Kinetic Energy, G-force and Speed Change
Kinetic energy is the energy an object possesses when it’s moving. For example, a 2,000-pound car traveling 65 mph has slightly more than 283,000 foot-pounds of energy¹. If that much kinetic energy were converted to heat, it would boil a quart of water.
During a normal, non-collision stop, most of a car’s energy is absorbed in the brake system. But in a vehicle crash, that energy is suddenly, cataclysmically dissipated — typically in less than 100 milliseconds — not by heating the brakes, but by crushing, tearing and twisting the vehicle. In the process, tremendous force is exerted on the vehicle’s occupants.
The kinetic energy equation is:
Kinetic energy = (½) M V²
in which M = mass
V = velocity
As speed increases, so does the amount of kinetic energy. However, because the equation has a velocity-squared term, the kinetic energy increase is exponential compared to the speed increase. For example, when the velocity or speed doubles, the kinetic energy quadruples, so even a small velocity increase results in a disproportionate increase in kinetic energy. Therefore, a 5 mph speed increase from 30 mph to 35 mph increases the kinetic energy by one-third.
High energy levels alone do not lead to disaster — consider how much kinetic energy a Boeing 747 has upon landing. It’s how that energy is dissipated that determines the outcome. We can throw a raw egg into the air and catch it without breaking it if we recoil our hands during the catch. In contrast, the egg will probably break if we let it smack into our hands without any give. In both cases, the egg’s kinetic energy is about the same, but the distances in which that energy is dissipated are drastically different, depending on whether or not we recoil our hands.
The following equation can be used to calculate the deceleration forces generated when an object slows from one speed to another in some given distance. The deceleration is measured in Gs.
To obtain the forces exerted, the weight of the object is multiplied by the Gs. For example, a 10-pound object subjected to a 30-G deceleration will have an effective weight of 300 pounds.
It’s surprisingly easy to generate 30 Gs: a vehicle coming to a stop in four feet from 60 mph would generate a 30-G deceleration.
Imagine the outcome when an unrestrained 10-pound baby slams headfirst into the dashboard in a 30-G collision, concentrating 300 pounds of force on its skull and spinal column. In a similar vein, when someone is ejected from a moving car and crashes into an unyielding object such as a utility pole, the deceleration distance is minimal. Therefore, the Gs are very high and the resulting force levels are many multiples of a person’s body weight.
How many Gs can the human body take? The answer is complex and depends on where and how rapidly the Gs are applied on the body, direction (head-to-toe, front-to-back, side-to-side, etc.) and duration.
All car designs sold in the U.S. must be tested on the G-forces registering on a safety-belted, front-seat dummy when its car crashes into an immovable barrier at 30 mph. To pass the test, the dummy’s chest deceleration cannot exceed 60 Gs for more than three milliseconds during the crash, according to the Federal Motor Vehicle Safety Standard 208.
High G loads are common even at low speeds, which explains why child safety seats are so vital. For example, the dummies in the federally mandated 30 mph crash test routinely register 40 to 50 Gs. Even though a restrained passenger would probably survive the collision, a 10-pound baby would weigh 400 to 500 pounds. Could someone be expected to hold onto a baby in such a crash? The answer is no.
The question of equivalent crashes often arises and can be solved by comparing each vehicle’s speed change and the amount of kinetic energy it dissipates.
The above collisions are equivalent. At impact, each vehicle will decelerate to a stop from 35 mph and each vehicle will absorb an equal amount of kinetic energy. For example, if each vehicle weighs 2,000 pounds, each one will absorb 82,208 foot-pounds of energy.
It’s often been said that two vehicles colliding head-on (Collision 2) is the equivalent to one of those vehicles impacting a solid object (Collision 3) at twice the head-on speed, which is incorrect. The kinetic energy absorbed by the vehicle in Collision 3 is four times greater than that of Collision 2, so Collision 3 is much more severe.
¹Force has been expressed in pounds and energy in foot-pounds. If you refer to a physics text, you’ll find Systeme International units, which are based upon the kilogram, meter and second. The units of force will be Newtons and kinetic energy will be in Joules.
A Newton is defined as the force necessary to accelerate a 1-kilogram object at a rate of 1 meter per second. A 1-pound force equals 4.45 Newtons. A Joule is a force of 1 Newton acting through a distance of 1 meter. 1 foot-pound equals 1.356 Joules.