So.. This is science.. I would ask the kids from my school but I have no friends and I don't know any of them.

8 N force

Explanation:

12 N forward, 4 N backwards

12-4=8

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.

the answer to this is proletariats.

hope this : )