First slide

Calorie

Question

From what height should an ice block at – 20°C be freely released so that 84 % of its mechanical energy is converted into heat and it just melts as it reaches the ground
(g = 10 ms^{– 2}specific heat of ice is 0.5cal/gm/°C and latent heat of fusion of ice is 80 cal/gm)

Moderate

A

4.05 m
B

450m
C

4.5 m
D

45 km

Solution

$mgh \times \frac{84}{100} = [m \times 0 .5 \times [0 - (- 20)] + m \times 80] \times 4 .2 \times 10^{3}$
→h = 45 km.

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