Accumulating a difference: relative velocities
Physics Narrative for 14-16
The rate of separation sets the relative velocity
Reporting any velocities unambiguously depends on making explicit the point of view that you've chosen. So all velocities are relative velocities – that is they report rates of separation. Usually it's the separation between the observer (Alice, Bob or Charlie) and what they're reporting on.
However, sometimes you'll want to work out, from what Alice reports and what you record about Alice's velocity, the velocity of the object that Alice is reporting on. That's when you need to add together the two rates of separation – the two velocities (→vAlice-Object and →vAlice-You). Of course, you'll remember that these are vectors that you're adding.
The ability to switch points of view by finding relative velocities is a powerful way of thinking, and you'll use it again in episode 03 to link the conservation of momentum to Newton's first law.