In this section, the following topics are discussed with examples

  • Downstream
  • Upstream
  • Basic Formulae
  • Examples

In streams there are two types of concepts,

DOWNSTREAM

One moving in the direction of water (i.e) the water will always flow down from a hill right ! So downstream is calculated when the boat is coming down from the hill.

UPSTREAM

The other is moving in the direction against the water. As I said the water will always flow down from the hill top so to reach a hill top through a stream we want to always go against the motion of water that is called as UPSTREAM.

Basic Concept

Let us consider

SB= Speed of the boat in still water

SS = Speed of water/stream/current

Speed of water downstream SD= SB + SS

Speed of water upstream SU=SB – SS

The speed of the boat in still water SB= (Sd + Su) / 2

The speed of the boat in stream SS =(Sd – Su) / 2

The ability to simplify seems to eliminate the unnecessary so that the necessary may speak.

Hans Hoffman

Example 1:

The speed of the boat with the current is found to be 15km/hr and the speed of the current is 5 km/hr.Find the speed of the boat moving against the current.

Sol:

Speed = 15 km/hr

Speed = 5km/hr

Speed of the boat in the current=speed of the boat+Speed of the current

15 = Speed of the boat + 5

Speed of the boat = 10 km/hr

Boat’s speed against the current=Speed of the boat-Speed of the current

=10-5

= 5 km/hr


Example 2:

A boatman goes 4km against the current of the stream in 4 hour and goes 1 km along the current in 20 minutes. How long it will take to go for 7 km in still water?

Given

Distance in upstream=4 km

Time taken in upstream=4 hr

Distance in downstream=1 km

Time taken in downstream=20 min

=20/60 hr

=1/3 hr

To find

Time taken for 7 km in still water=?

Sol

Speed in upstream =Distance/Time

=4/4

=1 km/hr

Speed in downstream=1/1/3

=3 km/hr

speed in still water=

=2 km/hr

The boat can travel 2km in 1 hr

i.e.,2 km= 1 hr

1 km=30 min

Therefore,7 km=3 hr 30 min


Example 3:

A man can go 24 km upstream and 36 km downstream in 6 hrs and also he can row 36 km upstream 24 km downstream in 61/2 hours. Find the rate of current.

Given

DU1 =24 km

DD1=36 km

T1=6 hrs

DU2=36 km

DD2= 24 km

T2=6.5 hrs

To find

Rate of current=?

Sol

W.K.T.

T=D/S

Let us consider SU and SD as x and y

24/x+36/y=6

36/x+24/y=6.5

Take 1/x= a and 1/y=b

24a +36b=6

4a+6b=1 ——-(1)

36a+24b=6.5 —–(2)

Solve (1) and (2)

(1) * 9 36a+54b=9

(2) 36a+24b=6.5

30b=2.5

b=2.5/30

b=1/12

Sub b in (1)

4a+6(1/12)=1

8a+1=2

8a=1

a=1/8

Therefore, x=8 and y=12

Rate of current=(12-8)/2

=2km/hr.