Question

A mason worker throws the bricks to a height of $10m$, at which the bricks reach with a velocity of $6$ $m{s}^{-1.}$. The percentage of energy he is wasting is nearly : $(g=10$ $m{s}^{-2})$

• A. $15\%$
• B. $18\%$
• C. $220\%$
• D. $36\%$

Answer 1

The correct option is A $15\%$

Given,

$h=10m$

$g=-10m/s^2$

$v=6m/s$

$m=$ mass

The kinetic energy , $K_f=\frac{1}{2}m{v}^2$

$K_f=\frac{1}{2}m\times (6{)}^2=10mJ$

From the 3rd equation of motion,

$v^2-u^2=2gh$

$(6{)}^2-u^2=-2\times 10\times 10=-200$

$36-u^2=-200$

$u^2=236$

The kinetic energy is,

$K_i=\frac{1}{2}m{u}^2=\frac{1}{2}m\times 236=118mJ$

The percentage of energy he is wasting is,

$\frac{18}{118}\times 100=15.25$%

The correct option is A.