Which of the following accurately describes the way in which a muscle moves?
a. no atp is needed in the contraction of a muscle.
b. when actin filaments are pulled toward the center of the sarcomere, the fiber shortens.
c. during contraction, cross-bridges pull the myosin to the center of the sarcomere.
d. interaction and attachment between myosin and actin cause a muscle fiber to relax.

When a charged balloon sticks to a wall, the downward gravitational force is balanced by an upward static friction force. the normal force is provided by the electrical attraction between the charged balloon and the equal but oppositely charged polarization induced in the wall's molecules. if the mass of a balloon is 1.2 g, its coefficient of static friction with the wall is 0.71, and the average distance between the opposite charges is 0.80 mm, what minimum amount of charge must be placed on the balloon in order for it to stick to the wall? express your answer to two significant figures and include appropriate units.