Pulley Mass Calculation: Atwood's Machine

[becca89]

Homework Statement

The two masses (m1 = 5.0 kg and m2 = 3.0 kg) in the Atwood's machine shown in Figure 10-23 are released from rest, with m1 at a height of 0.92 m above the floor. When m1 hits the ground its speed is 1.1 m/s. Assume that the pulley is a uniform disk with a radius of 12 cm.

Determine the pulley's mass.

Homework Equations

Conservation of Energy E_f=E_i
K= 1/2mv²+1/2Iw² (w = omega)
I = 1/2mr²

The Attempt at a Solution

I tried to cancel out as much as I could form the "K" equation and fill it in. Webassign keeps determining my answer wrong...

 

[Doc Al]

Show what you did. Hint: Don't forget gravitational potential energy.