Question

At a given instant, $A$ is moving with velocity of $5m/s$ upwards. What is velocity of $B$ at that time ?

• A. $15m/s\downarrow$
• B. $15m/s\uparrow$
• C. $5m/s\downarrow$
• D. $5m/s\uparrow$

Answer 1

The correct option is A $15m/s\downarrow$

Let the displacements of $A$ and $B$ be $X$ and $Y$ respectively.

Using work done by tension method, (net work - done by tension is zero, since it is an internal force)
$3TX+TY=0\Rightarrow Y=-3X$
Differentiating,
$V_B=-3V_A=-3(+5)=-15m{s}^{-1}=15m{s}^{-1}\text{downwards}$