How do we know galaxies are moving away from us

Because their light is red-shifted

Explanation:

Doppler effect is a phenomenon that occurs when the source of a wave is moving relative to an observed. In particular, two situations are possible:

- The wave source is moving towards the observer --> in this case, the frequency of the waves appear to be higher than the original one; as a consequence, the wavelength of the waves appears to be shorter

- The wave source is moving away from the observer --> in this case, the frequency of the waves appear to be lower than the original one; as a consequence, the wavelength of the waves appears to be longer

This phenomenon occurs also for light waves. In particular, the light coming from distance galaxies undergoes Doppler effect as well, since those galaxies are moving relative to us. We have again two possible situations:

- The galaxy is moving towards the Earth --> in this case, the frequency of the light appear to be higher than the original one; as a consequence, the wavelength of the light emitted appears shorter (so, it shifts towards the blue color --> this is known as blue-shift)

- The galaxy is moving away from the Earth --> in this case, the frequency of the light emitted appears to be lower than the original one; as a consequence, the wavelength of the light appears to be longer (so, it shifts towards the red color --> this is known as red-shift)

Since the Universe is expanding, almost all galaxies are moving away from us: therefore, we notice a red-shift for the light coming from nearly all of them, and this is how we know they are moving away from us.

with 5 seconds, no, but over time if the police car stays at 65m/s ad the speeding car stays at 60, then obviously yes, but the answer to your question would be, no.

hope this : )