15. A helicopter rotor blade is 7.80 m long and has a mass of 110 kg.
(a) What force is exerted on the bolt attaching the blade to the rotor axle when the rotor is turning at 320 rev/min?
(Hint: For this calculation the blade can be considered to be a point mass at the center of mass. Why?)
 (b) Calculate the torque that must be applied to the rotor to bring it to full speed from rest in 6.70 s. Ignore air resistance.
(The blade cannot be considered to be a point mass for this calculation. Why not? Assume the distribution of a uniform rod.)