When railroad tracks make a curve, the outer track is longer. This means a wheel that rides on the outer track needs to somehow
A) roll slower than the wheel on the inner track.
B) maintain the same speed as the wheel on the inner track.
C) roll faster than the wheel on the inner track.
Answer: C
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Two concentric spheres 0.2 m and 0.3-m in diameter, with the space between them evacuated, are to be used to store liquid air (133 K). If the surfaces of the spheres have been flashed with aluminum and the liquid air has a talent heat of vaporization of 209 kJ/kg, determine the number of kilograms of liquid air evaporated per hour.
GIVEN
? Two concentric spheres with the space between them evacuated and liquid air in the inner sphere
? Diameters
? D1 = 0.2 m
? D2 = 0.3 m
? Liquid air temperature (Ta) = 133 K
? Room temperature (T?) = 293 K
? Surfaces of the spheres have been flashed with aluminum
? Heat of vaporization of liquid air (hfg) = 209 kJ/kg = 209,000 J/kg
FIND
? The number of kilograms of liquid air evaporated per hour ( m )
ASSUMPTIONS
? Steady state
? Convective thermal resistance between the liquid air and interior sphere is negligible
? Thermal resistance of the sphere walls is negligible
? Natural convection on the exterior is negligible
? The room behaves as a blackbody enclosure
? The thickness of the sphere walls in negligible
SKETCH
PROPERTIES AND CONSTANTS
From Appendix 1, Table 5, the Stephan-Boltzmann constant
From Table 11.3, the hemispherical emissivity of the spheres will be approximated by that for
oxidized aluminum at 310 K: ? ? = ??1 ???2 = 0.11
In order to slide a 70.0 kg box across the floor at a constant speed, you must push horizontally with a force of 351 N. What is the coefficient of kinetic friction?
A. 0.149 B. 0.584 C. 0.337 D. 0.512 E. 0.678
The conservation of energy equation must include
A. Work done on the system B. Thermal energy added to the system C. Thermal energy leaving the system D. Work done by the system. E. All of these are correct.
A small metal sphere is suspended from the conducting cover of a conducting metal ice bucket by a non-conducting thread. The sphere is given a negative charge before the cover is placed on the bucket. The bucket is tilted by means of a non-conducting material so that the charged sphere touches the inside of the bucket. Which statement is correct?
a. The negative charge remains on the metal sphere. b. The negative charge spreads over the outside surface of the bucket and cover. c. The negative charge spreads over the inside surface of the bucket and cover. d. The negative charge spreads equally over the inside and outside surfaces of the bucket and cover. e. The negative charge spreads equally over the sphere and the inside and outside surfaces of the bucket and cover.