When you draw the free-body diagram for a single object, you must sketch
1.the forces that act on the single object along with any resulting accelerations that the object experiences.
2.only the forces that act on the object.
3.the forces that act on the object and the forces that the object exerts on other objects.
4.only those forces that the single object exerts on other objects.
2
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The electronic controls of a medical imaging device are housed in a compartment such that it dissipates heat to the ambient from a flat vertical plate surface, 15 cm wide and 20 cm high, to which fins are attached so as to enhance the heat transfer and improve the cooling of the electric systems. If 10 fins made of aluminum with a rectangular cross section are attached that extend over the full vertical height of the plate surface, where each fin is 0.15 cm thick with equal spacing of 1.5 cm along the width of the plate, determine the optimum fin height and the rate of heat transfer to the surrounding air. The room air temperature is at 20°C and the base temperature of the plate surface to which the fins are attached should not exceed 70°C.
GIVEN
? An electronic device with vertical aluminum fins in air
? Air temperature (T?) = 20°C
? Vertical Height of fin (H) = 0.2 m
? Width of surface (w) = 0.15 m
? Fin thickness(t) = 0.0015 m
? Maximum base temperatures: Tb= 70°C
? Fin spacing (s)= 1.5 cm=0.015 m
FIND
(a) Optimum fin height perpendicular to surface.
(b) Rate of heat transfer to surrounding air.
ASSUMPTIONS
? The fins are at a uniform temperature equal to the base temperature
PROPERTIES AND CONSTANTS
From Appendix 2, Table 28, for dry air at the mean temperatures for each of the base temperatures
From Appendix 2, Table 12, the thermal conductivity of aluminum in the range of around 70°C
(ka) = 240 (W/(m K))
Pulsars were discovered because they _____
a. provide the energy to ionize supernova remnants b. radiate gravitational waves c. absorb matter from a binary companion through an accretion disk d. emit short, regular bursts of radiation e. undergo nova eruptions on a regular period of a few years
Resolution: The two stars in a binary star system have an angular separation of 1.0 × 10-5 rad. If the wavelength of the light from the system is 500 nm, what is the smallest diameter aperture telescope that could just resolve this system to show that it contains two stars?
A. 0.50 cm B. 0.61 cm C. 5.0 cm D. 6.1 cm
If a single lens forms a virtual image of an object, then
A) the lens must be a diverging lens. B) the lens must be a converging lens. C) the lens could be either a diverging or a converging lens. D) the image must be inverted. E) the object must be between the lens and its focal point.