A 3.00-kg stone is dropped from a 39.2 m high building. When the stone has fallen 19.6 m, the magnitude of the impulse it has received from the gravitational force is
A.
s.B.
s.C.
s.D.
s.E.
s.Answer: D
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A quantity of an ideal gas is compressed to half its initial volume. The process may be adiabatic, isothermal, or isobaric. Rank those three processes in order of the work required of an external agent, least to greatest.
A. adiabatic, isothermal, isobaric B. adiabatic, isobaric, isothermal C. isothermal, adiabatic, isobaric D. isobaric, adiabatic, isothermal E. isobaric, isothermal, adiabatic
Two water waves, call them X and Y, have the same amplitude but different wavelengths: X has a longer wavelength than Y. Regarding their frequencies and their energies,
A) X has the larger (higher) frequency and carries the greater energy. B) Y has the larger frequency and X carries the greater energy. C) Y has the larger frequency and carries the greater energy. D) X has the larger frequency and Y carries the greater energy. E) Y has the larger frequency and the two waves carry the same energy.
An electronic circuit contains a power resistor that dissipates 1.5 W. The designer wants to modify the circuitry in such a way that it will be necessary for the resistor to dissipate 2.5 W. The resistor is in the shape of a disk 1 cm in diameter and 0.6-mm-thick. Its surface is aligned with a cooling air flow at 30°C and 10 m/s velocity, i.e., the vertical axis of the disc is in cross-flow with the cooling air. The resistor lifetime becomes unacceptable if its surface temperature exceeds 90°C. Is it necessary to replace the resistor for the new circuit?
GIVEN
• A heat generating resistor disk with its surface aligned with a cooling airflow
• Heat generation rate ( Q
G) = 2.5 W
• Disk diameter (D) = 1 cm = 0.01 m
• Disk thickness (t) = 0.6 mm = 0.0006 m
• Air temperature (T?) = 30°C
• Air velocity (U?) = 10 m/s
• Maximum surface temperature (Ts) = 90°C
FIND
• Is it necessary to replace the resistor?
ASSUMPTIONS
• Steady state
• Radiation is negligible
SKETCH
