Newton's three laws of motion, which he made public with the publication of Principia Mathematica in 1687, are as follows. The first says that an object in motion, or at rest, will remain in motion at the same rate of speed unless it is acted upon by an outside force. The second law says that when a body is acted upon by such a force, that the change in motion will always be proportional to that force. It will also be inversely proportional to the mass of the object being acted upon. Finally, the third and most famous of Newton's laws is that for every action, there is an equal and opposite reaction. One example might be that if two bodies of equal mass collide while traveling at the same velocity, they will exert the same amount of force on each other, but, obviously, in opposite directions.