Question

A block of mass $2\text{kg}$ is being brought down by a string. If the block acquires a speed of $1\text{m/s}$ in dropping down $25\text{cm}$, find the work done by the tension in the process.

• A. $-\text{16 J}$
• B. $4\text{J}$
• C. $-4\text{J}$
• D. $-4.5\text{J}$

Answer 1

The correct option is C $-4\text{J}$

By the FBD on the block

Applying kinematics, we get

$v^2=u^2+2as$ Where $v=1\text{m/s}$ is the final velocity, $s$ is the displacement covered in that time interval

$1=0+2a\times 0.25$

$\Rightarrow a=2{\text{m/s}}^2$

From $FBD$,

$20-T=ma=2\times 2$
$20-T=2a=2\times 2$
$T=16\text{N}$

Work done by tension,
$W=-TS=-16\times 0.25=-4\text{J}$
The string does negative work (because tension is acting in opposite direction of displacement)