Two blocks, joined by a string, have masses of 6.0 and 9.0 kg. They rest on a frictionless, horizontal surface. A second string, attached only to the 6.0-kg block, has horizontal force = 45 N applied to it. Both blocks accelerate. Find the tension in the string between the blocks
a. 18 N
c. 12 N
b. 27 N
d. 9.0 N
B
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Exhaust gases from a power plant are used to preheat air in a cross-flow heat exchanger. The exhaust gases enter the heat exchanger at 450°C and leave at 200°C. The air enters the heat exchanger at 70°C, leaves at 250°C, and has a mass flow rate of 10 kg/s. Assume the properties of the exhaust gases can be approximated by those of air. The overall heat transfer coefficient of the heat exchanger is 154 W/(m2 K). Calculate the heat exchanger surface area required if (a) the air is unmixed and the exhaust gases are mixed and (b) both fluids are unmixed.
GIVEN
• Cross-flow heat exchanger – exhaust gas to air
• Exhaust temperatures
? Te,in = 450°C
? Te,out = 200°C
• Air temperatures
? Ta,in = 70°C
? Ta,out = 250°C
• Air flow rate m a= 10 kg/s
• Overall heat transfer coefficient (U) = 154 W/(m2 K)
FIND
The heat exchanger surface area (A) if
(a) Air is unmixed, exhaust is mixed
(b) Both are unmixed
ASSUMPTIONS
• The properties of the exhaust gases can be approximated by those of air
• The air is in the tube
SKETCH
PROPERTIES AND CONSTANTS
the specific heat of air at the mean temperature of 160°C (cpa)
Two masses are suspended by cord that passes over a pulley with negligible mass. The cord also has negligible mass. One of the masses, 
has a mass of 5.0 kg and the other mass,
has a mass of 3.0 kg. The acceleration of 
is
A. 3.3 
upward.
B. 3.3 upward.
C. 2.5 upward.
D. 2.5 upward.
E. 1.1 upward.
A 50-N object falling in air experiences 30 N of air resistance. The net force on the falling object is
A) 0 N. B) 20 N. C) 50 N. D) 80 N.
The process of crystallization enriches a magma in
A) silica. B) minerals that have low melting points. C) silica and other minerals that have low melting points. D) all of the above.