Free fall describes a specific type of motion where an object moves solely under the influence of gravity. This scientific definition differs from the everyday understanding, which often includes any downward movement. In physics, the concept of free fall is precise, focusing on the forces acting upon an object.
Understanding the Concept
True free fall occurs when gravity is the only force acting on an object, meaning no other forces, such as air resistance, are present or negligible. This condition is observed in a vacuum, where there is no air to impede motion. An object in free fall continuously accelerates downwards at approximately 9.8 meters per second squared (m/s²) near Earth’s surface, a value known as the acceleration due to gravity.
A remarkable aspect of true free fall is that all objects accelerate at the same rate, regardless of their mass. This principle, famously demonstrated by Galileo, means a feather and a bowling ball dropped simultaneously in a vacuum would hit the ground at the exact same moment. The greater gravitational force on a more massive object is precisely offset by its greater inertia, resulting in identical acceleration for all objects in true free fall. This constant acceleration causes the object’s velocity to increase linearly over time.
The Influence of Air Resistance
In most real-world scenarios on Earth, objects falling through the atmosphere experience air resistance. This force opposes the object’s motion and becomes more significant as the object’s speed increases. Air resistance depends on factors such as the object’s shape, surface area, and the density of the air it moves through.
Eventually, the force of air resistance can become equal in magnitude to the downward force of gravity. When these two forces balance, the net force on the object becomes zero, and the object stops accelerating. At this point, the object reaches a constant maximum speed known as terminal velocity. An object at terminal velocity is no longer in true free fall because air resistance is also a significant factor.
Real-World Instances and Sensations
While true free fall is an idealized condition, many real-world experiences mimic its characteristics. Activities like skydiving or the rapid descent on a roller coaster provide a sensation akin to free fall, even though air resistance is present. During these moments, the body accelerates downwards, creating a feeling of weightlessness.
The sensation of weightlessness, often associated with astronauts in orbit, occurs because there are no external contact forces pushing or pulling on the body. Although gravity is still acting on astronauts aboard the International Space Station, they experience weightlessness because they and the station are continuously falling around Earth. This constant state of falling removes the sensation of being supported.