How Much Mass is Needed to Prevent the Wheel from Rotating?

[hey123a]

Homework Statement

A large wheel has a radius of 7.1 m. A rope is wrapped around the edge of the wheel and a 7.6 kg box hangs from the rope. A smaller disk of radius 1.38 m is attached to the wheel. A rope is wrapped around the edge of the disk as shown. An axis of rotation passes through the center of the wheel-disk system. What is the value of the mass M that will prevent the wheel from rotating?

Homework Equations

The Attempt at a Solution

all moments of inertia should equal zero so
m1r1^2 + m2r2^2 = 0
m1 = -m2r2^2/r1^2
m1 = -(7.6)(7.1)^2/1.38^3
m1 = 201

answer is actually 39kg

 

[ehild]

Homework Statement

A large wheel has a radius of 7.1 m. A rope is wrapped around the edge of the wheel and a 7.6 kg box hangs from the rope. A smaller disk of radius 1.38 m is attached to the wheel. A rope is wrapped around the edge of the disk as shown. An axis of rotation passes through the center of the wheel-disk system. What is the value of the mass M that will prevent the wheel from rotating?

Homework Equations

The Attempt at a Solution

all moments of inertia should equal zero so
m1r1^2 + m2r2^2 = 0

[/QUOTE]

The torques have to sum up to zero.

ehild