What is wave interference, and the two types of wave interference?
What will be an ideal response?
Answer: Wave interference is the result of two or more waves combining together. When the waves combine in phase, with crests overlapping crests and troughs overlapping troughs, the resulting wave has greater amplitude. This is constructive interference. When crests overlap troughs, cancellation or diminishing of the wave results. This is destructive interference.
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How do we know that there is much more mass in the halo of our galaxy than in the disk?
A) There are so many globular clusters in the halo that their total mass is greater than the mass of stars in the disk. B) Stars in the outskirts of the Milky Way orbit the galaxy at much higher speeds than we would expect if all the mass were concentrated in the disk. C) Although the question of mass in the halo was long mysterious, we now know it exists because we see so many brown dwarfs in the halo. D) The recent discovery of photinos, combined with theoretical predictions, tells us that there must be a huge mass of photinos in the halo. E) We don't know that there is more mass in the halo; it is only a guess based on theory.
At what speed is the mass of an electron double its rest mass?
A) 0.500c B) 0.650c C) 0.707c D) 0.866c E) 0.960c
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
Buoyancy: A waiter fills your water glass with ice water (containing many ice cubes) such that the liquid water is perfectly level with the rim of the glass. As the ice melts,
A. the liquid-water level remains flush with the rim of the glass. B. the liquid water level rises, causing water to run down the outside of the glass. C. the liquid-water level decreases.