4. A 5.0 g marble is fired vertically upward using a spring gun. The spring must be compressed 8 cm if the marble is to reach a target 20 m above the marble’s position on the compressed spring. (a) What is the change ∆Ug in the gravitational potential energy of the marble-Earth system during the 20 m ascent? (b) What is the change ∆Us in the elastic potential energy of the spring during it’s launch of the marble? (c) What is the spring constant of the spring?