Like the “spherical cow” joke, that captures something of the physicist mindset. The reason why we treat cows as spheres is that it’s often good enough for practical purposes. By the time you figure in measurement uncertainties and all the rest, the difference between the spherical-cow model and what you actually see probably isn’t too far off. You can see a lot of this in action at Rhet’s awesome Dot Physics blog, where he does a lot of simple calculations making these kinds of approximations, and gets pretty close. – Chad Orzel, “Think Like a Physicist,” Uncertain Principles, July 9, 2013
Note: “The reason why we treat cows as spheres is that it’s often good enough for practical purposes.” In physics, that is, not in dairy farming. 😉