The physics of a collision

When we drive our vehicles down the road, we are driving them at a certain velocity and direction. All vehicles have weight and mass to the order of a few hundred pounds for a motorcycle, a few thousand pounds for a compact car, an SUV, luxury car or several tons for a tractor-trailer truck. When our vehicles move at speed down the roadway, they create energy by virtue of their mass and forward velocity. This energy is called Kinetic Energy mathematically described as KE = 1/2mv2 where m = mass (lbm) and v = velocity (ft/s). Assuming the mass of a vehicle is assumed to be constant, it will create a certain quantity of kinetic energy while in motion at speed. When one accelerates a vehicle to a higher velocity, the vehicle's kinetic energy increases by the square of the velocity divided by 2. For example, if we consider a vehicle that weighs approximately 4000 pounds that is traveling at 60 mph (88 ft/s), this vehicle will produce approximately 480,994 ft•lbf of kinetic energy. If the velocity of this vehicle is increased to 70 mph (102.67ft/s) which is a 10 mph increase and 16.7% increase in speed, the vehicle's kinetic energy increases to 654,728 ft•lbf which is an increase of approximately 36.1%.

The physics of a collision between two bodies is governed by Newton's Second Law of Motion F = ma/gc where F = force in pounds force or (lbf), m = mass in pound mass (lbm), a = acceleration in feet per second squared or (ft/s2) and gc = a proportionality constant of 32.2(lbm•ft/lbf•s2). When vehicles are moving at interstate highway speeds, they possess much kinetic energy. When they collide, the time of impact is very short. An example is when we watch a NASCAR race on TV, and a race car collides with a retaining wall or another race car at say 150 mph. The forces generated due to these very short impact time intervals are called impulsive forces. Newton's Second Law of motion can be restated algebraically as F = ma/gc = mv/tgc where v = velocity in ft/s, t = time in seconds or s and v/t = “a” the average acceleration in ft/s2. Due to this very brief elapsed time interval during a collision, it can be seen mathematically from this equation that this very brief impact elapsed time interval causes these instantaneous impact forces between two vehicle bodies to be very high. This is due to the time variable "t" being in the denominator of the equation. Forces can be generated at impact to the order of tens of thousands to hundreds of thousands of pounds depending on the mass of the vehicles and their velocity at impact.