Why True Free Fall is so Rare for Objects Falling on Earth

Most of us imagine a dropped object falling freely, pulled only by gravity. But in our world, that’s almost never the case. Objects falling near Earth’s surface are constantly battling an invisible force: air resistance. This interaction prevents them from being in a state of true free fall, where gravity is the only force at play. Understanding this helps explain why a feather and a bowling ball fall so differently in the real world.

What Exactly is Meant by Free Fall?

In physics, the term “free fall” has a very specific meaning. It describes the motion of an object where gravity is the one and only force acting upon it. There are no other forces, like air resistance or friction, to interfere with its downward journey.

Under these ideal conditions, any object, regardless of its mass or shape, would accelerate towards the Earth at the same rate. This constant rate of acceleration is approximately 9.81 meters per second squared (m/s²).

This means that for every second an object is in true free fall, its speed increases by 9.81 m/s. However, achieving this perfect state requires a complete vacuum, which is a space entirely empty of matter, including air. Since Earth is surrounded by a thick atmosphere, true free fall is incredibly rare outside of a controlled laboratory setting.

The Main Culprit is Air Resistance

The primary reason objects on Earth don’t experience true free fall is the presence of our atmosphere. As an object falls, it has to push through countless air molecules, creating a form of friction known as air resistance or drag.

This force always acts in the opposite direction of the object’s motion. While gravity pulls the object down, air resistance pushes it up. This upward push counteracts some of the gravitational force, slowing the object’s acceleration. The faster an object moves, the stronger the force of air resistance becomes.

Think about sticking your hand out of a moving car’s window. When the car is moving slowly, you barely feel the air. But as the car speeds up, the force of the air pushing against your hand becomes much stronger. The same principle applies to any falling object.

How an Object’s Shape and Size Affect its Fall

Not all objects are affected by air resistance in the same way. An object’s physical characteristics play a huge role in how much drag it experiences. This is why a crumpled piece of paper falls much faster than a flat sheet of the same paper.

The flat sheet has a much larger surface area exposed to the air, so it catches more air molecules and experiences greater resistance. The crumpled ball, on the other hand, is more streamlined and has a smaller surface area, allowing it to cut through the air more easily.

Several key factors determine the amount of air resistance:

• Surface Area: Objects with a larger surface area relative to their mass will experience more drag. This is why parachutes are so effective.
• Shape: Aerodynamic or streamlined shapes encounter less resistance than flat or irregularly shaped objects.
• Speed: As mentioned, the force of drag increases significantly as an object’s speed increases.

This explains the classic example of a feather and a hammer. In the air, the feather’s large surface area and low mass cause it to be dominated by air resistance, making it float down slowly. The hammer, being dense and more compact, is far less affected and plummets quickly.

Reaching the Limit with Terminal Velocity

As a falling object picks up speed, the upward force of air resistance continues to grow. Eventually, the object can reach a speed where the force of air resistance becomes equal to the downward force of gravity.

When these two forces are perfectly balanced, the net force on the object is zero. According to the laws of motion, this means the object stops accelerating altogether. It continues to fall, but at a constant maximum speed known as terminal velocity.

Every object has a unique terminal velocity based on its mass, shape, and size. A skydiver, for example, can change their terminal velocity simply by changing their body position.

Once an object reaches terminal velocity, it is no longer accelerating, so by definition, it is not in free fall.

Can We Ever See True Free Fall on Earth?

While true free fall is not something we see in our everyday lives, it can be created in special environments. Scientists use large vacuum chambers to pump out all the air, removing air resistance from the equation.

Inside one of these chambers, you can witness a feather and a bowling ball being dropped at the same time. Without air resistance to slow the feather down, both objects fall side-by-side and hit the ground at the exact same moment. This beautifully demonstrates the principle that gravity accelerates all objects at the same rate.

The most famous demonstration of this occurred not on Earth, but on the Moon. During the Apollo 15 mission in 1971, Commander David Scott dropped a hammer and a feather. Because the Moon has virtually no atmosphere, there was no air resistance. Just as physics predicted, the hammer and the feather fell at the same rate and hit the lunar surface simultaneously.

Frequently Asked Questions

Why does a heavier object seem to fall faster than a light one?
A heavier object experiences a stronger gravitational force. This means it needs to reach a higher speed before the force of air resistance is strong enough to balance gravity, so it accelerates for longer and has a higher terminal velocity than a lighter object of the same shape.

What is the difference between weightlessness and free fall?
Astronauts in orbit are in a constant state of free fall. They are falling towards Earth, but their high sideways velocity keeps them from ever hitting it. This continuous state of falling is what creates the sensation of weightlessness.

Does air density affect how an object falls?
Yes, absolutely. Air is denser at sea level than it is at high altitudes. An object falling from a great height will experience less air resistance in the thin upper atmosphere and will be able to fall faster than it would closer to the ground.

Is an object in free fall as soon as you throw it upwards?
Yes. As soon as an object leaves your hand, the only major force acting on it is gravity (ignoring air resistance for a moment). It is in free fall as it travels up, slows down, and falls back down.