max shearing stress problem

Pepsi24chevy

Hey guys, I got a problem that goes like: A shaft with a circular cross section is subjected to a torque of 120ft-lb. Its shaft's diameter is 0.750 in and its length is 15 in., determine the maximum shearing stress.

I did the following: I tried to use shearing stress = Tr/J in which i plugged in as followed (120*.750/2)/(pi/32)*.750^4 and i get 1448, but the answer should be17.39ksi. I assume i am not getting the right answer because I am not factoring in the length but I am not sure. Any help is appreciated, thanks

Pepsi24chevy

nm, i figured it out, Stupid me forgot to convert the ft-lb to inch-lb.