Question

A man throws the bricks to the height of $12m$ where they reach with a speed of $12m/s$. If he throws the bricks such that they just reach this height, then what percentage of energy will he save?

• A. $19$%
• B. $38$%
• C. $57$%
• D. $76$%

Answer 1

Answer: (B)

$38$%

In first case, $W_1=\frac{1}{2}m{\left(12\right)}^2+mg\times 12$
$=\frac{1}{2}m\times 144+m\times 10\times 12$
= $192m$
In second case, $W_2=mgh=120m$
Therefore, percentage of energy saved $=\frac{192m-120m}{192m}\times 100$
$=38$%