# Homework Help: Rotational KE. , help pls

1. Mar 15, 2009

### makeAwish

Rotational KE. urgent, help pls!!

1. The problem statement, all variables and given/known data

Two metal disks, one with radius = 2.54cm and mass = 0.820kg and the other with radius = 4.98cm and mass = 1.68kg , are welded together and mounted on a frictionless axis through their common center.

a.What is the total moment of inertia of the two disks?

I have found the answer to be 0.00235 kgm^2

b. A light string is wrapped around the edge of the smaller disk, and a 1.50-kg block, suspended from the free end of the string. If the block is released from rest at a distance of 2.10m above the floor, what is its speed just before it strikes the floor?

c. Repeat the calculation of part B, this time with the string wrapped around the edge of the larger disk.

The attempt at a solution

How do u find part 2?? i use consevation of energy,

where initial KE = 0 and final U = 0
initial U = mass of block * g * 2.1
KE of block = 0.5mv^2
KR of wheels = 0.5 * total inertia * omega ^2, where omega = v^2/radius of small wheel

but i cant get ans. where have i gone wrong?
can someone help me pls? urgent!!

thanks.

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2. Mar 15, 2009

### tiny-tim

Hi janettaywx!

(what happened to that ω i gave you in the other thread? )