What Is Acceleration Due To Gravity?
QUICK ANSWER
Acceleration due to gravity is the rate at which a freely falling object speeds up under the pull of gravity alone. Near Earth's surface, this acceleration is approximately 9.8 meters per second squared, meaning a falling object gains 9.8 m/s of downward speed every second.
Drop a rock and a feather at the same time from the same height in a vacuum, and they land together. That counterintuitive result is the heart of acceleration due to gravity, the property of nature that makes gravity work the same way for every object regardless of mass. The number 9.8 m/s² shows up in every physics class from middle school onward.
Why do all objects fall at the same rate?
Ignoring air resistance, a feather and a hammer fall at the same rate because gravitational acceleration does not depend on the falling object's mass. Galileo first demonstrated this in the late 1500s, contradicting Aristotle. While a more massive object experiences more gravitational force, it also requires proportionally more force to accelerate. The two effects cancel exactly, so every object accelerates at the same rate in a vacuum. Apollo 15 astronauts famously verified this on the moon by dropping a hammer and a feather together.
Does air resistance change the acceleration?
Yes, in real-world conditions. Air resistance slows falling objects, and lighter or larger objects are affected more by drag because they have less mass per unit area. A feather falls slowly through air because drag overwhelms gravity almost immediately. A bowling ball falls fast because gravity overwhelms drag at the same speeds. In a true vacuum, both would accelerate identically at 9.8 m/s². This is also why skydivers eventually reach terminal velocity, the point where air resistance balances gravity and acceleration stops.
How is the 9.8 m/s² value measured?
Scientists measure gravitational acceleration by timing falling objects in vacuum chambers, observing pendulum swing periods, or using atomic interferometers that track individual atoms in free fall. The value varies slightly across Earth's surface, from about 9.78 m/s² at the equator to 9.83 m/s² at the poles due to Earth's spin and equatorial bulge. The standard value used in physics problems, 9.80665 m/s², is an internationally agreed average that works for most calculations to good precision.
How does acceleration due to gravity vary elsewhere?
Each celestial body has its own gravitational acceleration. The moon's is about 1.62 m/s², roughly one sixth of Earth's, which is why astronauts could bounce around so much on the lunar surface. Mars is about 3.71 m/s². Jupiter's surface gravity, at the cloud tops, is about 24.79 m/s². The sun's surface gravity is over 270 m/s². Acceleration due to gravity depends on the body's mass and radius, not on the falling object, so the value is a fixed property of the planet. More on gravitational acceleration across different bodies from Britannica's reference.
Acceleration due to gravity at 9.8 m/s² is one of the most useful constants in everyday physics. From timing a free fall to designing safety harnesses, the value applies the same to every object on Earth, regardless of mass, size, or composition. That universal sameness is what made Galileo's discovery so remarkable.
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