Why should the wavelength scale of spectroscope be calibrated?

What does si stand for,when referring to a system of measurement

Ben (55kg) is standing on very slippery ice when junior (25kg) bumps into him. junior was moving at a speed of 8 m/s before the collision and ben and junior embrace after the collision. find the speed of ben and junior as they move across the ice after the collision. give the answer in m/s. describe the work you did to get the answer.

Smallest intensity of a stimulus that you can detect 50 percent of the time is