(d) A spaceship of mass 2m originally at rest burns some matter–antimatter fuel until the ship’s mass is m. The light energy ejected from the ship in this process has total energy m as well.
A. C
B. F
B. F
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An electric room heater has been designed in the shape of a vertical cylinder 2-m-tall and 30 cm in diameter. For safety, the heater surface cannot exceed 35°C. If the room air is 20°C, find the power rating of the heater in watts.
GIVEN
• An electric heater in the shape of a vertical cylinder
• Heater height (H) = 2 m
• Heater diameter (D) = 30 cm = 0.3 m
• Room air temperature (T?) = 20°C = 293 K
FIND
• The power rating of the heater (q) in watts
ASSUMPTIONS
• Radiation is negligible
• Heat transfer from the top and bottom of the tank is negligible
SKETCH
PROPERTIES AND CONSTANTS
for dry air at the mean temperature of 27.5°C
Thermal expansion coefficient (?) = 0.00332 1/K
Thermal conductivity (k) = 0.0256 W/(m K)
Kinematic viscosity (?) = 16.4 × 10–6 m2/s
Prandtl number (Pr) = 0.71
Radioactive Decay: Given that the half-life for radium-226 is 1600 y, how long is the "quarter-life" (that is, the time for the radiation to decay to one-fourth of its initial level)?
Fill in the blank(s) with the appropriate word(s).
A system consists of two thermal reservoirs in contact with each other, one at temperature 300°C and the other at temperature 200°C. 600 J of heat transfers from the 300°C reservoir to the 200°C reservoir
What is the change in entropy of this system? A) 0.221 J/K B) 1.00 J/K C) 5.00 J/K D) -1.00 J/K E) 2.31 J/K
What prevents gravity from pulling you through the ground to the center of the Earth? Choose the best answer
1.They repel each other. 2.They attract each other. 3.They may attract or repel each other, depending on whether the charge on the insulator is positive or negative. 4.They exert no electrostatic force on each other. 5.The charged insulator always spontaneously discharges.