wow! relative velocity for p5 math ?!


first thing first:

1. when going downstream,
speed of raft includes the speed or flow rate of current

2. when going upstream,
speed of raft has to go against the speed or flow rate of current. Tends to be slower than downstream speed, much slower.


downstream speed = 24/2 = 12 km/h

upstream speed = 18/3 = 6 km/h

***

there r two speeds here in the qn
(a) speed of raft in still water
(b) speed(flow rate) of current

sum of 2 speeds = downstream speed
= 12 km/h

difference of 2 speeds = upstream speed = 6 km/h

so
12+6 = 18 km/h would give u
2 times the speed of raft in still water

therefore,
speed of raft in still water
= 18/2 = 9 km/h


extra:
speed of current = 12 - 9 = 3 km/h


check!
1. upstream is the difference of 2 speeds = 9 - 3 = 6 km/h


...

This is a net speed question that uses common logic.

Downstream : 24km ÷ 3 h = 12km/h
Upstream : 18km ÷ 3h = 6km/h

Upstream is slower than downstream by 6km/h.

So this means the current flows downwards. It is aiding you going downstream but working against you going upstream.

Now current speed doesn't change.The extent it slows you when going upstream is the same as the extent it speeds you up when going downstream.

Upstream speed
= Normal speed - current's speed

Downstream speed
= Normal speed + current's speed

Difference = 2 x current's speed
Difference also = 6km/h

Current's speed = 6km/h ÷ 2 = 3km/h


So normal speed (in still water, no current)
= 6km/h + 3 = 9km/h

Or 12km/h - 3 km/h = 9km/h