Our boy, AG, is referring to a joke: a dairy farmer asks a physicist how to estimate milk production. The physicist begins the calculations with, “Assume a spherical cow,” and takes it from there.
Physicists are famous for this. They call it simplifying the model. Sometimes they have a problem that’s too complicated to be calculated from the bottom up — say, climate change. So they make a model in which they simplify the parts — an average atmosphere, an average land, an average ocean. Change one of these averaged parts — make the atmosphere more likely to trap heat — and see how the whole climate responds.
I think in the case that AG presents, the problem is not milk production but something like the distance a cow can be catapulted. On the whole, simplifying the model is good for finding out roughly where the answer might be. But it might be hard on the cow.