Momentum science experiment help

[Illumination]

Hello, I am a ninth grade student in high school and I have a science fair project experiment idea. There will be a homemade marble shooter made with a rubber band. the rubber band will be pulled about half of a foot back and will propel a marble through a tube. the marble will hit a book and the marble's momentum will cause it to roll back. The distance of away from the book to the marble shooter will be a foot. The levels of independent variable will be marbles with completely different weights. This experiment will be used to find out momentum. How do I calculate do momentum for this experiment. Remember I am a ninth grader so please don't get too technical.

 

[haruspex]

the marble's momentum will cause it to roll back.

No, the elasticity of the impact with the book will make the marble roll back.
If it rolls back very little, nealy all the momentum has been imparted to the book, making it move a bit. But to find the momentum we need its mass and how fast it started moving. This is difficult because the speed will be small and very quickly fall to zero due to friction.
In short, this is not an effective way to find momentum.

 

[TomHart]

Remember I am a ninth grader so please don't get too technical.

Now that you're a freshman in college we should be able to discuss this on a more technical level. :)

P.S. Welcome to Physics Forums.