75) A semi-circular disc rests on a horizontal surface with its top flat surface horizontal and circular portion touching down. The coefficient of friction between semi-cricular disc and horizontal surface is \i. This disc is to be pulled by a horizontal force applied at one edge and it always remains horizontal. When the disc is about to start moving, its top horizontal force will

76) A particle inside a hollow sphere of radius r, having coefficient of friction -rr can rest upto height of

77) The algebraic sum of moments of the forces forming couple about any point in their plane is

78) A single force and a couple acting in the same plane upon a rigid body

79) If three forces acting in one plane upon a rigid body, keep it in equilibrium, then they must either