Newton's First Law

Every material body persists in its state of rest or of uniform motion in a straight line if and only if it is no acted upon by a net external force.

Define velocity

Definition includes the direction of the velocity.

Then First Law says if the net force is zero, the velocity of an object remains constant.

If the net force on Mars were zero, what path would it follow?

The old view:
The natural motion of a body on earth is stopped.
The natural motion of a body in the heavens is motion in a circle or a combination of circles.

The new view:
the natural motion of a body anywhere in the universe is motion in a straight line with constant velocity.

ASTR 121 Home

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@bovine.uoregon.edu