A 4 kg box is next to a 2 kg box on a rough horizontal surface. The coefficient of static friction is 0.3 and coefficient of kinetic friction is 0.2. a) What minimum horizontal force is needed to be applied to the 4 kg box in order to start to move both boxes horizontally? b) What is the acceleration of the boxes when they are moving by this force? c) Find the contact force between the boxes.

Suppose a force of 60 n is required to stretch and hold a spring 0.1 m from its equilibrium position. a. assuming the spring obeys hooke's law, find the spring constant k. b. how much work is required to compress the spring 0.5 m from its equilibrium position? c. how much work is required to stretch the spring 0.6 m from its equilibrium position? d. how much additional work is required to stretch the spring 0.1 m if it has already been stretched 0.1 m from its equilibrium? a. kequals 600