a very light ideal spring with a spring constant (force constant) of 250 n/m pulls horizontally on an 18-kg box that is resting on a horizontal floor. the coefficient of static friction between the box and the floor is 0.65, and the coefficient of kinetic friction is 0.45. (a) how long is the spring just as the box is ready to move? (b) if the spring pulls the box along with a constant forward velocity of 1.75 m/s, how long is the spring? (c) how long is the spring if it pulls the box forward at a constant 2.75 m/s?