  Question

# A boat B is moving upstream with velocity 3 m/s with respect to ground. An observer standing on boat observes that a swimmer S is crossing the river perpendicular to the direction of motion of boat. If river flow velocity is 4 m/s and swimmer crosses the river of width 100 m in 50 sec, thenDrift of swimmer along river is zeroVelocity of swimmer w.r.t ground is √13 m/sDrift of swimmer along river will be 100 mVelocity of swimmer w.r.t. ground is 2 ms−1

Solution

## The correct option is B Velocity of swimmer w.r.t ground is √13 m/s Here, width of the river =100 m, Velocity of boat w.r.t ground −−→VBG=−3^i m/s (Direction is opposite to +ve  x-axis) Velocity of river w.r.t ground −−→VRG=4^i m/s From the boat, the swimmer appears to cross the width of the river (100 m) in 50 seconds perpendicular to the direction of boat (along Y− axis). Hence, −−→VSB=100/50=2^j m/s  We have,  ⇒ −−→VSB=−−→VSG−−−→VBG ⇒ −−→VSG=−−→VBG+−−→VSB=−3^i+2^j |−−→VSG|=√(32+22) = √13 m/s  Suggest corrections   