Question

In the figure shown the velocity of lift is $2$ m/s while string is winding on the motor shaft with velocity $2$ m/s and block A is moving downwards with a velocity of $2$ m/s, then find out the velocity of block B.

• A. $2$ m/s $\uparrow$
• B. $2$ m/s $\downarrow$
• C. $4$ m/s $\uparrow$
• D. none of these

Answer 1

The correct option is D none of these

Given,

Velocity of lift $V_L=2m/s\uparrow$

Velocity of string $V_{string}=2m/s$

Let length of string is $l$

Here,

$-\frac{dl}{dt}=2m/s$

Velocity of Block $V_B=2m/s\downarrow$

Let displacement of block B is y and displacement of block A is x then,

$x=y+(l-x)$

$y=2x-l$

Differentiate both side with respect to time.

$\frac{dy}{dt}=2\frac{dx}{dt}-\frac{dl}{dt}{V}_B=2\times 2+2=6m/s$