The sun is 150,000,000 km from earth; its diameter is 1,400,000 km. a student uses a 5.2-cm-diameter lens with f = 10 cm to cast an image of the sun on a piece of paper. where should the paper be placed relative to the lens to get a sharp image?

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.