Assume that you are in a boat which moves with a speed of 13 km/hr (in still water) against the water flow. And the speed of the stream is 9 km/hr. Then what is the effective speed of your boat in this case?
4 km/hr
If the speed of an object is affected by the speed of another, then the net speed can be said to be Effective speed.
Consider, a boat moving at a speed of S1 and the speed of the water stream is S2.
If the stream and boat are moving in the same direction(Downstream or along the stream), the Effective speed of the boat = S1 + S2
And, If the stream and boat are moving in the opposite direction(Upstream or against the stream), the Effective speed of the boat = S1 - S2
Here, Boat and stream go in the opposite direction(upstream), So, Effective speed = 13 km/hr - 9 km/hr = 4 km/hr