What was the "best" dark matter percentage?
A.) 40%.
B.) 50%.
C.) 60%.
D.) 70%.
E.) 80%.
Ans: D.) 70%.
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The human body is typically modelled as a vertical cylinder that is 1.8 m high and is 30 cm in diameter, as shown in the figure. Calculate the average rate of heat loss from this body, which is maintained at 37°C, on a windy day when the airstream has a 5 m/s velocity and is at 35°C. To ascertain “wind chill” effects, compare this result with the heat loss that would occur in “stagnant” conditions, or when it is not windy and the heat transfer is only by natural convection (consider an average heat transfer coefficient of 3.6 W/(m2 K) for free convection). What is the wind chill effect if the wind got stronger (10 m/s) and colder (25°C)? Even though the natural convection heat transfer coefficient also changes somewhat (as discussed later in Chapter 8), for this calculation
consider it to remain the same. Moreover, compare the heat loss in both cases with the typical energy intake, or metabolic heat production from consumption of food, of about 1033 kcal/day and comment upon your results.
GIVEN
• Human body modeled as a cylinder in an air stream
• Body surface temperature (Ts) = 37°C
• Air velocity (V?) = 5 m/s
• Air temperature (T?) = 35°C
• Cylinder diameter (D) = 30 cm = 0.3 m
• Cylinder height (H) = 1.8 m
FIND (a) The heat loss from the idealized human body
(b) Heat loss if the wind speed is 10 m/s and its temperature is 250C. (c) Compare with the free convection results of Problem 5.8 and with the typical food
consumption rate of 1033 kcal/day
ASSUMPTIONS
• Air velocity is perpendicular to the axis of the cylinder
• Air flow approaching cylinder is laminar
• Heat transfer from the ends can be neglected
SKETCH
PROPERTIES AND CONSTANTS
Thermal conductivity (k) = 0.0262 W/(m K) Kinematic viscosity (?) = 17.1 × 10–6 m2/s Prandtl number (Pr) = 0.71 At the surface temperature of 37°C Prs = 0.71
According to modern scientific dating techniques, approximately how old is the solar system?
A) 10,000 years B) 4.6 million years C) 4.5 billion years D) 14 billion years
How large a pressure increase (in atm) must be applied to water if it is to be compressed in volume by 1.0%? The bulk modulus of water is 2.0 × 109 N/m2 and 1 atm = 1.0 × 105 N/m2
a. 50 atm b. 100 atm c. 1 100 atm d. 400 atm e. 200 atm
Measuring the Doppler shift of material at various distances from the center of a galaxy can be used to construct a(n) ____________________ for that galaxy
Fill in the blank(s) with correct word