"In fact the laws of physics state that an abject/particle in motion will stay in motion unless acted upon by another force."--Danny

First, those pistons travel fastest at mid stroke... and slow down to almost no movement for several degrees of the crank at both top and bottom of the stroke... which is why determining the TDC using the piston travel is a matter of measuring and marking several degrees before the top and finding the same distance the other side... then just taking half way between those spots to call TDC..

But concerning the above quote.... you have left out air pressure. That is the OTHER force.... and it rules the day down in that bore... even more so on a turbocharged engine...