A 50.0-kg box is resting on a horizontal floor. A force of 250 N directed at an angle of 30.0° below the horizontal is applied to the box. The coefficient of static friction between the box and the surface is 0.400
What is the force of friction on the box? A) 31.9 N
B) 196 N
C) 217 N
D) 246 N
E) 616 N
C
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A shell-and-tube heat exchanger is to be used to cool 25.2 kg/s of water from 38°C to 32°C. The exchanger has one shell-side pass and two tube side passes. The hot water flows through the tubes and the cooling water flows through the shell. The cooling water enters at 24°C and leaves at 32°C. The shell-side (outside) heat transfer coefficient is estimated to be 5678W/(m 2 K). Design specifications require that the pressure drop through the tubes be as close to 13.8 kPa as possible and that the tubes be 18 BWG copper tubing (1.24 mm wall thickness), and each pass is 4.9 m long. Assume that the pressure losses at the inlet and outlet are equal to one and one half of a velocity heat (?V2/gc), respectively. For these specifications, what tube diameter and how many tubes are needed?
GIVEN
A water-to-water shell-and-tube exchanger, hot water in tubes, cooling water in shell One shell and two tube passes Hot water flow rate m
h= 25.2 kg/s Water temperatures
? Hot: Th,in = 38°C Th,out = 32°C
? Cold: Tc,in = 24°C Tc,out = 32°C Shell-side transfer coefficient h o= 5678 W/(m2 K) Pressure drop (?p) = 13.8 kPa Tube wall thickness (t) = 1.24 mm = 0.00124 m Tube length per pass (Lp) = 4.9 m
FIND
The tube diameter (Do) and number of tubes (N)
ASSUMPTIONS
Pressure losses at inlet and outlet (?pii) = 1.5 (? V2/gc) Variation of thermal properties with temperature is negligible Fouling resistance is negligible Thermal resistance of the tube walls is negligible
SKETCH
PROPERTIES AND CONSTANTS
for water at 30°C Density (?) = 995.7 kg/m3
Specific heat (cp) = 4176 J/(kg K)
Thermal conductivity (k) = 0.615 W/(m K)
Kinematic viscosity (?) = 0.805 × 10–6 m2/s
Prandtl number (Pr) = 5.4
Astronomers believe that most of the meteorites come from broken up asteroids and comets, but some of the meteorites are fragments from the Moon or Mars.
Answer the following statement true (T) or false (F)
The fate of the world's uranium supply is to eventually become
A) alpha and beta particles. B) iron. C) lead.
Some of the gaps in Saturn's rings, such as the Cassini Division, are caused by ____ with moons
a. tides b. ripples c. rotations d. resonances e. forward scattering