Physics - W.E.P. Question with Solution | TestHub

Let the mass of the bob be $m$.

The initial total energy of the bob at point P (taking the potential energy at Q as zero) is purely potential energy:

$E_i=mgh$

The bob loses $20\%$ of its total energy due to air resistance as it moves from P to Q. Therefore, the remaining energy at Q is $80\%$ of the initial energy:

$E_f=0.8E_i=0.8mgh$

At point Q, all the remaining energy is in the form of kinetic energy:

$\frac{1}{2}m{v}^2=0.8mgh$

$v^2=1.6gh$

Substituting the given values $g=9.8{\text{ m/s}}^2$ and $h=2\text{ m}$:

$v^2=1.6\times 9.8\times 2$

$v^2=31.36$

$v=\sqrt{31.36}=5.6\text{ m/s}$

Answer: $5.6\text{ m/s}$