A Circular bar 1m long and of 8mm diameter is rigidly clamped at one end in a vertical position. A couple of magnitude of 2.5Nm is applied at the other end. As a result, a mirror fixed at this end deflects a spot of light by 15cm on a scale 1m away. Calculate the rigidity modulus of the bar? Torque,ζ=(∏n[r][/4]∅)/2l where r is the radius of the rod,l is length,n is the rigidity modulus,∅is the angle of twist. Applying given values in equation =>n=4.1*10 raised to 10 N/m square But the answer provided by this question is 8.3*10 raised to 10 N/m square. Similarly another problem using same equation Two cylinders P and Q of radii in the ratio 1:2 are soldered co axially. The free end of P is clamped and the free end of B is twisted by an angle of 17 degree. Determine the twist at the junction,if the cylinders are of the same length and made of the same material? The answer I got is 17*16,i.e twist=272 degree. But here also answer provided is wrong from my point of view.It is given only 16 degree. If I have made any mistake,kindly correct me with explanation.