Q. A convex lens is held over a piece of paper outdoors on a sunny day. When the paper is held 20cm below the lens, sunlight is focused on the paper and the paper ignites. What is focal length of the lens?
Q. A 35 g bullet moving at 475 m/s strikes a 4.20 kg bag of flour that is on ice, at rest. The bullet passes through the bag, leaving at 270 m/s. How fast is the bag moving when the bullet exits?