Dynamics Packet

This packet is intended for use in a mechanical engineering dynamics course. Impact injuries are discussed, and a homework problem is presented. Energy methods are encouraged for the homework problem, which compares the impact energy to a construction worker's head with and without a hard hat.

Time for presentation is approximately 15 minutes.

Objective:

To use dynamic energy concepts to introduce mechanical engineering students to basic safety concerns involving impact injuries.

This packet includes the following items:

• Lecture material for the instructor
• Overheads for use during the lecture
• Homework problems

Download the Dynamics Module in printable Adobe Acrobat Format (pdf).  This includes overheads in a ready to use format.

Homework problem solutions are available to qualified recipients. Send an email with request information to Dr. Donald Bloswick.

Dynamics Lecture Outline

1. Impact Injuries (OVERHEAD 1)
1. There is a need to control objects that could strike a person, especially when head impacts are a possibility.
2. A falling object with an energy of 50 ft-lbs (Hammer, 1989) can cause a skull fracture, so some form of protective head gear is recommended for those in situations where a head impact could occur.
3. The body can be impacted by projected solid materials such as
1. fragments from rupturing pressurized equipment, like high-pressurized tires or tanks,
2. windblown solids, or
3. debris projected from rotating equipment.

Machine guards should be provided on equipment that has the capability of causing impact injuries. As a minimum precaution, protective clothing should be worn.

1. Impact Injuries from Projectiles (OVERHEAD 2)
1. Small projectiles can cause injuries ranging from slight skin lacerations, to fracture of large bones, depending upon mass, velocity, and the location of impact. The table (adapted from Hammer) illustrates various masses, velocities, their kinetic energies, and the corresponding possible injury.
2. (OVERHEAD 3) Kinematics and energy methods can be used to determine impact conditions, such as energies and velocities.
3. For example, a common injury involves small rocks or pieces of metal being thrown by lawn mower blades. A lawn mower blade typically has a peripheral velocity of 150 to 300 feet per second.
1. If a 0.1 lb. (45.36 g) rock was thrown by a lawn mower blade, and the velocity was approximately 300 ft/s, the rock would have a kinetic energy of 139.8 ft-lbs, or approximately 190 J. As shown in the table, this energy would be enough to fracture large bones. (Table on Overhead 2.)
2. The need for machine guards becomes readily apparent when energy is considered.
4. (OVERHEAD 4) The homework problem uses energy methods in a head impact situation.
1. The injuries resulting from impact are compared to a situation where a hard hat is worn. You will find the differences to be significant.
2. The applicability of energy methods is explored with respect to head impact injury assessment.
2. Conclusion (OVERHEAD 5)
1. Impact injuries can be extremely harmful to people.
2. Head protection should be worn if an impact injury could occur.
3. The use of dynamics can help to determine the energies and velocities involved in an impact, and can therefore be useful in estimating the extent of an injury.

OVERHEAD 1

Impact Injuries

• A falling object with an energy of 50 ft-lbs (Hammer, 1989) can cause a skull fracture.

• The body can be impacted by projected solid materials such as
• fragments from rupturing pressurized equipment, like high-pressurized tires or tanks,
• windblown solids, or

• debris projected from rotating equipment.

• Machine guards should be provided on equipment that has the capability of causing impact injuries.

• As a minimum precaution, protective clothing should be worn.

Overhead 2

Impact Injuries from Projectiles

Small projectiles can cause injuries ranging from slight skin lacerations, to fracture of large bones, depending upon mass, velocity, and the location of impact. The table illustrates various masses, velocities, their kinetic energies, and the corresponding possible injury.

Effects of Missiles on Human Cadavers

(Adapted from Hammer, Occupational Safety Management and Engineering)

Overhead 3

• A lawn mower blade typically has a peripheral velocity of 150 to 300 feet per second.
• If a 0.1 lb. (45.36 g) rock was thrown by a lawn mower blade, and the velocity was approximately 300 ft/s, the rock would have a kinetic energy of 139.8 ft-lbs, or approximately 190 J. As shown in the table, this energy would be enough to fracture large bones.

• The need for machine guards becomes readily apparent when energy is considered.

Overhead 4

Impact Injuries from Projectiles

The homework problem uses energy methods in a head impact situation.

• •The injuries resulting from impact are compared to a situation where a hard hat is worn. You will find the difference to be significant.
• •The applicability of energy methods is explored with respect to head impact injury assessment.

Overhead 5

• Impact injuries can be extremely harmful to people.



• Head protection should be worn if an impact injury could occur.



• The use of dynamics can help to determine the energies and velocities involved in an impact, and can therefore be useful in estimating the extent of an injury.

Dynamics Homework Assignment

A construction worker is standing at a construction site when a 4 pound tool falls off a scaffold from a height of 20 feet, hitting the worker on the head. Assume the tool only accelerates due to gravity. The construction worker is NOT wearing a hard hat, against company safety standards. Use energy methods to determine the energy transferred to the worker's head, assuming that the tool hits the head over a small area. The worker is 6 feet tall. Compare this solution to a worker wearing a hard hat. A typical hard hat can absorb 40 ft-lbs of energy through the hat and the webbing. Without a hard hat, an energy of 50 ft-lbs is considered to be able to cause a skull fracture.

Determine the tool's velocity when it impacts the worker's head. The impact information listed in this assignment is typical of that found through a basic literature review. Discuss your solutions, with emphasis on the attenuation of energy which the hard hat provides. Also list three steps that could be taken to reduce the chance of this type of injury occurring.