There's an example question from my textbook that I'm confused about: A 500 g block on a spring is pulled a distance of 20 cm and released. The subsequent oscillations are measured to have a period of 0.80 s. At what position or positions is the block's speed 1.0 m/s? The book solves this problem by using 20 cm as the amplitude. What I don't understand is, how do we know that this is the amplitude? I know it's stretched that far but couldn't the amplitude result in something different?