Physics
Free Fall
Quick fact
In a vacuum, a feather and a bowling ball fall at exactly the same rate and hit the ground at the same time.
Why this is interesting
What if you dropped a feather and a hammer on the Moon? Which would hit the ground first? The answer might surprise you.
Read the full explanation
Understanding Free Fall
Imagine dropping a sock and a shoe from the same height. On Earth, the shoe hits first because air pushes upwards on the light sock more, slowing it down. But if you removed all the air (a vacuum), both would land simultaneously. That's free fall: the motion of an object under gravity alone, with no air resistance. The key idea is that gravity pulls on all objects with a force proportional to their mass, and more massive objects need more force to accelerate. These two effects exactly cancel, so every object gets the same acceleration—a constant 9.8 m/s² on Earth. We call this 'g'. This acceleration happens regardless of whether the object is light or heavy, or even if it's moving horizontally.
A deeper explanation
Free fall works because of the equivalence principle—the same property that makes an object heavy (gravitational mass) also makes it resistant to acceleration (inertial mass). When you drop a ball, the gravitational force is F = mg. Newton's second law says F = ma. Equating these gives a = g, so mass cancels out. This is why a feather and a hammer fall together in a vacuum. Free fall leads to weightlessness: an object in free fall is accelerating downward at g, so it exerts no force on a scale—like astronauts in orbit who are constantly falling toward Earth but missing it. This principle underlies projectile motion (a thrown object is in free fall after leaving the hand) and orbits (a satellite is a projectile that falls around the planet). Understanding free fall is fundamental to mechanics and the gateway to Einstein's general relativity, which treats gravity as the curvature of spacetime.