9. Can the total work done on an object during a displacement be negative? Can frictional forces do positive work? (A) yes, yes (B) yes, no (C) no, yes (D) no, no. 10. If work W is required to stretch a spring a distance x from its unstretched length, what additional work is required to stretch the spring an additional distance x? (A) W (B) 2W (C) 3W (D) 4W 11. The potential-energy function for a force F is U = axs, where a is a positive constant. What is the direction of F? (A) positive x-direction; (B) negative x-direction: (C) changes with position