Health Update: Whiplash
Low Speed Collisions – Where Does All That Energy Go?
You may have heard the comment, “If there’s no damage to the car, then there’s no injury.” Unfortunately, that does not always seem to be the case.
There are MANY factors that affect the dynamics of a collision and whether or not injury occurs. A short list includes: vehicle type and design, speed, angle of collision, momentum, acceleration factors, friction, kinetic and potential energy, height, weight, muscle mass, seat back angle and spring, head position upon impact, etc.
Consider Sir Isaac Newton’s Third Law of Motion: “For every action there is an equal and opposite reaction.” This law applies to a car accident at any speed. Using the analogy of hitting a pool ball into the corner pocket straight on, when the cue ball stops, its momentum is transferred to the target ball which accelerates at the same speed…hopefully into the corner pocket!
This example is not quite the same as an automobile collision because the energy transfer is very efficient due in part to the two pool balls not deforming (crushing or breaking) on impact with one another. If either ball did deform, more energy absorption would occur and the acceleration of the second ball would be lower.
In fact, in the United States, vehicle bumpers are tested at 2.5 mph with impact equipment of similar mass with the test vehicle’s brakes disengaged and the transmission in neutral. National Highway Transportation Highway Safety Administration (NHTSA) vehicle safety standards demand that no damage should occur to the car in this scenario.
However, energy transfer occurs very quickly and with a greater amount of force when there is no vehicle deformation (damage). As a result, a greater amount of energy (described as G-force) is directly transferred to the occupants inside the vehicle—increasing the risk of injury. A 1997 Society of Automobile Engineers article provided an example in which the same 25 mph (12 m/s) collision resulted in a five-times greater force on the occupants of the vehicle when the crush distance of the impact fell from 1 meter to .2 meters.
So be aware that even low-speed impacts can still place quite a bit of force on your body, even if the bumper of your car doesn't have a scratch on it.