The Flash, a comic superhero, is able to run close to the speed of light. Two people, with synchronized watches, stand several kilometers apart and time the Flash as he runs a straight course. The Flash has his own stopwatch and uses it to independently time his run. The Flash also observes the speed at which the finish line moves toward him, and upon finishing his run he calculates the length of the course by multiplying this speed by the time on his stopwatch. Who measures (I) the Flash's proper time for the race, and who measures (II) the rest or proper length for the race?
A. Both the Flash and the two people measure (I) and (II)
B. Only the Flash measures both (I) and (II)
C. The two people measure (I) and Flash measures (II)
D. Neither the Flash nor the two people measure (I) or (II)
E. The Flash measures (I) but the two people measure (II)
Answer: E
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The standard kilogram is a platinum-iridium cylinder 39 mm in height and 39 mm in diameter. What is the density of the material?
Conservation of ___________ is observed in all interactions
a. baryon number b. lepton number c. strangeness d. charm e. beauty
A 500-N block, on a 30.0° incline, is being held motionless by friction. The coefficient of static friction between the block and the plane is 0.63 . The force due to friction is:
a. 0 N. b. 433 N. c. 250 N. d. 500 N. e. 354 N.
An electronic circuit board is the shape of a flat plate 0.3 m × 0.3 m in plan-form and dissipates 15 W. It is placed in operation on an insulated surface in a horizontal position or at an angle of 45 degrees to horizontal, in both cases; it is in still air at 25°C. If the circuit fails above 60°C, determine if the two proposed installations are safe.
GIVEN
• A flat plate with insulated back, horizontal or at an angle of 45 degrees in still air
• Plate size (s × s) = 0.3 m × 0.3m
• Heat generation rate (qG) = 15 W
• Air temperature (T?) = 25°C
• Maximum plate temperature (Ts) = 60°C
FIND
• If the two plate positions are safe
ASSUMPTIONS
• Radiative heat transfer is negligible
SKETCH
PROPERTIES AND CONSTANTS
for dry air at the mean temperature of 43°C
Thermal expansion coefficient (?) = 0.00316 1/K
Thermal conductivity (k) = 0.0267 W/(m K)
Kinematic viscosity (?) = 17.9 × 10–6 m2/s
Prandtl number (Pr) = 0.71