Buoyancy: A piece of wood is floating in a bathtub. A second piece of wood sits on top of the first piece, and does not touch the water. If the top piece is taken off and placed in the water, what happens to the water level in the tub?
A. It goes up.
B. It goes down.
C. It does not change.
D. It cannot be determined without knowing the volumes of the two pieces of wood.
Answer: C
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Compare the rate of condensate flow from the pipe in Problem 8.28 (air pressure = 200 kPa) with that for a 3.89-cm-OD pipe and 200 kPa air pressure. What is the rate of condensate flow if the 2 cm pipe is submerged in a 20°C constant-temperature water bath?
GIVEN
• A long horizontal copper pipe carrying saturated steam within an environmental testing chamber or a water bath
• Steam pressure = 120 kPa
• Ambient pressure (P) = 2 atm
• Ambient air or water temperature (T?) = 20°C
FIND
Rate of condensate flow for
(a) Diameter (D) = 3.89 cm = 0.0389 m Fluid is air at 2.0 atm
(b) Diameter (D) = 2 cm = 0.02 m Fluid is water at T? = 20°C
ASSUMPTIONS
• Pressure change has no effect on absolute viscosity, thermal conductivity, or specific heat of the air
• Air is still
• Convective thermal resistance on the inside of the pipe is negligible
• Thermal resistance of the copper pipe is negligible
• The air behaves as an ideal gas
SKETCH
PROPERTIES AND CONSTANTS
For saturated steam at 0.12 MPa, the heat of vaporization (hfg) = 2238 kJ/kg, and the temperature (Ts) = 105°C.
for dry air at the mean temperature of 62.5°C and one atmosphere
Thermal expansion coefficient (?) = 0.00298 1/K
Thermal conductivity (k) = 0.0281 W/(m K)
Prandtl number (Pr) = 0.71 at P = 2.0 Atm,
Kinematic viscosity (?) = 9.83 × 10–6 Ns/m2
If a proton and an electron have the same momentum, the longer wavelength belongs to the
A) proton. B) electron. C) both the same
A plane convex lens is made of glass (n = 1.5) with one flat surface and the other having a radius of 20 cm. What is the focal length (in cm) of the lens?
a. 20 b. 30 c. 40 d. 10 e. 50
At t = 0, a wheel rotating about a fixed axis at a constant angular acceleration has an angular velocity of 2.0 rad/s. Two seconds later it has turned through 5.0 complete revolutions. What is the angular acceleration of this wheel?
a. 17 rad/s2 b. 14 rad/s2 c. 20 rad/s2 d. 23 rad/s2 e. 13 rad/s2