That's a pretty strange list of Newton's laws you've got there.
Usually the first law is given as the law of inertia, i.e. if
something has no forces acting on it then it keeps going at a steady
speed in a straight line.
The second law is often given as F=ma or a= F/m, i.e. the
acceleration is just determined by the force on the object divided by
its mass. There would be no point in listing the same law twice in
slightly different form. Of course you can think of the first law as a
special case of the second, when F=0.
The third law is that the forces which two objects exert on each
other are always the same size and pointing in exactly opposite
directions. Since the force is just the rate of change of the momentum,
this says that if you have a collection of objects exerting forces on
each other, but not feeling any forces from the outside, then the total
momentum of the collection doesn't change. This way of stating the law
actually holds up best in modern physics.
There's no particular reason which we know of for leaving gravity
off the list of Newton's force laws. Unlike the first two laws, and
maybe the third, it actually is due to Newton and not his predecessors.
Newton also investigated a great variety of other phenomena. For
example, he made great strides in our understanding of the spectrum of
visible light. He also made tremendous advances in mathematics,
inventing calculus along the way. Any short list of "Newton's Laws"
leaves out a huge number of his contributions.
Mike W. (and Tom)
(republished on 07/11/06)