# Group Problem #21 ```Group Problem #21 Physics 111-4 Spring 2012 Page 1 of 4
Group problem #21: Angular momentum
and angular velocity
A cockroach with mass, m, rides on a disk of mass 6.0m and radius R. The disk rotates around it’s central
axis with an angular sleep of ωi = 1.50 rad/s. The cockroach is initially at radius r=0.8R, but then it
crawls out to the rim of the disk. What is the final angular speed of the system?
Group Problem #21 Physics 111-4 Spring 2012 Page 2 of 4
At one instant a force 𝐹⃗ = 4.0𝑗̂𝑁 acts on a 0.25kg object that has a position vector 𝑟⃗ = (2.0𝑖̂ − 3.0𝑘̂)𝑚
and velocity vector 𝑣⃗ = (−5.0𝑖̂ + 5.0𝑘̂ )𝑚/𝑠 . About the origin and in unit vector notation, what are the
object’s angular momentum and the torque acting on the object?
Group Problem #21 Physics 111-4 Spring 2012 Page 3 of 4
A light rope tied around the circumference of a disk-shaped pulley is pulled downward with a constant
force of 1.30N. The pulley rotates freely, has a mass of 0.950kg, and a radius 0.150m. How much work is
done on the pulley as it rotates through half a revolution? If the pulley starts at rest, what is its angular
speed after half a revolution?
Group Problem #21 Physics 111-4 Spring 2012 Page 4 of 4
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