Monday, 2 May 2016

Aptitude Questions - Boats and Streams Concept and Shortcuts

                                                     Boats and Streams - Concept and Formulae
This Chapter is based on the speed of an object in still water or in flowing water.
There are two most important terms :

 i) Still water: It implies that that speed of water in the river is zero. When speed of the water or stream is zero, then it is considered as still water.

  ii)   Stream or Current: It implies that the water of the river is moving. Stream or Current or flow or water or river all are used interchangeably.

Speed of an object:
In this boats and stream aptitude questions, object can be man or boat or ship.
Speed of object means speed of object in non-flowing or still water.
In other words, if speed of the object is given, then that particular speed is the speed of that object in still water.
If speed of water is zero, then the water is considered to be still.

Upstream motion:
 If the motion of object is against the direction of the current, then such motion is known as upstream motion

Downstream motion:
If the motion of the object is along the direction of the current, then such motion is known as downward motion

Downstream Speed: It is speed of an object along the flow of stream.
If the speed of an object in still water is x kmph and speed of the stream is y kmph, then
Downstream speed = (x+y) kmph

Example: The speed of a man in still water is 6 kmph and speed of the still water is 2 kmph, then downstream speed = 6+2 = 8 kmph

Upstream Speed :  It is speed of an object against the flow.
If the speed of an object in still water is x kmph and speed of the stream is y kmph, then Upstream speed= (x-y) kmph

Example : The speed of a boat in still water is 12 kmph and the speed of the stream is 8 kmph, then upstream speed = (12-4)=8 kmph

Important Points on Boats and streams :
1)    Speed of an object when downstream speed (DSS) and upstream speed(USS) are given
 If downstream speed of an object is DSS kmph and upstream speed of that object is USS kmph , then Speed of the boat in still water=
(DSS + USS ) / 2


Example: The downstream speed of a boat is 4.5 kmph and speed of the upstream is 1.5 kmph, then speed of the boat in still water=
(4.5 + 1.5) / 2
= 3 kmph


2)    Speed of the current or stream when downstream speed(DSS) and upstream speed(USS) are given
If downstream speed of an object is DSS kmph and upstream speed of that object is USS kmph , then
Speed of the stream=
(DSS - USS) / 2

Example: The downstream speed of a man is 6 kmph and upstream speed of man is 2 kmph then, speed of stream=
(6-2) / 2
= 2 kmph