Physical Science: Kinetic Energy

Energy is something that can be neither created, nor destroyed. Since the energy cannot be created or destroyed, it can only move from one form to another form. Kinetic energy is one of the nine different forms of energy. It is the energy that is created from movement. Another form of energy, known as potential energy, is closely related to kinetic energy. Potential energy is the energy of an object/system relative to a lower level.

When it comes to fall protection, our engineers need to consider the potential energy of workers since they are increasing their height. An increase in height increases the potential energy of the worker. If a worker falls, the Earth’s gravity accelerates the body faster and faster throughout the fall event. As a result, their potential energy has turned into kinetic energy.

In classical mechanics, the kinetic energy of a non-rotating rigid object is dependent upon the mass and speed of the object. Where m=mass and v=velocity, the formula looks like this: Kinetic Energy in foot-pounds=1/2 × m × v²

Where m=mass, H=height, and g= gravity (or 32.2ft/sec²), the formula for potential energy looks like this: Potential Energy in foot-pounds=m × g × H

Engineers will use these formulas (along with many others) to calculate the kinetic energy and the potential energy of different heights for the different weights of workers. The key to a good fall protection system is to transfer the kinetic energy of a falling person into another form of energy (usually friction through a deceleration device) while simultaneously limiting the maximum amount of force on the person’s body so they do not get injured. The goal is to decelerate the worker to a stop as quickly as possible and in the shortest distance as safely as possible. These calculations can get pretty complicated and this is why ANSI mandates that they are performed by a qualified person.

Force is a huge factor during a fall event as the force behind a person hitting the ground is actually what causes the harm after a fall. The formula for calculating force is: force=mass × acceleration. When a fall is decelerated or arrested, the person in the system will experience the Maximum Arresting Force, much like the bungee jumper from our previous blog. The mass of the falling person will play a role in the amount of acceleration and hence the force that the worker experiences.

ANSI has developed different equations that can be used for different types of fall protection systems. Each type of system (whether it’s an energy absorbing lanyard or a self-retracting lifeline) must meet the calculation requirements as set forth by the ANSI Fall Protection Code, Z359. The general requirements for most ANSI equipment are that all solutions must support a worker who weighs between 310 pounds and 130 pounds. Then, any worker attached to a typical system can only experience a maximum freefall distance of 6 feet. Using those weights and distances, a qualified person must use the formulas in the ANSI standard to determine if the entire fall arrest system is compatible.

ANSI outlines the different equations that can be used to analyze custom fall protection systems in ANSI Z359.6-2009. For more specific information about how to calculate the various aspects of different types of fall arrest solutions, please refer to this portion of the ANSI code. And remember, always consult with a qualified person when preparing to implement a fall arrest system.

Thanks for reading and stay safe!