If a radioactive isotope has a one year halflife, what fraction will remain after 5 years?
A) None
B) Half
C) 1/32
D) 1/5 [one fifth]
E) 1/64
C
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"Seeds" around which galaxies, clusters and walls grew may have been ___________ before inflation and recombination
a. caused by hot dark matter b. caused by baryons c. the result of the separation of the electromagnetic and weak forces d. the result of freezing water molecules shortly after the Universe became transparent to photons e. caused by initially tiny quantum mechanical fluctuations in space-time
Scientific models are used to ________
A) make miniature representations of the universe B) present the scale of the solar system to the general public C) prove that past paradigms no longer hold true D) make specific predictions that can be tested through observations or experiments
A Ferris wheel starts from rest and builds up to a final angular speed of 1.9 rad/s while rotating through an angular displacement of 4.9 rad. What is its average angular acceleration?
a. 9.3 rad/s2 b. 0.37 rad/s2 c. 2.6 rad/s2 d. 0.39 rad/s2
A journal bearing is idealized as a stationary flat plate and a moving flat plate that moves parallel to it. The space between the two plates is filled by an incompressible fluid. Consider such a bearing with the stationary and moving plates at 10°C and 20°C respectively, the distance between them is 3 mm, the speed of the moving plate is 5 m/s, and there is engine oil between the plates.
(a) Calculate the heat flux to the upper and lower plates
(b) Determine the maximum temperature of the oil.
GIVEN
Journal bearing: Two flat plates, one stationary, one moving with oil between them
Stationary plate temperature (Ts) = 10°C
Moving plate temperature (Tm) = 20°C
Distance between plate (H) = 3 mm = 0.003 m
Speed of the moving plate (Up) = 5 m/s
FIND
(a) Heat flux (q/A) for the plates
(b) The maximum temperature of the oil
ASSUMPTIONS
Steady state
Constant physical properties
Negligible edge effects
Oil is incompressible
SKETCH
PROPERTIES AND CONSTANTS
From Appendix 2, Table 17, for engine oil at 15°C
Thermal conductivity (k) = 0.145 W/(m K)
Absolute viscosity (?) = 1.561 (Ns)/m2
Prandtl number (Pr) = 196