Physics - Kinematics Question with Solution | TestHub

PhysicsKinematicsRelativeEasy2 minPYQ_2020

PhysicsEasysingle choice

A boat which has a speed of $5 km h^{- 1}$ in still water crosses a river of width $1 km$ along the shortest possible path in $15$ minutes. The velocity of the river water in $km h^{- 1}$ is

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Answer:

Solution:

Vertical displacement = $1 km$

$t = \frac{15}{60} = \frac{1}{4} h$

$∴ V_{b} \cos θ = \frac{1}{(\frac{1}{4})} = 4 km h^{- 1}$

$\begin{array}{l} \Rightarrow \cos θ = \frac{4}{V_{b}} = \frac{4}{5} \\ \Rightarrow \sin θ = \frac{3}{5} \end{array}$

By velocity triangle ABC, $\sin θ = \frac{V_{r}}{V_{b}}$

$\begin{array}{l} ∴ \frac{V_{r}}{V_{b}} = \frac{3}{5} \\ \Rightarrow \frac{V_{r}}{5} = \frac{3}{5} \\ \Rightarrow V_{r} = 3 km h^{- 1} \end{array}$

Stream:JEESubject:PhysicsTopic:KinematicsSubtopic:Relative

⏱ 2mℹ️ Source: PYQ_2020

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