Suppose that 6 J of work is needed to stretch a spring from its natural length of 36 cm to a length of 50 cm. (a) How much work (in J) is needed to stretch the spring from 38 cm to 46 cm?
(b) How far beyond its natural length (in cm) will a force of 45 N keep the spring stretched?

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.