A 2.4 g marble is fired vertically upward using a spring gun. The spring must be compressed 8.0 cm if the marble is to just 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 its launch of the marble? (c) What is the spring constant of the spring?