Which of the following are true statements about friction? check all that apply. a. the amount of friction depends on the type of surface and the force of contact. b. friction is a force that acts between two surfaces that are in contact. c. frictional forces cannot cause acceleration of an object. d. the direction of the frictional force varies such that it always opposes motion.

Point charges q1=+2.00μc and q2=−2.00μc are placed at adjacent corners of a square for which the length of each side is 1.50 cm . point a is at the center of the square, and point b is at the empty corner closest to q2. take the electric potential to be zero at a distance far from both charges. part a what is the electric potential at point a due to q1 and q2? express your answer with the appropriate units.