A horizontal 52-N force is needed to slide a box across a flat surface at a constant velocity of 3.5 m/s. What is the coefficient of kinetic friction between the box and the floor?
A) 0.11
B) 0.09
C) 0.10
D) 0.13
A
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For the aluminum alloy 7075-T6, shown in Figure 11.23, a stress amplitude of 250 MPa is applied for 106 cycles to a group of 100 specimens. How many of these specimens should fail in this test?
What will be an ideal response?
An engineer is designing a heating system that consists of multiple tubes placed in a duct carrying the air supply for a building. She decides to perform preliminary tests with a single copper tube, 2-cm-O.D., carrying condensing steam at 100°C. The air velocity in the duct is 5 m/s and its temperature is 20°C. The tube is placed normal to the flow, but it may be advantageous to place the tube at an angle to the air flow and thus increase the heat transfer surface area. It the duct width is 1 m, predict the outcome of the planned tests and estimate how the angle ? will affect the rate of heat transfer. Are there limits?
GIVEN
• A copper tube carrying condensed steam in an air duct
• Tube outside diameter (D) = 2 cm = 0.02 m
• Steam temperature (Ts) = 100°C
• Air velocity (U?) = 5 m/s
• Air temperature (T?) = 20°C
• Duct width (w) = 1 m
FIND
• Is it more advantageous to have the tubes normal to the air flow or at some angle to the air flow?
ASSUMPTIONS
• Steady state
• Air velocity in the duct is uniform
• Thermal resistance due to steam condensing is negligible
• Thermal resistance of the tube wall is negligible
SKETCH
PROPERTIES AND CONSTANTS
Thermal conductivity (k) = 0.0251 W/(m K) Kinematic viscosity
(?) = 15.7 × 10–6 m2/s Prandtl number (Pr) = 0.71
Which of these graphs would be correct for the speed of a ball thrown upward? 
A. A B. B C. C D. D
It is not possible to increase the efficiency of thermal power plants because of thermodynamics
What will be an ideal response?