Internal injuries in vehicular accidents may be due to what is called the "third collision" The first collision is the vehicle hitting the external object. The second collision is the person hitting something on the inside of the car, such as the dashboard or windshield. This may cause external lacerations. The third collision, possibly the most damaging to the body, is when organs, such as the heart or brain, hit the ribcage, skull, or other confines of the body, bruising the tissues on the leading edge and tearing the organ from its supporting structures on the trailing edge.

A) Why is there a third collision? In other words, why are the organs still moving after the second collision?

B) If the vehicle was traveling at 60 mph before the first collision, would the organs be traveling more than, equal to, or less than 60 mph just before the third collision?

Collisions phenomenon can be understood from kinetic energy and linear momentum conservation. Depending on what magnitude is conserved after the collision, there are two kinds; either elastic or inelastic. In case of elastic collisions both, energy and linear momenta quantities are conserved, whereas the inelastic collision the kinetic energy is not conserved.

(a) Most of the organs are located into cavities, which might be contained certain liquids to maintain the stability and other organs do not require this kind of environment. Now, suppose a car A will impact another car B under the following conditions: Car A in motion - Car B in rest. When the car A collides with B - e.g full elastically - most of the kinetic energy and linear momentum from car A is transferred to B and all objects and human beings inside of B will undergo the transferring. In human body, the organs undergo an aggressive change of motion, which collides into the cavities at the same speed of the first collision, just after of this collision and the car B stops, the organs undergo a deceleration producing a muscular or bone trauma that can indirectly/directly affect them.

(b) Depending on which type of collision is taking place, part of the linear momenta and kinetic energy can be transferred and this is not higher than 60mph, but that can be equal or less of 60. In real accidents, the collision can be absorbed by the car structure acting as a "spring", which converts the energy in other kind and reduce the speed of collision