A farmer noticed that his chickens were sick, and called in a biologist, a chemist, and a physicist to help diagnose the problem.
The biologist studied the chickens and concluded that there was something wrong with them, but did not know what was causing it.
The chemist took fluid samples from the chickens back to his lab, and found that they had an infection, but didn't know how they got it.
Meanwhile, the physicist had been sitting on the floor, scribbling in his notebook. Suddenly, he jumped up, exclaiming, "I have the answer, but it only works for spherical chickens in a vacuum!"
As a side-note, physics is pretty good at making true accurate predictions without assumptions eg. assuming no friction etc, but for undergrads or school physics you will probably recognise this!