I sure can’t. But why would I want to measure speed increase internally, when it’s due to an external field? Quick question, how would I even measure speed its

I sure can’t. But why would I want to measure speed increase internally, when it’s due to an external field? Quick question, how would I even measure speed itself internally? I thought motion and speed was always measured relative to something else, why would an increase of these properties then have to be internal? Not trying to be a dick, I’m genuinely curious. And yes, I obviously do not know anything about physics. Any explanation or link to a source to help me understand?