Physics
Free Fall Motion
Quick fact
In a vacuum (or on the Moon), a feather and a hammer fall at exactly the same rate because free fall acceleration is independent of mass.
Why this is interesting
Drop a feather and a hammer at the same time—which hits the ground first? On Earth, the hammer wins, but on the Moon, they land together. Why does gravity treat them the same in one place but differently on another?
Read the full explanation
Understanding Free Fall Motion
Imagine you're in an elevator and the cable snaps. As you plummet, you feel weightless—that's free fall. In free fall, gravity is the only force acting on an object. On Earth, air resistance often complicates things: a feather floats while a rock drops quickly. But if you remove the air (like in a vacuum chamber), both fall side by side. This happens because gravity pulls all objects with the same acceleration, about 9.8 meters per second squared (m/s²) near Earth's surface. That means every second, your downward speed increases by 9.8 m/s—whether you're a marble or a bowling ball. The effect of mass cancels out: more mass means more gravitational force, but also more inertia, so the acceleration remains constant.
A deeper explanation
Free fall motion arises from Newton's law of universal gravitation and his second law of motion. The gravitational force on an object is F = mg, where m is mass and g is the gravitational field strength. According to Newton's second law, F = ma, so mg = ma, and a = g. This shows acceleration is independent of mass. The motion is described by kinematic equations (e.g., Δy = v₀t + ½gt²), assuming constant g and no air resistance. In reality, air resistance creates a drag force that opposes motion, leading to terminal velocity when drag equals gravity. Understanding free fall is crucial for designing parachutes, launching rockets, and analyzing the motion of projectiles. It also underpins Einstein's equivalence principle, which connects gravity with acceleration.