If a book is pushed across the table with a force of 20 Ne and the friction that occurs pushes back in the opposite direction of 15 Newtons, what is the net force on the book?

You throw a small rock straight up from the edge of a highway bridge that crosses a river. the rock passes you on its way down, 7.00 s after it was thrown. what is the speed of the rock just before it reaches the water 28.0 m below the point where the rock left your hand? ignore air resistance.