Q3.5
The law of conservation is useful whenever a system of particles is isolated, i.e., the system experiences no net external force. In this case, the momentum of the system is conserved.
During collisions, the external forces may be so small compared to internal forces that they can be ignored with little loss of accuracy.
Q3.6
This term is used in the context of systems of particles. The momentum of a system of particles is conserved if the net external force is zero.
In this context, an external force is a force exerted on any particle of the system except those coming from other particles in the system.
By Newton’s 3 rd Law, if one particle in the system exerts a force on a second, the second exerts an equal and opposite force on the first, so that the sum of the two forces on the system is zero.
Q3.7
The player will move southeast, in the direction of the net force.
P3.10
P3.11
P3.12
C3.1.
Collision 1 will do the least damage. The second car can recoil, so that the first car will experience less damage. If we look at things from a center of mass reference frame, collision one is like collision three, except that the two cars are going only half as fast, clearly a less damaging situation.
Collisions 2 and 3 will do exactly the same damage. Symmetry and conservation of momentum tell us that one car hitting an identical car head on means that the point(s) of contact do not move, so that is the same as hitting an immovable barrier.