According to the well-known equation E = mc2,
A) mass and energy travel at the speed of light squared.
B) energy is actually mass traveling at the speed of light squared.
C) mass and energy travel at twice the speed of light.
D) mass and energy are related.
E) none of the above
Answer: D
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Bottles of perfume sometimes have thick glass (n = 1.5) walls which give the impression the volume is larger than it is. Assume a cylindrical bottle has an inner radius of 1.0 cm and an outer radius of 2.0 cm. How thick (in cm) would the wall appear if you could see a mark on the inside surface?
a. 0.80 b. 0.51 c. 0.72 d. 80 e. 12
A sphere 20 cm in diameter containing liquid air (–140°C) is covered with 5-cm-thick glass wool (50 kg/m3 density) with an emissivity of 0.8. Estimate the rate of heat transfer to the liquid air from the surrounding air at 20°C by convection and radiation. How would you reduce the heat transfer?
GIVEN
• A sphere containing liquid air covered with glass wool
• Sphere diameter (Ds) = 20 cm = 0.2 m
• Liquid air temperature (Ta) = – 140°C = 133 K
• Surrounding air temperature (T?) = 20°C = 293 K
• Insulation thickness (s) = 5 cm = 0.05 m
• Insulation emissivity (?) = 0.8
FIND
• Rate of heat transfer from liquid air to surrounding air
(q)
• How can this be reduced?
ASSUMPTIONS
• Steady state conditions
• The surroundings behave as a black body enclosure at T?
• Surrounding air is still
• Thermal resistance of the convection inside the sphere and of the container wall are negligible
SKETCH
PROPERTIES AND CONSTANTS
the Stephan-Boltzmann constant (?) = 5.67 × 10–8 W/(m2 K4).
the thermal conductivity of glass wool (ki) = 0.037 W/(m K)
Structure 1 with member BC of length L/2 has force P1 applied at joint C. Structure 2 with member BC of length L has force P2 applied at C. Both structures have constant flexural rigidity EI. If the ratio of the applied forces P1/P2 5 5/2, the ratio of the joint B rotations ?B1/?B2 is approximately:
(A) 1
(B) 5/4
(C) 3/2y
(D) 2
The relaxation time for tensile stress relaxation is equal to the elastic modulus divided by three times the _________.
Fill in the blank(s) with the appropriate word(s).