Inertia

Today my friends and I went to play pool.  While we were playing, I realized that physics played a huge role in the game.  Inertia is the tendency to resist changes in velocity.  It basically means that all objects want to keep doing what they're doing, whether its staying at rest or moving.  Newton's 1st Law of Motion is the Law of Inertia.

The white cue ball is at rest on the table because two equal forces are acting on it.  The weight of the cue ball (mass times gravity) is the downward force and the normal force from the table is the upward force.  Because the upward and downward forces cancel each other out, the net force is zero (equilibrium).  

After the cue ball is hit, it is now in motion.  The weight of the cue ball is still cancelled by the normal force from the table.  Because of inertia, the ball will want to continue rolling at a constant velocity, but because the table has friction, it will eventually stop.