Velocity of river,
Vr=5 m/min
Velocity of man w.r.t river,
Vm=10 m/min
Let the width of the river be
d and assume the man heads in a direction making angle
θ with the bank of the river.
On breaking component of
Vm along and perpendicular to river flow
We can see that component
Vmsinθ will be responsible for crossing the river.
Time taken to cross river,
t=width of riverperpendicular component of velocity
∴t=dVmsinθ
For
t to be minimum,
sinθ should be maximum, and maximum value of
sinθ is
1 at
θ=π2=90∘.
∴ Man should head at angle of
90∘ with the bank of river i.e. in North direction.