Video Demo: Apple Inertia

Inertia is the tendency of an object to resist a change in its motion.  An apple and a knife are moving downward.  The counter-top applies an unbalanced force upward on the knife.  The knife decelerates, but the apple does not.  The apple decelerates only when the knife handle applies a force upward on it.

Video Demo: Forces and Motion

According to Newton's first law of motion, an object moving at constant velocity will continue to move at constant velocity unless an outside unbalanced force is applied.  This is why moving objects on Earth will always slow down and stop.  The outside unbalanced force that does it is friction.  Watch the video to see how friction affects the motion of a marble on a track.