Bouncing Bungee Bodies

One of the most fascinating uses of gravity is the sport of bungee jumping. I can appreciate the thrill of voluntarily hurling yourself off of a ledge and enjoying the adrenaline of hurtling toward the ground, only to be bounced to a life-affirming halt. However, in any situation where people are toying with the natural laws of physics, there is always the potential for the physics to fight back. In the case of free-fallers, once the possibility of death has been removed, the next biggest concern is the amount of gravitational force (or g-force) that is placed upon the body. Calculations for the amount of g-forces a human experiences are based upon an individual’s body weight in relationship to the amount of force the safety system will generate.

Gravity can be painful when we drop something on our foot or when we embrace our clumsiness with a journey to the floor. But, at the end of the day, gravity is the ultimate force that keeps things grounded. People dealing with heights in their lives need to consider the amount of force that is being placed on the body when humans attempt to halt a plummet. In order to fully understand the force of gravity, we must first understand the factors at hand. Let’s start with some gravitational basics.

Courtesy of Sir Isaac Newton, humanity learned about the various forces of gravity. Newton discovered that the Earth and the human body exert an equal amount of gravitational force upon one another. In addition to Newton’s Law of Gravity, he also devised the Laws of Motion.

The three laws of motion state:
1. A body at rest will stay at rest, unless otherwise influenced by an external force. A body in motion will stay in one direction and in motion, unless otherwise influenced by an external force.
2. The force of a body in motion is directly proportional to its weight (mass) and the speed (acceleration) at which it’s moving. F=m(a)
3. Every action has an equal and opposite reaction.

All three of these laws influence our daily lives. When we consider different elements of fall protection, the laws of motion become even more significant. For the sake of simplicity, we are going to avoid discussing the influence of air resistance in fall arrest (but, please note that air resistance is a greater factor in bungee jumping than fall protection). With the exclusion of air resistance, the gravitational pull of the Earth causes objects of any weight to accelerate during a fall at a rate of 32.2 feet per second². This means that when a person falls, they are constantly being pulled down by gravity, accelerating to an increasingly faster speed. Consequently, this means that in order to stop, they need to be decelerated with a force higher than what gravity will generate, essentially, they will feel more than one “G” when slowing to a stop.

In many ways, the Rigid Lifelines™ products are similar to bungee jumping equipment. When a person performs a free-fall from a ledge, they need to have many of the same fall protection devices that an industrial worker would need at a worksite. Two of the most fundamental differences between these experiences is that one action is performed voluntarily for sport, whereas the other action is unexpected and a workplace hazard. Also, bungee jumpers want to experience a free fall for the maximum amount of time, while fall prevention desires immediate termination of downward travel. When either event occurs, the same fall arrest elements need to be involved.

As a thrill seeker is preparing for a bungee jump, they need to find an anchorage point, a device that will slow their descent, and a means of attaching the deceleration device to themselves. In more familiar terms, a bungee jumper needs a secure place to tie-off, a bungee cord, and a harness. All of those elements create a complete fall prevention system that matches the components of a Rigid Lifelines™ fall protection system.

Gravitational forces exerted on the body during bungee jumping are reduced thanks to the elasticity of the bungee cord. Because the cord is so flexible, it does an excellent job of absorbing the shock of gravitational force. All bungee cords are designed for 200% extension of its slack-length. Since the cords are designed to double their relaxed length during the deceleration process, the deceleration forces are more evenly distributed over a (relatively) long period of time.

Almost all bungee jumping facilities require that jumpers weigh between 90 and 300 pounds. Most bungee products are designed to generate a maximum force of 900 pounds of force when employed properly. This means that the cords are designed to generate a force of about 3 times the maximum amount of weight attached to them. That being said, we can use a very simple equation to determine the amount of gravitational forces exerted upon the human body:

900/Human Mass = gravitational force

For example, if a person weighing 100 pounds voluntarily leaps off of a building and experiences 900 lbs of deceleration force, they will experience 9 “G’s” of force. Meanwhile, a person who weighs 300 pounds will experience 3 “G’s” of force.

This same equation is applied to Rigid Lifelines™ products. Once again, the only major differences between bungee jumping equipment and Rigid Lifelines™ products are the length of time in which the deceleration occurs, the amount of time the person is falling, and the physical object that supplies the deceleration (bungee cord vs. rip-stitch/Self-Retracting Lanyards). The bungee cord provides a gradual stop, whereas Rip-Stitch and Self-Retracting Lanyards provide a much shorter stop time. However, since both products can be tensioned to handle three times the weight they support, they can create approximately the same amount of G-forces on the human body during the deceleration process.

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Hannah Addison