The energy associated with a moving vehicle is one of the most important concepts that apparatus drivers must come to understand. Any moving vehicle that is driving down the road has “kinetic energy.” Don’t be intimidated by the big words; kinetic energy is simply a function of how fast the vehicle is traveling and how large it is. Take a 60,000-pound fire truck traveling at 50 mph and you can imagine the amount of energy that is associated with the vehicle as it is weaving through rush hour traffic. It is this energy that causes tires to skid, metal to crush and, at times, injury and death.
As a vehicle drives down the road, it gathers kinetic energy. How much energy a vehicle gathers depends on how fast the vehicle is going and how large it is. In order for an apparatus driver to bring the vehicle to a stop, he or she must first find a way to disperse all of this kinetic energy.
Imagine that your fire truck is a bucket and it is full of water. The water that is inside the bucket is the equivalent to the kinetic energy that your fire truck has created as it is driving down the street at a high rate of speed. If some of the water is dumped out (and some is still left in the bucket), the fire truck will slow down but continue to travel at a slower speed. The only way to bring the fire truck to a complete stop is to dump all of the water out of the bucket, or in other words, get rid of all of your kinetic energy.
Drivers must remember that kinetic energy can’t just be destroyed; it has to be converted into a different type of energy. This means that you just can’t dump the water out of the bucket; it has to go somewhere. The question is, “Where does it go?” About 99 percent of the time the kinetic energy of your vehicle is converted into heat energy. This conversion takes place when the operator applies the brakes on the truck. The brake pads rub against the brake drums or discs and create tremendous amounts of heat energy. This essentially burns off the kinetic energy of the vehicle. As this energy is burned off, the vehicle can slow down and eventually come to a stop.
Now, what happens when a car pulls out in front of you and you have to slam on your brakes, causing your wheels to lock? In this case, the wheels stop turning and your brake pads are no longer rubbing on the discs or drums. As a result, the vehicle must find another way to burn off its energy so that it can come to a stop.
How does this occur? As the locked tires slide across the roadway, the friction between the tire and the road surface create the heat that uses up the energy. The only problem with this scenario is that as the tires slide across the road surface, it creates a tremendous amount of heat. This heat energy actually liquefies the oils in an asphalt roadway and causes them to float to the surface. As these oils float up to the surface, your fire truck is actually sliding on a thin layer of oil with absolutely no steering control at all. This scenario results in what we commonly refer to as a “skid mark.”
A skid mark is caused by the residual oils that have been brought to the surface of the road by heat energy. If you are lucky, your vehicle will skid in a straight line and eventually come to a stop. But your vehicle could just as easily slide into a fixed object or another vehicle on the road.
Ideally, you will “burn off” your kinetic energy by simply applying the brakes and coming to a stop. But you may have to slam on your brakes and lock your tires, causing you to skid to a stop. If you aren’t able to burn off all of your energy before another car pulls out in front of you, you could strike another vehicle with your bucket still half full of water (energy). Instead of converting this leftover kinetic energy into heat via the brakes or skidding tires, nature “burns off” this excess energy by crushing the metal that you are riding in. What you see in the aftermath of a motor vehicle crash — the crushed cars — is actually evidence of how much kinetic energy was left in the vehicles at the time of the crash. The bigger the vehicle and the faster it is traveling, the more crush damage it will sustain during a crash.
Apparatus drivers must understand how the size and weight of their vehicle, in conjunction with the speed that they are driving, directly affects the amount of energy that they must dissipate in order to bring a fire truck to a safe stop. In an ideal situation, this energy will be dissipated safely by simply applying the brakes. In less than ideal situations, the energy may be dissipated by skidding tires or crushing metal. Hopefully, we can all stick to using the brakes.