Ahollow steel shaft is to transmit 4200 n. m of torque and is to be sized so that the torsional stress does not exceed 120 mpa

the displacement produced by the body.  

the amount of force subjected to the body.  

the angle between the direction of force and displacement

initial speed when it is dropped is 150 ft/s

now at the same moment the acceleration due to gravity is perpendicular to its velocity

so as we know that

now we have centripetal acceleration a = 32 ft/s^2 as its perpendicular to velocity

while tangential acceleration at the same moment will be zero

so

now radius of curvature is given as

so the radius of curvature just after it is dropped is 703.125 ft