At about 05.30, Dr. Lewin makes a small calculation to find the centripetal acceleration one experiences due to Earth's spin (at the equator). He then says that because of how small the magnitude of this acceleration is, we could consider the Earth's surface to be an inertial reference frame (approximately). My question is, what if the centripetal acceleration were much larger? Wouldn't the frame be (approximately) inertial as well? For instance, two people standing somewhere near the equator would have the same acceleration relative to the centre of the Earth, and so they would be stationary relative to one another. Is there something I'm not getting right? I'm actually quite confused about the definition of an inertial reference frame; "a frame that is not accelerating", not accelerating relative to what?