The question is often posed to physics students who have always given answers under the assumption that Earth has uniform mass. But now, Alex Klotz, a McGill University grad student, has come up with a new calculation that challenges this concept.

His findings were published in the American Journal of Physics - a publication of the American Association of Physics Teachers.

Professor David Jackson of Dickinson College is the editor of the journal. He tells Here & Now's Jeremy Hobson why Klotz's answer - the answer now accepted as right - is 38 minutes and 11 seconds, four minutes less than originally thought.

Guest