Question

A brick weighing 4.0 kg is dropped into a 1.0 m deep river from a height of 2.0 m. Assuming that 80% of the gravitational potential energy is finally converted into thermal energy, find this thermal energy is calorie.

Answer 1

Given:
Mass of the brick, m = 4 kg
Total vertical distance travelled by the brick, h = 3 m

Percentage of gravitational potential energy converted to thermal energy = 80

Total change in potential energy of the brick = mgh = 4 × 10 × 3 = 120 J
$\mathrm{Thermal}\mathrm{Energy}=120\times \frac{80}{100}=96\mathrm{J}$

Thermal energy in calories is given by
$U=\frac{96}{4.2}=22.857\mathrm{cal}\approx 23\mathrm{cal}$