Question

A 70 m wide river flows at 0.80 m/s [E]. A girl, starting on the south shore, swims across it at 1.4 m/s relative to the water.

a) What is the least time she requires to cross the river?

b) How far downstream will she be when she lands on the opposite shore?

c) At what angle to the shore would she have to aim, in order to arrive at a point directly opposite the starting point?

Answer 1

a) Answer;

50 seconds

Solution;

t = d /v

where t is the time taken, d is the distance and v is the speed

d= 70 m and s= 1.4 m/s

Therefore;

t = 70/1.4

= 50 seconds

b) Answer;

40 meters

Solution;

dc = Vc × t

Where dc is the distance down stream, Vc is the velocity downstream and t is the time taken

Vc = 0.8 m/s

t = 50 seconds

Therefore;

dc = 0.8 × 50

= 40 meters

c) Answer;

55 degrees

Solution;

cosA = 0.8/1.4

A = 55.15 ° with the horizontal (shore) upstream

Therefore; the angle to aim from the shore is 55°