Free-falling Through Earth
Imagine the Earth were hollow. How long would it take us to fall through the planet? First off, says minutephysics host Henry Reich, assuming the Earth is perfectly spherical and has the same density everywhere, it turns out that the gravitational attraction from any spherical, symmetric object is the same as if all of its mass was concentrated at the center of that object. Phew! That was a mouthful. Tune in as Reich turns to his wheelhouse–math and physics–to give us an answer.