A radar receiver indicates that a pulse return as an echo in 20 ?s after it was sent. How far away is the reflecting object? (c = 3.0 × 108 m/s)
A) 1.5 km
B) 3.0 km
C) 6.0 km
D) 9.0 km
B
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In the late 1800s, Kelvin and Helmholtz suggested that the Sun stayed hot due to gravitational contraction. What was the major drawback to this idea?
A) It predicted that the Sun would shrink noticeably as we watched it, but the Sun appears to be stable in size. B) It is physically impossible to generate heat simply by making a star shrink in size. C) It predicted that the Sun could shine for about 25 million years, but geologists had already found that Earth is much older than this. D) It predicted that Earth would also shrink in size with time, which would make it impossible to have stable geology on our planet.
What is the net power that a person with surface area of 1.20 m2 radiates if his emissivity is 0.895, his skin temperature is 300 K, and he is in a room that is at a temperature of 290 K? The Stefan-Boltzmann constant is 5.67 x 10-8 W/(m2•K4)
A) 60.3 W B) 62.6 W C) 65.7 W D) 68.4 W E) 64.8 W
A 10-kg block is pushed across a horizontal surface with a horizontal force of 30 N against a friction force of 10 N. The acceleration of the block is
A) 1 m/s2. B) 2 m/s2. C) 5 m/s2. D) 10 m/s2. E) none of the above
Which has the larger entropy: a gas that occupies only half of a container, with the other half a vacuum, or the same gas occupying the entire container?
What will be an ideal response?