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.

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.

Speed is the rate at which an object covers a distance.  How do you know if that rate changes or not?  The video shows how to determine if the speed of an object is constant.

Velocity is speed in a given direction.  How is a change in velocity (acceleration) detected?  Accelerometers.  Most smartphones have accelerometers to detect changes in velocity to monitor certain tasks.  The video demonstrates a very basic accelerometer and how it works.

A simple demonstration of centripetal force. A marble is moving in a circular path. The wall of the roll of tape is applying a force on the marble toward the center of the circular path. That is centripetal force. Once the force is removed, the marble, because it has inertia, moves in a straight line.