This summer we had gone to USA for a fun trip. For a change, we decided to take a motor boat along Niagra river. I hand heard a lot about Niagra water falls is 50 m high. What an adventure it would be to fall freely with the water fall. We started about 20 km upstream of the watyer fall with a motor boat which could travel at 18 km/hr relative to the water. the Biagra river is a slowly flowing river, flowing at pleasant 3.6 km/hr. The river is almost 300 m wide and we decided to go downstream with engine of motor boat on. Hardly we had gone 3-4 km when we noticed a long tree of length almost 10 m flowing 50 m away from us. our line of motion was passing through the centre of the tree which was oriented perpendicular to river flows. sensing our uneasiness, the boatman took a left turn by an angle `theta` (as seen by us) and we just managed to cross the tree. thereafter, the boat moved only downstreas as before. as we reached Niagra falls. the velocity of boat and the river was horizontal. the boat flew in the air and struck the water below at an angle to the horizontal. vow! That an experience of life.
What was the time interval between seeing the tree and crossing it ?

Options

A. 10 sec. app
B. 1 sec. app
C. `25/3` sec. app
D. 50 sec. app

Answer

C

Explanation

`tan theta=1/10`
`v_(br) sin theta=5/t`
`5xx1/(sqrt(101)) =5/t`
t=10 sec . Approx.