Circular Motion/Work Homework: Finding Work

[laurenwings5]

Homework Statement

A spring has a force constant of 548 N/m and
an unstretched length of 6 cm. One end is
attached to a post that is free to rotate in the
center of a smooth table, as shown in the top
view below. The other end is attached to a
8 kg disk moving in uniform circular motion
on the table, which stretches the spring by
4 cm.
Note: Friction is negligible.

1. W = 8.26365 J
2. W = 4.13182 J
3. W = 1.3152 J
4. W = 0.00473472 J
5. W = 0 J

Homework Equations

Sum of forces = m*v^2/r
Work = ?

The Attempt at a Solution

I don't know.

 

[Andrés85]

mmmm i think in this case the centripetal force is the one applied by the spring, so

(constant)x(4cm) = (m)x(v^2/r)

solve and get v, and then

work of the disk = (1/2)x(m)x(v^2) + (1/2)x(I)x(W^2)
= (1/2)x(m)x(v^2) + (1/2)x(1/2x m xR^2)x(W^2)
= (1/2)x(m)x(v^2) + (1/2)x(1/2x m x v^2)
= (3/4)x(m)x(v^2)

 

[Andrés85]

i was wrong, i think the work is 0 because the centripetal force is perpendicular to the direction of v