Free fall
Motion under only gravity, producing uniform acceleration and weightlessness.
Free fall is any motion of a body where gravity is the only force acting upon it. In classical mechanics, a freely falling object may not necessarily be falling downward; if subject only to gravity, it is said to be in free fall even when moving upward. The Moon is thus in free fall around the Earth. In a roughly uniform gravitational field, free fall results in the sensation of weightlessness when no other forces act on the body.
- field
- Classical mechanics, general relativity
- known_for
- Motion under only gravity; uniform acceleration; weightlessness
- key_contributors
- Aristotle, John Philoponus, Abu'l-Barakāt al-Baghdādī, Domingo de Soto, Galileo Galilei, Isaac Newton
- acceleration_near_Earth
- 9.8 m/s²
Lore & Background
In the Western world prior to the 16th century, it was generally assumed that a falling body's speed was proportional to its weight. Aristotle discussed falling objects in Physics (Book VII). In the 6th century, John Philoponus challenged this, stating that by observation two balls of very different weights fall at nearly the same speed. In 12th-century Iraq, Abu'l-Barakāt al-Baghdādī gave an explanation for gravitational acceleration, which Shlomo Pines described as the oldest negation of Aristotle's fundamental dynamic law. Galileo credited De Soto as his inspiration. He repeated experiments 'a full hundred times' until achieving accuracy within one-tenth of a pulse beat.
Reader's Guide
Free fall is a foundational concept in physics, central to understanding gravity and motion. Historically, it challenged Aristotelian physics, which held that heavier objects fall faster. The work of Domingo de Soto, Galileo Galilei, and later Isaac Newton established that in a vacuum all objects accelerate uniformly regardless of mass. This principle is essential for orbital mechanics—the Moon, for example, is in free fall around Earth. In general relativity, a body in free fall has no force acting on it, as gravitation is reduced to space-time curvature. The concept also explains weightlessness: astronauts in orbit experience free fall, as do skydivers before reaching terminal velocity (though aerodynamic drag prevents full weightlessness). The equations of motion for free fall near Earth's surface—v(t) = v₀ − gt and y(t) = v₀t + y₀ − ½gt²—are standard in classical mechanics. The demonstration on the Moon by David Scott visually confirmed Galileo's discovery. Free fall remains a key example in teaching physics and a basis for understanding gravitational phenomena.
Did You Know?
- The Moon is in free fall around the Earth, though its orbital speed keeps it in a very far orbit from the Earth's surface.
- A skydiver's 'free fall' after reaching terminal velocity produces the sensation of weight being supported on a cushion of air, not full weightlessness.
- On the Moon, gravitational acceleration is approximately 1.63 m/s², about 1/6 that on Earth.
Frequently Asked Questions
What exactly is free fall in mechanics?
Free fall is any motion in which gravity is the only force acting on a body, with no air drag, thrust, or contact forces present. Under those conditions the object experiences a single, uniform acceleration throughout its path.
Does free fall always mean an object is moving downward?
No — an object thrown straight up is still in free fall the entire time only gravity acts on it. The Moon circling Earth is a standard example: it is perpetually accelerating toward Earth under gravity alone, which qualifies as free fall even though it never 'falls' to the surface.
Why does free fall produce the sensation of weightlessness?
When no normal or contact force pushes back on a body, the body and everything inside it accelerate together at the same rate. That shared acceleration eliminates the internal stresses we normally read as weight, creating the familiar weightless feeling.
Who were the key figures in establishing the physics of free fall?
The concept evolved from Aristotle's early (and ultimately incorrect) claims through John Philoponus, Abu'l-Barakāt al-Baghdādī, and Domingo de Soto, before Galileo Galilei demonstrated uniform acceleration and Isaac Newton codified it within his laws of motion.
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