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beth12345
A boy and girl both swim at 3.0m/s. They jumpinto a river 1.0km across, with a current of 2.0m/s (E). She swims straight across relative to the shore. How long (in minutes) does each take to cross the river
You are given the velocity of boy and girl w.r.t the ground and velocity of current w.r.t ground. Velocity of girl w.r.t ground, V(g,G) = 3 m/s Velocity of current = V (c,G) = 2 m/s Now, velocity of girl w.r.t current = V(g,G) + V(c,G) [ vector sum!! ] Hope you may try to solve it now...
No... I said the vector sum, as the direction of speeds are different you can't just add both the velocities..
oh would I use a squared +b squared = c squared
But, think.. For calculating time she required to cross the river if she propells herself perpendicular to the shore, would in anyway be affected by velocity of current.
Think why time depends on both components...
uhm, I don`t understand
|dw:1331693378113:dw| this is all I understand so far...
Right. See, She propells herself perpendicular to the bank at 3 m/s and at the same time she is getting pushed sideways at 2 m/s. So, her resultant velocity will be directed at an angle b/w the two.
Now, see 3m/s contributes to her getting across the bank. Time she requires = 1000 m / 3 m/s = 333.33 sec.
and then it makes an angle triangle like this |dw:1331693918116:dw| ?
and then it is 5.6 minutesÉ
RIght, you may shift vectors in a plane, without changing the directions and magnitude.
oh i think i get it, does the 2m per s affect it?
No, it doesn't effect the time, but other parameters like where she would land on the other bank does.
ok, is it the same for the boy then?
Yes, Gender doesn't effect the time as well.. lol..
oh right! haha well thanks for helping :)