Energy Conservation With Conservative Forces: Two identical grasshoppers jump into the air with the same initial speed and experience no air resistance. Grasshopper A goes straight up, but grasshopper B goes up at a 66° angle above the horizontal. Which of the following statements about these grasshoppers are correct? (There could be more than one correct choice.)
A. At their highest point, both of them have the same amount of gravitational potential energy.
B. At their highest point, both of them have the same amount of kinetic energy.
C. At their highest point, both of them have the same amount of mechanical energy.
D. At their highest point, grasshopper B is moving faster than grasshopper A.
E. At their highest point, grasshopper A has more gravitational potential energy than grasshopper B.
Answer: E
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A radiation source is to be built, as shown in the diagram, for an experimental study of radiation. The base of the hemisphere is to be covered by a circular plate having a centered hole of radius R/2. The underside of the plate is to be held at 555 K by heaters embedded in its surface. The heater surface is back. The hemispherical surface is well-insulated on the outside. Assume gray diffuse processes and uniform distribution of radiation. (a) Find the ratio of the radiant intensity at the opening to the intensity of emission at the surface of the heated plate. (b) Find the radiant energy loss through the opening in watts for R = 0.3 m. (c) Find the temperature of the hemispherical surface.
GIVEN
• A radiation source as shown above
• Radius of hole = R/2
• Temperature of underside of plate (T2) = 555 K
• Underside of plate is black
• Hemispherical surface is well insulated on the outside
FIND
(a) The ratio of the radiant intensity at the opening to the intensity at the surface of the heated plate (G1/ Eb2)
(b) The radiant energy loss through the opening (q1) in watts for R = 0.3 m
(c) The temperature of the hemispherical surface (T3)
ASSUMPTIONS
• Gray diffuse processes
• Uniform distribution of radiation
• Radiation entering A1 is negligible, i.e., A1 as a black body at 0 K Heat loss through insulation is negligible
PROPERTIES AND CONSTANTS
The Stephan-Boltzmann constant (?) = 5.67 × 10–8 W/(m2 K4)
A low density gas must be hot in order to produce an absorption line
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Large meteoroids are thought to collide with Earth:
A) at least once a decade. B) about once a century. C) a few times in every million-year period. D) once every billion years. E) never; only comets are thought to collide with Earth.
A color-coded ______________ Index is used to indicate how good or bad the air is, in terms of pollution
Fill in the blank(s) with correct word