EMT Exam: Connecting MOI and Force - dummies

By Arthur Hsieh

You will need to know about MOI’s and force for the EMT exam. A mechanism of injury is a description of a force that can cause an injury. Remember Isaac Newton and his three basic laws of physics? They may sound complicated, but in trauma they’re pretty easy to understand.

Say that someone is driving a car down a country road. Newton’s first law says that an object in motion stays in motion unless acted upon by an outside force. In this example, the car keeps moving down the highway until the driver goes off the road after being distracted by his mobile phone. The car strikes a tree (the outside force) and quickly comes to a stop.

However, the driver continues to move forward for another couple of milliseconds, until he, too, is acted upon by an outside force — an airbag and a seat belt, for example.

Hopefully, the driver was smart enough to have this safety equipment, because it was designed to slow him down more gradually than, say, the steering wheel. This slower change in velocity is Newton’s second law, which says the faster things move, the more force they have. If the driver was only moving at 25 miles per hour, you might not expect much damage to the car (and the driver.)

Change the speed to 50 mph, and now you’re talking some serious problems!

Newton’s second law actually states that force is a product of the mass of the object and its acceleration. However, what really generates force is the speed of the object and not its mass. Consider the concept of kinetic energy (KE), the force a moving object has. The formula for KE is

KE = (mass/2) × velocity2

You don’t have to do the math; just recognize that the velocity is squared, meaning that the force increases much more quickly with speed than mass.

Unfortunately for the driver, he was moving at highway speed when he crashed into the tree. He was careless and forgot to put on his seat belt. And he was driving a car made in the early 1970s, before there were airbags.

All of these factors result in the driver crashing his chest and abdomen into the steering wheel and his forehead into the windshield. The steering wheel bends, and the windshield cracks wildly under the force. More importantly, an opposite force is created by the steering wheel and windshield, causing injuries to the driver’s skull, brain, chest cavity, and abdomen.

This is Newton’s Third Law: For every action, there is an opposite and equal reaction.

There you have it — Newton’s three laws. The MOI is the car crashing into a tree at a high rate of speed. When you size up the scene, you notice that the steering wheel is deformed and the windshield in front of the driver is badly damaged.

You also notice he’s not wearing a seat belt and there is no airbag. Before you even begin your assessment of the driver, you already have a high index of suspicion that he has serious head, chest, and abdominal injuries based on the apparent MOI.

Don’t be distracted by the injuries you can see — it’s the ones you don’t see that can be lethal. That’s why you want to have a clear idea of the MOI. The driver in this example may only have a small laceration to his head, but he may also have a brain injury that doesn’t appear until later.