The video shows an animated billiards experiment in which a cue ball strikes a glued-in-place eight-ball. Which of the following explains why the momentum of the eight-ball is conserved?

Choose 1

The collision is elastic.
The cue ball rebounds with the same speed it had coming in.
The eight-ball is an isolated system.
The "glue force" cancels the collision force.

Part B

Which statement must be true for the momentum of a system to be conserved?

Choose 1

The net external force on the system is zero.

The internal forces sum to zero.

There are no external forces acting on the system.

The net external force on the system is non-zero.

A)  The correct answer is the first.   The shock is elastic

B)  For the momentum to be preserved the external force on the system must  be zero

Explanation:

Part A

Let's analyze the problem, this is a case of collisions, if the eight ball is stuck, it is as if it had a very high mass, so the cue ball must bounce, since the collision is elastic

The correct answer is the first.

The shock is elastic

Part b

Momentum is preserved if the whole body system is isolated

For the momentum to be preserved the external force on the system must  be zero

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =