# Direction of swim

## Homework Statement

A girl swim one bank of a river with 100m when she swims perpendicular to the water current. She reaches the other bank 50m down the stream. The angle at which she should swim ti reach a point directly opposite on the other bank of the river is?
(Velocity of girl=u, velocity of river=v)

## The Attempt at a Solution

I have a problem, please tell why the question give the velocities to find the angle.
It can easily taken by applying tan value to the displacements.

then it can be gain as
tanα = 50/100 = 1/2
α = tan-(1/2)

Please tell me is this right

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tiny-tim
Homework Helper
hi kalupahana!

no, you're misunderstanding the question

first she swims directly across, but the current makes her go at an angle which (as you say) is tan-1(1/2)

ok, so that means that the ratio of her speed and the speed of the current is … ?

now use that ratio to find the angle she must swim upstream to land exactly opposite

hi kalupahana!

no, you're misunderstanding the question

first she swims directly across, but the current makes her go at an angle which (as you say) is tan-1(1/2)

ok, so that means that the ratio of her speed and the speed of the current is … ?

now use that ratio to find the angle she must swim upstream to land exactly opposite
Then it come as

1/2 = u/v
v/2u = tanα
α = tan-1(v/2u)

is it come like this

It's a good question, and I think tiny tim is right and kalupahana is not (yet). I don't know how you people can solve these problems without drawing vector addition triangles.

tiny-tim
Homework Helper
hi pongo38!

yup, kalupahana, you should have drawn a second triangle by now …

you seem to be still on the first one.

what does your second triangle look like?

hi pongo38!

yup, kalupahana, you should have drawn a second triangle by now …

you seem to be still on the first one.

what does your second triangle look like?
To horizontal 4km/h. VRG is constant. Therefore The VRG of both triangles are equal.
VRM is 60o inclined downwards from VMG to opposite direction.

tan60o = x/4
4√3 = VRG
cos60 = 4/y
1/2 = VRM
Now its okay?

tiny-tim
Homework Helper
hi kalupahana!

(just got up :zzz: …)

i think you've got it, but i'm finding it difficult to understand what you've written

you should have got an equilateral triangle … did you?

hi kalupahana!

(just got up :zzz: …)

i think you've got it, but i'm finding it difficult to understand what you've written

you should have got an equilateral triangle … did you?

oh, sorry tiny tim, i posted two posts at one time. When I check them, I reply a 2nd questions tags in here.

okk, i can understand it

thnx a lot