Consider the forces involved in pushing a thumbtack into a board.
Applying the laws of motion shows that the forces on the thumbtack are all
the same size. In particular, the force of the finger on the head of
the thumbtack is the same as the force of the board pushing back on the point
of the thumbtack.
So why not try it with the thumbtack reversed?
The difference between pushing on the head and the point is that the force
is spread over a large area or concentrated into a small one. The relevant
concept is
Pressure = Force/AreaThe area of the head of the tack is much larger than the area of the point; when you divide the same force by these very different areas you find that the pressure is very large at the point and relatively smaller at the head. This is why the point pushes things out of the way and penetrates into the board (assuming that you have remembered to push on the head. Otherwise, you will soon get the point).
The chapter on balanced forces