Question

A man can swim at 4 ft/s in still water. He wishes to cross tje 40-ft wide river to point B, 30 ft downstream. If the river flows with a velocity of 2 ft/s, determine the speed of the man and the time needed to make the crossing. Note While in the water he must not direct himself toward point B to reach the point.

Answer 1

Answer:

$v \approx 4.472\,\frac{ft}{s}$, $t = 10\,s$

Explanation:

Since man and river report constant speeds and velocities are mutually perpendicullar, the absolute speed of the man is calculated by the Pythagorean Theorem:

$v = \sqrt{(4\,\frac{ft}{s} )^{2}+(2\,\frac{ft}{s} )^{2}}$

$v \approx 4.472\,\frac{ft}{s}$

The required time to make the crossing is:

$t = \frac{40\,ft}{4\,\frac{ft}{s} }$

$t = 10\,s$