Normally, during a truck-car collision, there is a net external force exerted on the system due to the friction force of the ground on the vehicles. However, this force is relatively small compared to the internal forces of the vehicles on each other. We can use momentum conservation for the short time during the collision. For this particular simulation, there is no friction. So momentum is conserved--the sum of the x components of momentum of objects in the system is constant, as is the sum of the y components. For two-object systems

Momentum Before = Momentum After
m_truckv_{truck ox} + m_carv_{car ox} = m_truckv_{truck x} + m_carv_{car x}
and
m_truckv_{truck oy} + m_carv_{car oy} = m_truckv_{truck y} + m_carv_{car y}