Momentum exchange
Newton considers momentum in his 3 principles, it is a feature of all accidents and an understanding of Newton’s principles are essential to this as all features of accident mechanics. Velocity or acceleration and mass or weight are all important the relationship between them is a multiplication so a small mass travelling at a high velocity has considerable momentum. Likewise a large mass travelling at a slow velocity has a considerable momentum.
The formulae is momentum = mass * velocity
thus p (momentum) = m (mass) * v (velocity)
When a force is applied to a body for a short period of time it is known as an ‘impulsive force’. The change in momentum produced by this force is called the ‘impulse’. When 2 bodies collide the force on one is equal to the force on the other at the point described by Newton in his third law, it is this force at this point in the collision that is calculated using the momentum formulae.
Therefore at this point the momentum before the accident is equal to the momentum after the accident providing no other force other than those of the collision act upon the vehicles involved. This represents the force of the vehicles which reflects their speed at this point. This rule is known as the ‘Conservation of Linear Momentum’.
If we consider 2 cars, A and B, with respective mass Ma and Mb and velocity Ua and Ub the momentum of each prior to the accident is represented by:
a = MaUa whilst b = MbUb
Following the accident the velocities will have changed and will be represented by:
a = MaVa whilst b = MbVb
The formulae is therefore:
MaUa + MbUb = MaVa + MbVb
This formulae is applicable to impacts at 90 degrees, the system becomes much more complicated when vehicles collide or depart at angles other than this. The calculation again is not for the innocent and should be referred. By rearranging the formulae most situations can be dealt with to some extent by the appropriate expert.
Vectors
Finally to conclude this section as I have mentioned the above deals with 90 degree collisions and variables, the more common result of an accident is that the vehicles concerned enter at right angles to each other but depart at more acute angles, known as vectors. These angles are coped with in an extended form of calculation that accommodates them. The basis for these calculations is within triangles, it would be fair to say that these situations should be left strictly with the expert and not even attempted by the unwary. Suffice it to know that it can be done and is difficult and that you need to now someone special when they turn up.
