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Mechanics

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 Joined: 27 Jan 2007 Post: 3
27 Jan 2007 15:47:35 IST
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Kinematics
Engineering Entrance , Medical Entrance , AIPMT , JEE Main , AIIMS , JEE Advanced , Physics , Mechanics

I want to ask a few questions:
1.A man can swim with a speed of 4 km/hr in still water.How long does he take to cross a river 1 km wide if the river flows steadily at 3km/hr and he makes strokes normal to the river current?How far down the river does he go when he reaches the other bank?
2.A boatman can row with a speed of 10km/hr in still water.If the river flows steadily at 5km/hr,in which direction should the boatman row in order to reach a point on the other bank directly opposite to the point from where he started?The width of the river is 2 km.

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Joined: 23 Jan 2007
Posts: 373
27 Jan 2007 16:10:17 IST
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Dear Prerna, nice question, envolves both relative vel and some concept of vectors... m feelin sorry that i cant explain it with a diagram.. but hope u understad without a diag.

The most important thing to understand in vectors is that everything.. displacement, velocity, acceleration, force.. all along x axis is unaffected by what is happening along y axis and vice versa.

now, Q1.  Swimming in normal dirn, man's vel is 4kmph, so he takes 1/4 hr.
and the river current gives the man a vel of 3kmph in a perpendicular                   dirn, so it takes him (1/4)3 km along the flow.

Q2. To reach from pt A to pt B, the resultant vel should be in direction of vector AB.. (from A to B) isnt it?? so the resultant of the vel of river + man (vector additon) should be in the normal dirn, as here vector AB is in normal dirn.
Just use vector to get the answer.. tell me if u don get it.

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