A demonstration that forces between two bodies always occur in pairs: They are opposite in direction and equal in magnitude. These facts were summarized by Newton with the statement "action = reaction". Two people are standing on flat carts on a level floor, a few meters apart. The forces are exerted by means of a rope. The experiment shows that in both cases the forces are equal in magnitude and opposite in direction, in short: action = reaction. It also shows that the total momentum of the system is conserved.