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

Thermal energy = 22.8 cal

Explanation:

Given: Mass of brick = 4 kg

Height = 3 m

Let g be 10 m/s

Potential energy = mgh = 4 * 10 * 3 = 120 J

Amount of energy converted to thermal energy  = 80%

= 80 / 100 * 120

= 96 J

Thermal energy in calories =  96 / 4.2 = 22.8 cal

Answer 2

Thermal energy is 23 cal.

Explanation:

Given data in the question

$\begin{aligned}&m=4 \mathrm{kg}\\&h=3 m\end{aligned}$

m is Mass of the brick,  

h is Total vertical distance traveled by the brick,  

Percentage of gravitational energy transferred to heat $=80$

Total change in Brick's potential energy$=m g h=4 \times 10 \times 3=120 J$

Thermal energy is  $=120 \times \frac{80}{100}$

$=12 \times 8$

$=96 \mathrm{J}$

Thermal energy is given in calories

$U=\frac{96}{4.2}=22.857 \mathrm{cal} \approx 23 \mathrm{cal}$