Italian astronomer and scientist Galileo Galilei (1564–1642) performs his legendary experiment, dropping a cannonball and a wooden ball from the top of the Leaning Tower of Pisa, circa 1620. This was designed to prove to the Aristotelians that objects of different weights fall at the same speed, but wound up demonstrating a number of important physics principles. (Hulton Archive/Getty Images)

We All Learned Physics’ Biggest Myth: That Projectiles Make A Parabola

It’s an incredibly useful approximation. But the truth takes us far deeper.

Anyone who’s ever taken a physics course has learned the same myth for centuries now: that any object thrown, shot, or fired in the gravitational field of Earth will trace out a parabola before striking the ground. If you neglect external forces like wind, air resistance, or any other terrestrial objects, this parabolic shape describes how the center-of-mass of your object moves extremely accurately, no matter what it is or what else is at play.

But under the laws of gravity, a parabola is an impossible shape for an object that’s gravitationally bound to the Earth. The math simply doesn’t work out. If we could design a precise enough experiment, we’d measure that projectiles on Earth make tiny deviations from the predicted parabolic path we all derived in class: microscopic on the scale of a human, but still significant. Instead, objects thrown on Earth trace out an elliptical orbit similar to the Moon. Here’s the unexpected reason why.

If the gravitational acceleration of Earth always pointed exactly ‘downward,’ the shape of a projectile on Earth would always make a parabola. But given that the Earth is curved and the gravitational acceleration is oriented towards its center, that cannot be exactly true. (Cmglee / Wikimedia Commons)