The frequency of the sound waves detected by a detector is exactly equal to the frequency of the sound waves emitted by a source. what can we say about the relative motion of the detector and source?

a.
both are at rest.
b.
both are moving with the same velocity.
c.
they are moving with opposite velocities.
d.
both are either at rest or moving with the same velocity.

Uranium has two naturally occurring isotopes. 238u has a natural abundance of 99.3% and 235u has an abundance of 0.7%. it is the rarer 235u that is needed for nuclear reactors. the isotopes are separated by forming uranium hexafluoride uf6, which is a gas, then allowing it to diffuse through a series of porous membranes. 235uf6 has a slightly larger rms speed than 238uf6 and diffuses slightly faster. many repetitions of this procedure gradually separate the two isotopes. what is the ratio of the rms speed of 235uf6 to that of 238uf6? express your answer to five significant figures.