Question

A man rows across a river with velocity 8km/h.If his resultant velocity is 10km/h then what is the speed of river water?

Answer 1

vboat=8 km/hResultant velocity, vres=10 km/hSpeed of river water, vr=?vres2=vr2+vboat2102=vr2+82or vr2=102−82=100−64=36or vr=6 km/h

Answer 2

     Average velocity is defined to be the change in position divided by the time of travel.

The average velocity of an object does not tell us anything about what happens to it between the starting point and ending point .

                                       $V_R$ = Velocity of river

Apply Pythagoras theorem,

Resultant velocity,         $V_R^2 =V_B^2+V_R^2$

                                           $10^2=8^2+V_R^2$

                                            $100 = 64+V_R^2$

                                        $V_R^2= 36$

                                        $V_R= 6 km/h$