The engine of the plane rumbles loudly, but you seem to feel it more than hear it. The wind rushing by the open bay drowns out all other sounds. Yet, there's one sound that you seem to hear above all others: the pounding of your racing heart.

The parachute on your back weighs heavily on your shoulders. Right now, before the jump, it feels more like a millstone than a best friend. When you make that leap into the great unknown, though, you know you'll be glad it's strapped to your back, ready to float you safely back to Earth.

The time has come. You feel the tap on your leg that tells you it's time to jump. You take the few steps to the open bay. Looking down, your heart leaps into your throat. You close your eyes and jump!

You feel yourself in free fall, hurtling toward Earth. You open your eyes and see the ground slowly getting closer. You seem to be picking up speed as you fall. Soon, though, you know you'll hit terminal velocity.

What's that? Is it the point when you must open your parachute or face a certain death upon impact with Earth? Could it be the speed at which you'll pass through a portal to another world? Nope! It's just the velocity at which you'll no longer be accelerating.

With a cool name like "terminal velocity," many people assume it means something sinister or extraordinary. In reality, though, it simply describes a physical reality based upon Newton's First Law of Motion.

When an object (like a skydiver!) falls freely through a medium, such as water or air, the force of gravity pulls it toward Earth. As the object falls, its velocity increases as it accelerates toward Earth. In other words, it begins to fall faster and faster toward Earth thanks to gravity.

Gravity isn't the only force working on the object, though. Air molecules collide with the falling object, pushing it upward against gravity. Scientists call this force air resistance. As the velocity of the falling object increases, so does air resistance.

Eventually, air resistance will equal the weight of the object in free fall. When this occurs, the object reaches terminal velocity. This means the falling object has reached its maximum velocity and acceleration is now zero. The object will continue to fall at the same speed (terminal velocity) for the remainder of its free fall…until it hits Earth or pulls the ripcord on the parachute in the case of a skydiver!

Terminal velocity can be affected by a few different factors. For example, a heavier object will generally have a higher terminal velocity. In the case of skydivers, terminal velocity can also depend upon the orientation of the body during free fall. A smaller surface area (body tucked into a ball) will have a higher terminal velocity than a larger surface area (arms and legs outstretched).

So what speeds do skydivers reach during free fall? Although it varies by person, most skydivers reach a terminal velocity of around 125 miles per hour. Experienced skydivers who streamline their bodies during free fall have reached speeds of over 200 miles per hour!

Amazingly, 200 miles per hour isn't even close to the fastest speed ever reached during a skydive, though. In October 2012, Austrian skydiver Felix Baumgartner jumped from a helium balloon in the stratosphere over 24 miles above Earth.

Because the atmosphere at that height is so thin, there is next to no air resistance at the beginning of the free fall. This allows the skydiver to reach a much higher terminal velocity before encountering Earth's normal atmosphere far below.

On Baumgartner's skydive, he reached an estimated top speed of approximately 843.6 miles per hour. Since that's faster than the speed of sound, Baumgartner created his own sonic boom during his skydive!