A river flows due south with a speed of 2.0 ms^-1. A man strees a motorboat across the river , his velocity relative to the water is 4 m s^-1 due east. The river is 800 m wide. (a) What is his velocity (magnitude and direction) relative to the earth ? (b) How much time is required to cross the river ? ( c) HOw far south of his starting points will he reach the opposite bank ?