A CUBICAL BLOC OF MASS M AND EDGE a SLIDES DOWN ON A ROUGH INCLINATION PLANE OF ANGLE @ WITH UNIFORM SPEED. FIND THE MAGNITUDE OF THE TORQUE OF THE NORMAL FORCE ACTING ON THE BLOCK ABOUT ITS CENTRE

ANS 1/2MGASIN@

MY DOUBT IS THAT SINCE THE NORMAL FORCE INTERSECTS THE CENTRE ITS TORQUE SHOULD BE 0

MG COS @ IS ALSO INTERSECTING THE CENTRE SO THE TORQUE AGAIN WILL BE 0

THE ONLY FORCE WHICH IS LEFT IS MGSIN@ AND ITS DISTANCE FROM THE CENTRE IS a/2 SO

THE TORQUE CAN BE 1/2 MGIN@

BUT MGSIN@ IS NOT THE NORMAL FORCE PLS EXPLN