Figure shows a rod of length l resting on a wall and the floor. Its lower end A is pulled towards left with a constant velocity u. As a result of this, end A starts moving down along the wall. Find the velocity of the other end B downward when the rod makes an angle `theta` with the horizontal.

Explanation

In such type of problems, when velocity of one part of a body is given and that of other is required, first find the relation between the two displacement, then differentiate them with respect to time.Here if the distance from the corner to the point `A` is `x` and upto `B` is `y`, then `x^(2)+y^(2)=l^(2)`
Differentiating with respect to time `t`
`2xdx/dt+2ydy/dt=0`
where `v_(A)=v=dx/dt & v_(B)=v=-dx/dt`
(-sign denotes that `y` is decreasing )
`xv=yv_(B)`
`v_(B)=(V)x/y=v cot theta`