Newton's first law states that an object will move at a constant velocity -- that 
is, in a straight line at a constant speed -- unless acted upon by an outside 
force.  This is, in other words, the principle of the conservation of momentum.  
Newton's second law defines force: the force exerted on an object is given by the 
product of the object's mass and the object's acceleration. Newton's third law 
states that for every action there is an equal and opposite reaction. 
Momentum and force are defined mathematically as follows:

  p = mv      (momentum = mass * velocity)

  F = ma      (force = mass * acceleration)

So, to summarize:

Newton's First Law describes the Conservation of Momentum.
 Thus, if F = 0, then p = mv is constant.
 The typical units for velocity is m/s ("meter per second").
 The typical units for momentum is  kg m/s ("kilogram meter per second").

Newton's Second Law defines force as the change in momentum over time.
 Thus, if F is not 0, then F = ma.
 The typical unit for acceleration is m/s2 ("meter per second squared").
 The typical unit for force is  kg m/s2 ("kilogram meter per second squared").
    This unit of force is called a "newton" ("N") in Isaac Newton's honor.


Newton's Third Law describes the Reaction Principle.  This principle has no 
straightforward formula, but is a critical tool to evaluate any system of 
interacting objects.

NOTE: Momentum and force are vector quantities - their direction is just as 
important as their magnitude when computing them.  So adding and subtracting
momenta and forces requires the use of vector math and trigonometry, if the
collisions are not head-on.