I have done a spring-mass  system with solidworks motion of solidworks 2009 according to a textbook called "motion simulation and mechanism design with cosmotion 2007". I will upload my assembly. In this motion study, i defined a spring stiffness: 20 length: 3 force: 0 coil diameter: 0.75 number of coils: 8 wire diameter: 0.25 . And i also defined the initail position as 4. But when i output the plot of the displacement, i find it is not as the same as that in the textbook.

I don't know what the problems are.

Thank you !