Did You Figure it Out? Get the Answer to the Riddle from E-News Issue No. 2, April 2024
Boat Trip
How long would Lena be on the water?
On a hot and sunny summer day, Lena decides to take a trip with her boat. It takes five hours for her to row her boat down the river. If she continues to row at the same pace, she will need six hours to row back up the river. Now imagine that Lena is traveling the same distance with her boat on a lake (without a current).
How long would she be on the water if her boat traveled at a constant speed?
Solution
It is a uniform motion, so the following physical relationship is valid:
Speed x time = distance or s ∙ t = d
The time “t” is calculated accordingly: t = d/s
For our example this means:
5 = d/(S + s) → 5(S + s) = d, and
6 = d/(S - s) → 6(S - s) = d
Where “S” the speed of the boat and “s” is the speed of the current.
By setting the two equal to one another, you get:
5(S + s) = 6(S - s)
5S + 5s = 6S - 6s
11s = S
So, the speed of the boat is eleven times greater than the speed of the current.
t = d/s and the distance “d” can be calculated using
d 5(S + s) = 6(S - s),
which yields d = 5 [ S + (1/11)S ] = 6 [ S - (1/11)S ] = (60/11)S
and therefore
t = d/s = 60/11
Because you have to consider the distance there and back, the result is:
t = 120/11 = 10 (10/11)h = 10h 54,5 seconds
It is logical that a trip without a current is shorter, because traveling with the current is shorter than traveling against the current.
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