http://physics.stackexchange.com/questions/243457/about-the-formula-of-pendulum-simple [Broken] a(t) = a0 * sin ( sqrt(g/l) * t * Pi/2 ) - [ k/(mll) * cos ( sqrt(g/l) * t * Pi/2 ) * t ) ] a(t) : the angle in instant t . t : time g : gravity . Pi = 3.14 k = Fixed (Friction ) l : longer of pendulum but this formula is wrong , because the curve of this formula Remains constant , like this : I want to correct this equation to become true Means keeps decreasing curve, even non-existent ( = 0 ) . Specifically I want to change in the second part , in [ k/(m*l*l) * cos ( sqrt(g/l) * t * Pi/2 ) * t ) ] can you help me ? Are there any other equation?