One stone is dropped from the side of a bridge, and a second is dropped from the same place a short time later. What is true about the distance between the stones as a function of time after both have been dropped? Ignore air resistance

A. Increases, finally reaching a constant value
B. Always increases
C. Is constant
D. Decreases, and then reaches a constant value
E. Always decreases


B. Always increases

Physics & Space Science

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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)

Physics & Space Science

When a projectile is launched, the velocity may be described as:

a. increasing in the vertical direction. b. decreasing in the horizontal direction. c. constant in the vertical direction. d. constant in the horizontal direction.

Physics & Space Science

Elliptical galaxies are more likely to be found in clusters than are spiral galaxies

Indicate whether the statement is true or false

Physics & Space Science

Which of the following molecules is the most polar?

A) HCl B) BrF C) CO D) Image

Physics & Space Science