A 0.20-km wide river has a uniform flow speed of 4.0 m/s toward the east. It takes 20 s for a boat to cross the river to a point directly north of its departure point on the south bank. In what direction must the boat be pointed in order to accomplish this?
a. 23° west of north
b. 20° west of north
c. 24° west of north
d. 22° west of north
e. 17° west of north
D
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Predict the heat transfer per unit area for warm water at 70ºC placed in a large pan at 10ºC.
What will be an ideal response?
How do we know that the Universe's expansion is speeding up?
A.) We've measured it for a long time and seen that galaxies are moving faster with time. B.) We look at objects from different times in the past to see how expansion changes with time. C.) We can see the change in the expansion rate from year to year. D.) We can see the change in the expansion rate from month to month. E.) We can see the change in the expansion rate from week to week.
A copper wire, 0.8 mm OD, 5 cm long, is placed in an air stream whose temperature rises at a rate given by Tair = (10+ 14t)°C, where t is the time in seconds. If the initial temperature of the wire is 10°C, determine its temperature after 2 s, 10 s and 1 min. The heat transfer coefficient between the air and the wire is 40 W/(m2 K).
GIVEN
• A copper wire is placed in an air stream
• Wire diameter (D) = 0.8 mm=8*10-4 m
• Wire length (L) = 5 cm=0.05 m
• Air stream temperature is: Tair = (10 + 14t)°C
• The initial temperature of the wire (To) = 10°C
• The heat transfer coefficient ( )ch = 40 W/(m2 K)
FIND
• The wire temperature after 2 s, 10 s and 1 min
ASSUMPTIONS
• Constant and uniform heat transfer coefficient
SKETCH
PROPERTIES AND CONSTANTS
For copper at 127°C
• Thermal conductivity (k) = 383 W/(m K)
• Density (?) = 8933 kg/m3
• Specific heat (c) = 383 J/(kg K)
It takes 4.00 minutes to fill a gas tank with 50.0 liters of gasoline. If the pump nozzle is 1.35 cm in radius, what is the average speed of the gasoline as it leaves the nozzle? (1 000 liters = one cubic meter)
a. 3.64 m/s b. 0.364 m/s c. 21.8 m/s d. 1.36 m/s e. 12.5 m/s