Home » Ask & Discuss » Physics
« Back to Discussion

Mechanics

New kid on the Block

Joined:	27 Jan 2007
Post:	3

27 Jan 2007 15:47:35 IST

0 People liked this

527

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.

Comments (1)

SUDEEP KUMAR

Forum Expert

Joined: 23 Jan 2007

Posts: 373

27 Jan 2007 16:10:17 IST

Like 1 people liked this

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.

Quick Reply

Reply

	Some HTML allowed.
	Keep your comments above the belt or risk having them deleted.
	Signup for a avatar to have your pictures show up by your comment
	If Members see a thread that violates the Posting Rules, bring it to the attention of the Moderator Team