Relative Motion
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General Learning Resources
Relative Motion:
$ \vec{v}_{AB} \text{(velocity of A with resprect to B)} = \vec{v}_A - \vec{v}_B $
$ \vec{a}_{AB} \text{(acceleration of A with respect to B)} = \vec{a}_A - \vec{a}_B$
Relative Motion Along Straight Line:
$ \vec{x_{BA}} = \vec{x_{B}} -\vec{x_{A}} $
Crossing River:
A boat or man in a river always moves in the direction of resultant velocity of velocity of boat (or man) and velocity of river flow.
(i) Shortest Time :
Velocity along the river, $v_{x}=v_{R}$.
Velocity perpendicular to the river, $v_{f}=v_{m R}$
The net speed is given by $v_{m}=\sqrt{v_{m R}^{2}+v_{R}^{2}}$
(ii) Shortest Path :
velocity along the river, $\mathrm{v}_{\mathrm{x}}=0$
and velocity perpendicular to river $v_{y}=\sqrt{v_{m R}^{2}-v_{R}^{2}}$
The net speed is given by $v_{m}=\sqrt{v_{m R}^{2}-v_{R}^{2}}$
at an angle of $90^{\circ}$ with the river direction.
velocity $v_{y}$ is used only to cross the river, therefore time to cross the river, $t=\frac{d}{v_{y}}=\frac{d}{\sqrt{v_{m R}^{2}-v_{R}^{2}}}$
and velocity $\mathrm{v}_{\mathrm{x}}$ is zero, therefore, in this case the drift should be zero.
$\Rightarrow \quad v_{R}-v_{m R} \sin \theta=0 \quad \text{or} \quad v_{R}=v_{m R} \sin \theta$
or $\theta=\sin ^{-1}\left(\frac{v_{R}}{v_{m R}}\right)$
Rain Problems:
$ \vec{v}_{Rm} = \vec{v}_R - \vec{v}_m $
$ or \quad v_{R m}=\sqrt{v_{R^2} + v_{m^2}}$
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