Question

The ratio of the distance carried away by the water current, downstream, in crossing a river, by a person, making the same angle with downstream and upstream is 2: 1. The ratio of the speed of person to the water current cannot be less than
(A) $1 / 3$
(B) $4 / 5$
(C) $2 / 5$
(D) $4 / 3$

Answer 1

Hint: We should know that if the flow of data goes toward the original source, that flow is upstream. If the flow of data goes away from the original source, that flow is downstream. We should know that velocity is defined as the rate change of displacement per unit time. Speed in a specific direction is also known as velocity. Velocity is equal to displacement divided by time. Speed, being a scalar quantity, is the rate at which an object covers distance. The average speed is the distance which is a scalar quantity per time ratio. On the other hand, velocity is a vector quantity; it is direction-aware. An object which moves in the negative direction has a negative velocity. If the object is slowing down then its acceleration vector is directed in the opposite direction as its motion in this case. Based on this we have to solve this question.

Complete step by step answer
We know that the motion of the person making an angle (alpha) with the downstream.
The time taken to cross the river $=\dfrac{\mathrm{d}}{\mathrm{v} \sin \alpha}$………………. (i)
The distance caries away downstream in the same time = speed $\times $time
$\mathrm{x}_{1}=(\mathrm{u}+\mathrm{v} \cos \alpha) \dfrac{\mathrm{d}}{\sin \alpha}$
Motion of the person making $\alpha$ angle with upstream. The time taken to cross the river is equal to $\dfrac{\mathrm{d}}{\mathrm{v} \sin \alpha}$
So now the distance carried away downstream in the same time
$\mathrm{x}_{2}=\left[\mathrm{u}+\mathrm{v} \cos \left(180^{\circ}-\alpha\right)\right] \dfrac{\mathrm{d}}{\mathrm{v} \sin \alpha}$----------(ii)
$\Rightarrow \mathrm{x}_{2}=(\mathrm{u}-\mathrm{v} \cos \alpha) \dfrac{\mathrm{d}}{\mathrm{v} \sin \alpha}$
It is given that:
 $\dfrac{(\mathrm{u}+\mathrm{v} \cos \alpha) \dfrac{\mathrm{d}}{\mathrm{v} \sin \alpha}}{(\mathrm{u}-\mathrm{v} \cos \alpha) \dfrac{\mathrm{d}}{\mathrm{v} \sin \alpha}}=\dfrac{2}{1}$
After the evaluation we get that:
$\dfrac{(\mathrm{u}+\mathrm{v} \cos \alpha)}{(\mathrm{u}-\mathrm{v} \cos \alpha)}=\dfrac{2}{1} \Rightarrow 3 \mathrm{v} \cos \alpha=\mathrm{u}$
$\Rightarrow \dfrac{\mathrm{v}}{\mathrm{u}}=\dfrac{\sec \alpha}{3}$…………………………………………. (iii)
$\sec \alpha \geq 1 \Rightarrow \dfrac{\sec \alpha}{3} \geq \dfrac{1}{3}$
From eq (iii) $\dfrac{\mathrm{V}}{\mathrm{u}} \geq \dfrac{1}{3}$
So, value cannot be less than $1 / 3$.

Hence the correct option is option A

Note: We should know that if an object's speed or velocity is increasing at a constant rate then we say it has uniform acceleration. The rate of acceleration is constant. If a car speeds up then slows down then speeds up it doesn't have uniform acceleration. The instantaneous acceleration, or simply acceleration, is defined as the limit of the average acceleration when the interval of time considered approaches 0. It is also defined in a similar manner as the derivative of velocity with respect to time. If an object begins acceleration from rest or a standstill, its initial time is 0. If we get a negative value for acceleration, it means the object is slowing down. The acceleration of an object is its change in velocity over an increment of time. This can mean a change in the object's speed or direction. Average acceleration is the change of velocity over a period of time. Constant or uniform acceleration is when the velocity changes the same amount in every equal time period.