Two cars, one behind the other, are traveling in the same direction at the same speed. does either driver hear the other’s horn at a frequency that is different from that heard when both cars are at rest?

Yes they hear different frequencies when they are moving as compared to when they are at rest.

Explanation:

The difference in the frequencies of sound that we hear when an object moves and when it is at rest is a phenomenon known as Doppler's effect of sound and the difference in the heard frequencies is known as Doppler shift.

The frequency that we perceive if the source is moving is given by

where

'f' is the perceived frequency

is the original frequency

c is velocity of sound

are the velocity of source and velocity of receiver respectively

3. dark matter

empty space could also be a simple vacuum