Quasars are caused by galaxy collisions in the early Universe
a. True
b. False
Indicate whether the statement is true or false
True
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Vision Problems: What is the power of the lens that has a focal length of 40 cm and a diameter if 5.0 cm?
A. -2.5 diopters B. +20 diopters C. +0.20 diopters D. +2.5 diopters E. +2.7 diopters
A 55kg girl swings on a swing, whose seat is attached to the pivot by 2.5m long rigid rods (considered to be massless in this problem). As she swings, she rises to a maximum height such that the angle of the rods with respect to the vertical is 32 degrees. What is the average torque on the rods due to her weight, as she moves during one cycle of her swinging from the bottom of her swing path to the highest point?
A. 370 Nm B. 970 Nm C. 2400 Nm D. 2000 Nm
A small gray sphere having an emissivity of 0.5 and a surface temperature of 5370C is located in a blackbody enclosure having a temperature of 37°C. Calculate for this system: (a) the net rate of heat transfer by radiation per unit of surface area of the sphere, (b) the radiative thermal conductance in W/K if the surface area of the sphere is 95 cm2 , (c) the thermal resistance for radiation between the sphere and its surroundings, (d) the ratio of thermal resistance for radiation to thermal resistance for convection if the convective heat transfer coefficient between the sphere and its surroundings is 11 W/(m2 K), (e) the total rate of heat transfer from the sphere to the surroundings, and (f) the combined heat transfer coefficient for the sphere.
GIVEN
• Small gray sphere in a blackbody enclosure
• Sphere emissivity (?s) = 0.5
• Sphere surface temperature (T1) = 3370C= 810 K
• Enclosure temperature (T2) = 37°C = 310 K
• The surface area of the sphere (A) = 95 cm2=9.5*10-3 m2
• The convective transfer coefficient ( ch ) = 11 W/(m2 K)
FIND
(a) Rate of heat transfer by radiation per unit surface area (b) Radiative thermal conductance (Kr) in W/K (c) Thermal resistance for radiation (Rr) (d) Ratio of the radiative and conductive resistance (e) Total rate of heat transfer (qT) to the surroundings (f) Combined heat transfer coefficient ( cr h )
ASSUMPTIONS
• Steady state prevails
• The temperature of the fluid in the enclosure is equal to the enclosure temperature
SKETCH
In which of the following cases would the tragedy of the commons apply?
a. common use of land for grazing of sheep b. use of air in industrial processes c. use of water for power plant cooling d. Tragedy of the commons applies in all the above cases. e. Tragedy of the commons does not apply in any of the above cases.