Skydiving “Free Fall”

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The time a skydiver spends between leaving the aircraft and opening a parachute is often called the “free fall” time.  During a recreational skydive the “free fall” time is about one minute. The current record “free fall” time of about 5 minutes was set by Alan Eustace in 2014 when he fell from an altitude of more than 135,000 feet. According to the  Paragon Space Development Corporation, “Eustace reached top speeds of over 800 miles per hour. He was going so fast that his body broke the sound barrier, creating a sonic boom that could be heard on the ground.” The jump broke the previous record of 127,852 feet set by Felix Baumgartner in 2012. The 2012 jump was sponsored by GoPro cameras and the video has a much higher production value than the more recent 2014 jump:

Physics Free Fall

Now that we have introduced the skydiver’s use of the term free fall, we need to recognize that physics uses the term free fall in a completely different way, so we will need to be careful to avoid confusion. In physics, and in this book, we use the term free fall to describe the motion of an object when gravity is the only force acting on the object, or any other forces are small enough compared to gravity that we can ignore them without introducing too much error. Skydivers experience significant air resistance, so they are not actually in free fall.

Free-Fall Acceleration

In the absence of air resistance, heavy objects do not fall faster than lighter ones and all objects will fall with the same acceleration. Need experimental evidence? Check out this video:

It’s an interesting quirk of our universe that the same property of an object, specifically its mass, determines both the force of gravity on it and its resistance to accelerations, or inertia. Said another way,  the inertial mass and the gravitational mass are equivalent. That is why all objects have the same free-fall acceleration which near the surface of Earth happens be 9.8 m/s/s, downward (toward the center of earth). Coming up in a later unit we see for ourselves how this result is predicted by a combination of two physical laws.