Question

A man throws the bricks to a height of $12 \,m$ where they reach with a speed of $12 \,m / s$. If he throws the bricks such that they just reach that height, what percentage of energy will be saved : ( Take $g =9.8 \,m / s ^{2}$ )

• A. $29 \%$
• B. $46 \%$
• C. $38 \%$
• D. $50 \%$

Answer 1

Answer: (C)

Case-I.

Let speed with which the ball is thrown up is $u_{1}$
$(12)^{2} =u_{1}^{2}-2 \times 9.8 \times 12$
$u_{1}^{2} =379.2$
$K E_{1} =\frac{1}{2} m u_{1}^{2}=\frac{1}{2} m(379.2)$
Case-II.

$(0)^{2} =u_{2}^{2}-2 \times 9.8 \times 12 $
$u_{2}^{2} =235.2$
$K E_{2} =\frac{1}{2} m u_{2}^{2}=\frac{1}{2} m (235.2) $
$\%$ energy charge $=\frac{K E_{2}-K E_{1}}{K E_{1}} \times 100 $
$=-38 \%$
Negative sign shows that energy is saved.