Hello, my book explains detailed the proofs of these three formulas: y = Asin(ωt + φo) v = ωAcos(ωt + φo) a = -ω²Asin(ωt + φo) Where a is acceleration, v is velocity, ω is angular velocity, A is amplitude. The book uses the following figures: Figure a) --> http://tinypic.com/view.php?pic=30m8f9j&s=8 Figure b) --> http://tinypic.com/view.php?pic=2hrejde&s=8 Figure c) --> http://tinypic.com/view.php?pic=a0f9te&s=8 In the first case a) what I'm seeing is that the sine in the triangle whose hypotenuse is R and opposite side vector y is (vector y)/(vector R), in this case R is the amplitude. The rest b) and c) are just complicated, I don't understand them. Anyone out there to help me here? Thank you!! Edit: Sorry for posting this here, it's supposed to go to homework help, my fault.