Everybody may well remember their first acquaintance with Newton’s laws. At least I felt unforgettable surprise when at a lesson of physics my teacher wrote the following formula on the board, essentially without any proof:
F = mа
and said that it was Newton’s second law.
Then many examples had to be reviewed and quite a number of problems solved using this law before the notion of force somehow started getting more or less clear, besides, the force is easily felt by hand, whereas the meaning of mass was virtually impossible to understand.