1. The problem statement, all variables and given/known data A homogeneous semicircular disc of mass m and the radius r is released from rest in the vertical position shown in the figure. Determine a minimum value of the coefficient of friction μ between the disc and the horizontal surface that prevents sliding in the initial moment 2. Relevant equations Because it is a rigid body then: OG=4R/3π Force and torque equation: ex: m*agx = -F (where F is friction) ey: m*agy= N-m*g IG * α = N*OG - F*R I know how to solve rest but the only thing I really got stuck and do not understand, is the direction of the friction. What I learned early in physics was to friction used to be the opposite of the direction of movement. But I see some issues that friction has the same direction as the movement! why is it like that ?!