A drifting boat

A river is flowing at a uniform velocity of v = 10 m/s v = 10 \text{ m/s} . As shown in the diagram, P P , Q Q , and R R are three points on a line perpendicular to the river such that P Q = Q R = 100 m PQ =QR = 100 \text{ m} .

A boat starts from point P P and moves at a constant speed of v = 10 m/s v = 10 \text{ m/s} relative to the river. It moves in such a way that the direction of its resultant velocity is always perpendicular to the line joining the boat and point R R .

If the boat drifts a a meters and takes b b seconds to land on point S , S, submit your answer as a + b a + b to 3 decimal places.

P.S If you liked this problem, check this one out too!!!


The answer is 186.375.

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2 solutions

Nivedit Jain
Mar 10, 2017

Very happy to solve this awsome problem,

Awesome!!....Hua nhi mjhse😂

Shivanshi Chauhan - 4 years, 2 months ago

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Same here.... It was a bit tough..... :)

Pranav Saxena - 4 years, 2 months ago

Did it the same way. At first, I guessed the trajectory to be a circle, and immediately concluded it to be 200 200 , but even after getting the right solution, I wanted to prove it!

Vinay Shankar - 3 years, 11 months ago

Same here too!

Tan Kiat - 4 years, 2 months ago
Pranav Saxena
Mar 10, 2017

Since the resultant velocity is always perpendicular to the line joining the boat and R R , the boat is moving in a circle of radius 2 w 2w .

1) Thus the drift will be Q S QS = 4 w 2 w 2 \sqrt{4w^2 - w^2} = 3 w \sqrt{3}w

2) Suppose at any arbitrary time, the boat is at point B B .

V n e t V_{net} = 2 v cos θ 2v\cos\theta

d θ d t \dfrac {d \theta}{dt} = V n e t 2 w \dfrac{V_{net}}{2w} = v cos θ w \dfrac{v\cos\theta}{w}

Cross multiplying :-

w v sec θ d θ \dfrac{w}{v}\sec\theta d\theta = d t dt

Now integrating and using proper limits(at time t = 0 t = 0 , the line joining R and P makes an angle of 0 0^\circ , and at time t t , the angle becomes 6 0 60^\circ )

0 t d θ \displaystyle\int^t_0d\theta = w v 0 6 0 sec θ d θ \dfrac{w}{v}\displaystyle\int^{60^\circ}_0 \sec\theta d\theta

t t = w v [ l n ( sec θ + tan θ ) ] 0 6 0 \dfrac{w}{v}[ln(\sec\theta + \tan\theta)]^{60^\circ}_0

t t = 1.317 w v \dfrac{1.317w}{v}

So adding them we get :-

[ ( 1.317 ) ( 10 ) + 3 ] ( 100 ) [(1.317)(10) + \sqrt{3}](100)

= 186.375 \boxed{186.375}

P.S If someone could pls help me with this solution by providing an image, it would be really great as it may prove to be difficult to deduce what angle is θ \theta and some of the other things too.

ok will image in paint will be fine??

Nivedit Jain - 4 years, 3 months ago

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Yup.All I need is an image which labels all the points and angles mentioned in my solution.Thank you

Pranav Saxena - 4 years, 3 months ago

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ok when ever i get free time i will make it

Nivedit Jain - 4 years, 3 months ago

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@Nivedit Jain Thanks. I do hope there was a better way........

Pranav Saxena - 4 years, 3 months ago

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@Pranav Saxena which way is in your mind?? PLzzz share

Nivedit Jain - 4 years, 3 months ago

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@Nivedit Jain A better way so as to make images. Paint seems tiresome......... and making them in word and then converting them to jpg is also a lot of work.........

Pranav Saxena - 4 years, 3 months ago

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@Pranav Saxena OK as u wish

Nivedit Jain - 4 years, 3 months ago

@Pranav Saxena Why have you taken θ \theta as 60°?

Satwik Murarka - 3 years, 12 months ago

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.When the boat reaches the opposite shore, it forms an equilateral triangle with the point R.Its difficult to convey this without an image..

Pranav Saxena - 3 years, 12 months ago

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