A mass on a spring vibrates in simple harmonic motion at a frequency of 4.0 Hz and an amplitude of 4.0 cm. If a timer is started when its displacement is a maximum (hence x = 4 cm when t = 0), what is the speed of the mass when t = 5 s?
a. zero
c. 0.016 m/s
b. 0.0065 m/s
d. 0.024 m/s
A
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Estimate the rate at which heat needs to be supplied to an astronaut repairing the Hubble telescope in space. Assume that the emissivity of the spacesuit is 0.5. Describe your model with a simple sketch and clearly state your assumptions.
GIVEN
• An astronaut repairing the Hubble telescope in space.
• Emissivity of spacesuit (?) = 0.5
FIND
• Rate at which heat needs to be supplied to the astronaut.
ASSUMPTIONS
• The spacesuit is idealized as a cylindrical object of height 1.8 m and diameter of 0.4 m.
• Temperature of 370C is maintained inside the suit.
SKETCH
Reconsider the problem described in where water jets from a nozzle that has a diameter of 6 mm and impinges on a disk of 4 cm in diameter that is subjected to a uniform heat flux of 70,000 W/m2. Instead of water, consider using air and ethylene glycol as the cooling fluids with the same flow rate (0.008 kg/s) and temperature (20°C) in each case, and determine the temperature on the surface of the disk at radial distances of 3mm and 12 mm from the axis of the jet. Compare the results with those for water obtained in and comment on them.
GIVEN
FIND
(a) Temperature on the surface of disk at radial distance of 3 mm and 12 mm from axis of jet.
(b) Compare results with those of water obtained
PROPERTIES AND CONSTANTS
A jet has an angular length of 10 seconds of arc and flows from a star at a distance of 2,000 pc. How far does this jet extend away from the star?
a. 20,000 pc b. 20,000 AU c. 20, 000 ly d. 20,000 miles e. 20,000 km
The polar caps and dust storms of Mars can be seen with Earth-based telescopes
Indicate whether the statement is true or false