16. why is it that momentum is conserved between two objects just before and just after a collision?

Momentum is conserved when the net external force on the system is zero

Explanation:

Here we call 'system' the two objects together.

It is possible to demonstrate that the total momentum of a system is conserved when the net external force acting on it is zero. In fact, Newton's second law states that:

where F is the net external force, m the mass of the system, a its acceleration.

Re-writing the acceleration as rate of change of velocity:

we get:

However, assuming that the mass of the system does not change, the quantity at the numerator is just the change in momentum of the system:

So the equation becomes:

And therefore, if the net external force on the system is zero, F = 0, we get

which means that the total momentum is conserved.

the answer is 3.1 his answer is wrong mines is right

the total distance that martina travels that day is

(distance from home to mom's office) plus (distance from mom's office to school)

plus (distance from school to library) plus (distance from library to friend's house)

plus (distance from friend's house to martina's house).

(we don't know what any of these distance actually is.)