Block rising between two downward-moving wires at angle α

[phymatter]

Homework Statement

[URL=http://img18.imageshack.us/i/questionf.png/][PLAIN]http://img18.imageshack.us/img18/4880/questionf.png[/URL]

Uploaded with ImageShack.us[/PLAIN]

Homework Equations

if the 2 wires on both sides move with downward velocity u , with what velocity does the block rise , the angle with vertical is a ;

The Attempt at a Solution

i cannot figure out the solution, i think it should be 2ucos(a)
please help!1

 

[cupid.callin]

consider the triangle in red

use Pythagoras theorem on it and differentiate it.

dh/dt is what you need and dl/dt is speed of string
what you think of db/dt ?

 

[phymatter]

thanks cupid.callin !

 

[Kartikc]

so is 2ucos(a) right? i think it should be!

 

[cupid.callin]

so is 2ucos(a) right? i think it should be!

no its not ...

even i thought it should be but its not

try the way i told and you'll get the answer