Which is heaviest?
A) oxygen atom
B) hydrogen atom
C) electron
D) proton
E) water molecule
E
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As a ball falls, the action force is the Earth's pull on the ball. The reaction force is the
A) air resistance acting against the ball. B) acceleration of the ball. C) ball's pull on Earth. D) none of the above
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)
If the date is April 21, what zodiac constellation will be visible on your meridian at midnight?
A) Leo B) Aquarius C) Libra D) Aries
When strong solar winds are displaced poleward by our magnetic fields, we get
A) the Van Allen radiation belts. B) intense auroral displays. C) sunspots. D) hurricanes in the tropics. E) droughts and dust bowls in the American West.