Micro Life Zone
Asked by beth98 to Dirk, Elise, Janette, Renee, Yusuf on 15 May 2012.
Keywords: bottle, river, threw, wind
@Dirk is the official mailman for all bottles thrown in rivers or ocean, so will leave this question to him.
I am going to assume you mean this location in SA (see link 1) below). If you tossed the bottle from the ferry dock on the eastern shore into the river and if you throw anything like me, which is badly, then it’ll just make it about 3 meters out into the river. This is still pretty close to shore. You said the wind is going to the east so the bottle will head east, if it floats, you did drink the contents right? No sense wasting a nice bottle of carrot juice, you like carrot juice right? Anyway, now it is just a matter of calculating the speed of the bottle moving down river which is roughly due Southeast at the ferry crossing.
At this point in the river it flows slow but I have not found a decent number to use. Up river in Victoria it can travel 2-3 km/hr (see links 2) and 3) below) so lets 1/2 that to 1.5 km/hr. Next we’ll have to see how fast the wind is in the area. BOM’s observations for Murray Bridge (just Northwest up river, see link 4) below) indicates today’s wind was about 4 km/hr and from the west. So to the east, just like you wanted =:>
Now the idea is to add the two flows or vectors. One complication is that the bottle is small and will not feel the full wind. We will use a ‘rule of thumb’ here (one we use in coastal ocean areas anyway) that 3% of the wind is a reasonable estimate of surface currents. That means the bottle will be pushed 0.12 km/hr eastwards:
4 * 0.03 = 0.12
Since the bottle is 3 m from shore, or 0.003 km, it would only take 1.5 minutes to come back to you, albeit down river:
0.003 km / 0.12 km/hr = 0.025 hr
0.025 hr * 60 m/hr = 1.5 m
Now we calculate how far down the river it went.
0.025 hr * 1.5 km/hr = 0.0375 km = 3.75 m
hmm … 3.75 m almost as far as I tossed it.
I’ve simplified the situation a bit since the bottle was so close to shore. Ideally I should have added the two vectors 1st and then done the calculations but doing it step wise like I did is valid in this case.
BONUS: [extra credit potential teacher allowing] What would happen if you tossed the bottle off the western shore or dropped into the middle of the river while crossing on the ferry?
And finally don’t toss bottles into rivers. That is littering and not very nice =:<
Dirk – you da MAN!
What Dirk said! Plus I also agree with Renee.
I agree with everyone else…this is for Dirk!
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