Knowledge you can walk. Read the idea below, then step up to the general, across to peers or down to the specific.
Physical
Free Fall Acceleration
Free fall acceleration is the steady gain in speed of an object falling in a vacuum, where gravity is the only force acting on it.
Learn, discuss, create, buy or invest
The idea
Free fall acceleration is how quickly a falling object speeds up when gravity is the only force acting on it. Because the acceleration has the same size for all bodies, scientists usually write it with the single letter g. In a vacuum, every object falls at the same rate, no matter how heavy it is or what it is made of. Near Earth's surface, a falling object speeds up by about 9.8 meters per second every second.
If an object starts from rest, the total distance it has fallen grows with the square of the elapsed time, so it covers more ground each second. The value differs slightly from place to place on Earth, depending on altitude, latitude and longitude. On the Moon, free fall acceleration is about 1.63 meters per second squared, only about a sixth of Earth's.
In air, most falling objects do not stay in true free fall for long, because air resistance pushes back harder as their speed increases. Once the drag force equals the object's weight, it stops speeding up and has reached its terminal velocity. For a human skydiver, terminal velocity is around 53 meters per second, or about 190 kilometers per hour. Astronauts in orbit are in free fall too, which is why they feel weightless.
Check yourself
Question 1 of 3
Ignoring air, about how fast does a falling object speed up near Earth's surface?
Composition
what it's made of
- Gravity — The only force acting on an object in true free fall.
- Equal rate for all masses — Heavy and light objects gain speed at the same rate in a vacuum.
- Earth's rotation — The centrifugal effect of Earth's spin combines with gravitation to set the value at each spot.
- Standard gravity — A fixed reference value, written g0, of 9.80665 meters per second squared.
Connection
how it relates
Under Acceleration
Change
how and why it evolves
- Past
- In 1551, Domingo de Soto became the first to state that a body in free fall speeds up at a uniform rate. In 1901, the third General Conference on Weights and Measures adopted a standard value for gravity's acceleration. On August 2, 1971, astronaut David Scott demonstrated free fall on the Moon by releasing a hammer and a feather at once.
- Now
- Since 1901, standard gravity has been set at exactly 9.80665 meters per second squared for measurement work, even though real free fall acceleration varies from place to place.
- Next
- Satellite missions will likely keep mapping Earth's gravity field more precisely and tracking how it changes over time.