Free fall

In classical mechanics, free fall describes a specific type of motion. It occurs when gravity is the only force acting upon an object. While we often think of falling as moving downward, science defines it differently. An object moving upward is technically in free fall if gravity is its only influence.

Even the Moon is in a state of free fall. It orbits the Earth because its orbital speed keeps it from crashing into the surface. When no other forces, like the normal force from a surface, act on a body, the result is weightlessness. This sensation happens to astronauts in orbit. It also happens when a gravitational field is very weak, such as far from any planet.

The mechanism of free fall depends on whether an object is in a vacuum or an atmosphere. In a vacuum, gravity causes a constant acceleration. Near Earth's surface, this acceleration is approximately 9.8 m/s². This means the object's velocity increases steadily over time. If an object starts from rest, the distance it falls grows as the square of the elapsed time.

However, most falls on Earth happen through air. In these cases, a second force called aerodynamic drag acts on the object. This drag pushes against the direction of motion. As the object speeds up, the drag force increases. Eventually, the drag force becomes equal to the force of gravity. At this point, the object stops accelerating and reaches a steady speed called terminal velocity. For a human skydiver, this is roughly 53 m/s or 190 km/h.

There are distinct ways to categorize these motions. The first is the "textbook" case of uniform gravitational acceleration. This occurs in a uniform field without air resistance. In this scenario, all objects accelerate at the same rate regardless of their mass. The second type involves motion with air resistance. Here, the drag depends on the object's mass, surface area, and drag coefficient.

A third type involves objects in orbit. Two bodies orbiting each other are actually in free fall around one another. This includes artificial satellites falling around the Earth or planets falling around the Sun. These orbits follow specific mathematical paths called elliptical orbits.

Our understanding of these motions has changed significantly over centuries. Before the 16th century, many believed heavy objects fell faster than light ones. The Greek philosopher Aristotle argued that speed was proportional to weight. He suggested a 10 kg object would fall ten times faster than a 1 kg object. In the 6th century, John Philoponus challenged this by observing that different weights fall at nearly the same speed. Later, in 12th-century Iraq, Abu'l-Barakāt al-Baghdādī provided an explanation for gravitational acceleration. He anticipated the laws of classical mechanics by suggesting that continuous force produces acceleration. In 1551, Domingo de Soto became the first to state that free fall involves uniform acceleration caused by Earth's mass.

Galileo Galilei built upon these earlier ideas through careful experimentation. Though stories say he dropped weights from the Leaning Tower of Pisa, he likely studied objects rolling down ramps. Ramps allowed him to slow down the motion enough to measure it. He used water clocks and his own pulse to track time. Galileo repeated his experiments a full hundred times to ensure accuracy. He aimed for a deviation of no more than one-tenth of a pulse beat. His work, including the manuscript De Motu Antiquiora, laid the foundation for Isaac Newton's later studies of gravity.

Real-world examples demonstrate these principles clearly. A spacecraft with its propulsion turned off is in free fall. A person jumping off the ground is also in free fall for a brief moment. We can see the difference between free fall and other motions by looking at the forces involved. Flying in an aircraft is not free fall because lift provides an extra force. Standing on the ground is not free fall because the ground exerts a normal force against you. Even a skydiver with a parachute is not in true free fall. The parachute creates an aerodynamic drag force that balances gravity, preventing weightlessness.

A famous demonstration of these laws occurred on the Moon. On August 2, 1971, astronaut David Scott released a hammer and a feather simultaneously. Because the Moon has no atmosphere, there was no air resistance to slow the feather. Both objects fell at the same rate and hit the surface at the same time. This proved that gravity accelerates all objects equally in a vacuum. On the Moon, this acceleration is much lower than on Earth, at about 1.63 m/s². This experiment connected centuries of theory to physical reality.