Posted by Josh on Saturday, January 11, 2014 at 6:44pm.
Two uniform solid spheres have the same mass of 3 kg, but one has a radius of 0.25 m while the other has a radius of 0.8 m. Each can rotate about an axis through its center.
(a) What is the magnitude τ of the torque required to bring the smaller sphere from rest to an angular speed of 300 rad/s in 15 s?
N·m
(b) What is the magnitude F of the force that must be applied tangentially at the sphere's equator to provide that torque?
N
(c) What is the corresponding value of τ for the larger sphere?
N·m
(d) What is the corresponding value of F for the larger sphere?
N

Physics  Damon, Saturday, January 11, 2014 at 7:17pm
I sphere = (2/5) mR^2
see http://hyperphysics.phyastr.gsu.edu/hbase/isph.html
I1 = (2/5)3 (.25)^2 = .075
alpha = 300/15 = 20 radians/second^2
Torque = I alpha = .075*20 = 1.5 Nm
F1 = torque/R = 1.5/.25 = 6 N
now big sphere
I2 = (2/5)3 (.8)^2 = .768
torque = I alpha = .768*20 = 15.36 Nm
F2 = 15.36/.8 = 19.2 N