Feeling Acceleration in Free Fall: An Insight into the Physics of Falling

Many people wonder why they don't feel acceleration when they are free falling under the Earth's gravity. This question not only delves into the principles of physics but also has implications for our understanding of space-time. Let's explore this fascinating phenomenon in more detail.

Understanding the Lack of G-Force

When you sit in an accelerating car, you feel a force pressing you back against your seat. Similarly, during a bungee jump, you experience the deceleration sensation when the cord pulls you back. However, as you free fall due to gravity, you feel weightless but do not experience this "pushing back" sensation. This absence of G-force can be perplexing, especially considering Newton's laws of motion. Yet, this observation is key to understanding the nature of space-time and the subtle intricacies of acceleration.

The Role of Inertial Forces

According to Newton's first law, an object in motion remains in motion with the same speed in the same direction unless acted upon by an external force. When you are falling freely, there is no net force acting on your body, which means you are not experiencing inertial forces. As a result, you feel weightless, but there is no sensation of being "pressed back" against the seat. The sensation of weightlessness is due to the fact that you and the object you are touching are both accelerating at the same rate, so there is no relative acceleration.

Free Fall and Terminal Velocity

I have firsthand experience as a skydiver, and I can attest that during the early phase of a free fall, you do feel acceleration. But as the fall progresses, you eventually reach a constant velocity known as terminal velocity, which occurs when the force of air resistance balances the force of gravity. At this point, you stop accelerating.

During the initial phase of free fall, gravity increases your velocity. As your velocity increases, the air resistance or drag also increases. Eventually, the drag force equals the force of gravity, and you reach a stable velocity, known as terminal velocity. For a human body in a stable "boxman" position, this terminal velocity is approximately 53 meters per second. At this point, the acceleration sensation ceases, and you feel as though you are floating, akin to being on a quickly deflating, very noisy air mattress.

Implications for Physics and Space-Time

This idea has significant implications, as described by Albert Einstein's theory of relativity. According to Einstein, an object in free fall is stationary from the perspective of space-time. The acceleration we observe due to gravity is relative to the Earth, but from the point of view of space-time itself, there is no acceleration. Instead, it is space-time that is warping or flowing due to Earth's gravity, and free-falling objects are caught in this "stream" of space-time, which moves them toward the source of gravity.

Conclusion

While the absence of acceleration sensation during free fall may seem counterintuitive, it provides profound insights into the nature of space-time and our fundamental understanding of physics. The experience of a skydiver underscores the importance of understanding these principles, offering a tangible example of how objects behave in free fall.

Further Reading

For those interested in learning more about the physics of free fall and the implications of relativity, consider reading:

Einstein and Falling Apples: Understanding Gravity Why Do Objects Fall at the Same Rate? How Does Free Fall Work Near a Black Hole?