By book defines SHM as
Simple harmonic motion is defined as the projection on any diameter of a graph point moving in a circle with uniform speed.
but in the next line it says -
Moving back and forth along the line AB, the mass point is continually changing speed vx' Starting from rest at the end points A or B, the speed increases until it reaches C.
Can somebody explain - If the original particle is moving with a constant speed then how come it's mass point have different velocity ? Also for a simple harmonic motion resorting force should be proportional to displacement (towards mean position) .In this diagram i see no force acting on mass point?