PhanthomJay
Homework Helper
Gold Member
WOW. Thanks so much, I would never have known...