In the simple harmonic motion of a block on a spring, what is the restoring force proportional to?

Asun-sized star will spend most of its lifetime as a: white dwarf red giant protostar main-sequence star

Which explanation describes the forces involved for a person to walk down the sidewalk? a. the person's feet push backward on the sidewalk; the sidewalk pushes backward on the person. b. the person's feet push forward on the sidewalk; the sidewalk does not push on the person. c. the person's feet push backward on the sidewalk; the sidewalk pushes forward on the person. d. the person's feet push forward on the sidewalk; the sidewalk pushes forward on the person.

What are similarities and differences between refraction, reflection, diffraction and absorption?