At the same instant that a 0.50-kg ball is dropped from 25 m above earth, a second ball, with a mass of 0.25 kg, is thrown straight upward from earth's surface with an initial speed of 15 m/s. they move along nearby lines and pass each other without colliding. at the end of 2.0 s, the height above earth's surface of the center of mass of the two-ball system is:

People with normal vision cannot focus their eyes underwater if they aren't wearing a face mask or goggles and there is water in contact with their eyes. in a simplified model of the human eye, the aqueous and vitreous humors and the lens all have a refractive index of 1.40, and all the refraction occurs at the cornea, whose vertex is 2.60 cm from the retina.with the simplified model of the eye, what corrective lens (specified by focal length as measured in air) would be needed to enable a person underwater to focus an infinitely distant object? (be careful-the focal length of a lens underwater is not the same as in air! assume that the corrective lens has a refractive index of 1.62 and that the lens is used in eyeglasses, not goggles, so there is water on both sides of the lens. assume that the eyeglasses are 2.05cm in front of the eye.)