Every
body perseveres in its state of rest, or of uniform motion in a right line, unless it is
compelled to change that state by forces impressed thereon.
Projectiles persevere
in their motions, so far as they are not retarded by the resistance of the air,
or impelled downwards by the force of gravity.
The alteration of
motion is ever proportional to the motive force impressed; and is made in the
direction of the right line in which that force is impressed.
To
every action there is always opposed an equal and opposite reaction: or the
mutual actions of two bodies upon each other are always equal, and directed to
contrary parts.
Whatever
draws or presses another is as much drawn or pressed by that other. If you
press a stone with your finger, the finger is also pressed by the stone. If a
horse draws a stone tied to a rope, the horse (if I may so say) will be equally
drawn back towards the stone: for the distended rope, by the same endeavour to relax or unbend itself, will draw the horse as
much towards the stone, as it does the stone towards the horse, and will
obstruct the progress of the one as much as it advances that of the other. If a
body impinge upon another, and by its force change the motion of the other,
that body also (because of the equality of the mutual pressure) will undergo an
equal change, in its own motion, towards the contrary part. The changes made by
these actions are equal, not in the velocities but in the motions of bodies;
that is to say, if the bodies are not hindered by any other impediments. For,
because the motions are equally changed, the changes of the velocities made
towards contrary parts are reciprocally proportional to the bodies. This law
takes place also in attractions.