Let F be a force vector and a an acceleration vector. A vector v(x,y,z) in 3 dimensional space xyz is the set of objects ((x1,y1,z1),(x2,y2,z2)) with conditions : x2-x1=x , y2-y1=y , z2-z1=z Writing vectorial F=ma , the equality means an equality between the sets. When a ball falls in a gravitional field it follows the vectorial law F=ma. So now i have an equality between 2 sets with infinite elements, but only one ball ? Did i miss something ?