A super train is moving along a track at a speed close to the speed of light. You are watching the train from the ground. You observe lightning to strike in two places along the track, a mile apart, at precisely the same time
What would someone on the train say? A) The two bolts of lightning struck at different times, and they struck at places that are more than a mile apart.
B) The two bolts of lightning struck at the same time, and they struck at places that are less than a mile apart.
C) The two bolts of lightning struck at different times, and they struck at places that are precisely a mile apart.
D) The two bolts of lightning struck at different times, and they struck at places that are less than a mile apart.
D
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A hydrogen bomb may be approximated by a fireball at a temperature of 7200 K according to a report published in 1950 by the Atomic Energy Commission. (a) Calculate the total rate of radiant-energy emission in watts, assuming that the gas radiates as a blackbody and has a diameter of 1.5 km, (b) If the surrounding atmosphere absorbs radiation below 0.3 
determine the per cent of the total radiation emitted by the bomb that is absorbed by the atmosphere, (c) Calculate the rate of irradiation on a 1 m2 area of the wall of a house 40 km from the center of the blast if the blast occurs at an altitude of 16 km and the wall faces in the direction of the blast, (d) Estimate the total amount of radiation absorbed
assuming that the blast lasts approximately 10 sec and that the wall is covered by a coat of red paint, (e) If the wall were made of oak whose flammability limit is estimated to be 650 K and that had a thickness of 1 cm, determine whether or not the wood would catch on fire. Justify your answer by an engineering analysis stating carefully all assumptions.
GIVEN
- A hydrogen bomb fireball
- Fireball temperature (T1) = 7200 K
- Surrounding atmosphere absorbs radiation below 0.3
- The blast occurs at an altitude (H) of 16 km = 16,000 m
FIND
(a) The total rate of radiant-energy emission in watts (qr)
(b) The per cent of the total radiation absorbed by te atmosphere
(c) The rate of irradiation on a 1 m2 area of the wall of a house 40 km (40,000 m) from the center of the blast and facing the blast (G2)
(d) Total amount of radiation absorbed if the blast lasts 10 seconds and the wall is covered with red paint
(e) If the walls are oak with a flammability limit of 650 K and a thickness (s) of 1 cm, will the wood catch fire?
ASSUMPTIONS
- The gas radiates as a blackbody
- Diameter of the fireball (D) = 1.5 km
- The air and surrounding temperature (T?) = 10°C
- The surroundings behave as a blackbody enclosure
- The heat transfer from the oak walls to its surroundings during the 10 seconds of irradiation can be neglected
- The house wall is initially at the surroundings temperature
SKETCH
PROPERTIES AND CONSTANTS
the Stephan-Boltzmann constant
the emissivity of red paint at short wavelengths
the emissivity of red paint at long wavelengths
Specific heat
Thermal conductivity
Density
Thermal diffusivity
On the way to lower floors, an elevator begins its descent from rest at a constant acceleration, descending the first 5.0 m in 0.85 s. What is the apparent weight of a 75 kg man inside the accelerator during this time interval?
A. 0.74 kN B. 1.8 kN C. 0.30 kN D. 1.0 kN
The type of EM radiation that has wavelength just a bit longer than that of visible light is
a. ultraviolet. b. infrared. c. x-ray. d. microwave. e. gamma ray.
Write the balanced chemical equation for the complete combustion of pentane, C5H12