Question

A machine which is $70 \%$ efficient raises a $10\, kg$ body though a certain distance and spends $100\, J$ energy. The body is then released. On reaching the ground, the kinetic energy of the body will be

• A. 0 J
• B. 70 J
• C. 50 J
• D. 35 J

Answer 1

Answer: (B)

Energy spent by the machine to raise the body of mass $10\, kg$ at certain height, is $E=100\, J$
Since, machine is only $70 \%$ efficient.
Hence, energy used by machine,
$E'=70 \%$ of $E$
$=\frac{70}{100} \times 100=70\, J$
When body is released, then whole energy spent by machine is converted into kinetic energy on reaching the ground.
$\therefore $ Kinetic energy, $K=E'=70\, J$