Kepler's discovery that T2 /r3 = K applies
A) to orbits where K is a universal constant.
B) only to circular orbits.
C) to lunar motion provided K is the same for planetary motion.
D) to elliptical orbits where r is the average distance.
D
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When discussing stellar lives, astronomers divide stars by mass into low-mass stars (masses similar to the Sun or lower) and high-mass stars (masses more than about 8 times that of our Sun). Which category includes stars that die in supernova explosions?
A) high-mass stars only B) low-mass stars only C) They both die in supernova explosions. D) Neither: Supernovas only occur in binary star systems.
Charge of uniform density 12 nC/m is distributed along the x axis from x = 2.0 m to x = 5.0 m. What is the electric potential (relative to zero at infinity) at the origin (x = 0)?
A. 91 V B. 99 V C. 82 V D. 74 V E. 140 V
A 10 kg object has a moment of inertia of 1.25 kg 
If a torque of 2.5 N•m is applied to the object, the angular acceleration is
A. 10 
B. 8.0
C. 6.0
D. 4.0
E. 2.0
A circular ice skating rink, 20 m in diameter, is enclosed by a large hemispherical dome of diameter 30 m. Assuming that the ice is 2 cm thick, estimate the time it takes for the ice to melt if the refrigeration system of the rink fails. Make this calculation first with both the dome and the ice behaving as black bodies at respective temperatures of 20°C and 0°C. Then repeat with the water-ice having an emissivity of 0.3 and losing heat by natural convection to the air in the dome with a convection heat transfer coefficient of 15 W/(m2 K).
GIVEN
• Circular skating rink enclosed by hemispherical dome
• Diameter of the rink (D1) = 20 m
• Diameter of hemispherical dome (D2)= 30 m
• Temperature of dome and surrounding air (T2)= 200C
• Temperature of ice (T1) = 00C
FIND
Time it takes for ice to melt if the refrigeration system of rink fails considering (a) both dome and ice behaving as black bodies. (b) ice having emissivity of 0.3 and loosing heat by natural convection to air in dome with 2 15 W/(m K)
SKETCH
PROPERTIES AND CONSTANTS
Latent Stefan-Boltzman’s heat of fusion constant for ice (Lf)= (? 334 KJ/kg )=5.67*10-8 W/(m2 K4)