question_answer

A boat crosses a river with a velocity of 8 km/h. If the resulting velocity of boat is 10 km/h then the velocity of river water is                                                [CPMT 2001]

A)                 4 km/h 

B)                 6 km/h

C)                 8 km/h 

D)                 10 km/h

Correct Answer: B

Solution :

                \[\overrightarrow{{{v}_{br}}}=\overrightarrow{{{v}_{b}}}+\overrightarrow{{{v}_{r}}}\]                                 Þ\[{{v}_{br}}=\sqrt{v_{b}^{2}+v_{r}^{2}}\]                                 Þ\[10=\sqrt{{{8}^{2}}+v_{r}^{2}}\]                                 Þ\[{{v}_{r}}=6km/hr\].