 Rotation – when an object turns about an internal axis.  Think of this as spinning  Revolution – when an object turns about an external axis.  Think of this as going around something.  Example: Think of a bug on a record player.

The Earth rotates about its axis once every 24 hours – This is what creates a day – The axis of rotation is inside of the Earth (internal axis) The Earth revolves around the Sun once every days – This creates a year – The axis of revolution is the Sun (external axis)

 A merry-go-round has two rows of horses. One near the axis of rotation, and one nearer the outside of the ride.  Which horse moves faster?

 Linear speed is the distance an object moves per unit of time.  Therefore, the horse on the outside has greater linear speed since it moved a greater distance within the same amount of time as the inner horse.  In general, linear speed is greater for objects on the outer edge of a rotating object.

 Rotational speed is the number of rotations (or revolutions) per unit of time.  Therefore, both horses have the same rotational speed since they both complete one revolution in the same amount of time.  Units for rotational speed - RPM

 We are going to go outside for a moment and try an activity.  We are going to hold hands and spin in a circle, trying to keep a straight line of students.

Which student had to move the fastest in order to keep the line straight? Who did not have to move tangent to the circle at all? Tangential speed = (radial distance)*(rotational speed) An object that is twice as far from the center of rotation will have twice the tangential speed.

 Demonstration: Two cups taped together on meter sticks.

 Spinning an object on a string over your head.  How do you keep the object rotating?  In order for an object to be in rotation, there must be a force pulling it towards the center of the circular path.

 Definition: any force that is at a right angle to the path of a moving object that produces a circular motion.  Example: The Sun exerts a centripetal force on the Earth

 What force keeps a car from leaving the traffic circle in front of our school?

 During the spin cycle, the tub in the washing machine rotates at a high speed.  The inner wall exerts a centripetal force on the clothes keeping them in a circular path.  The holes in the tub allow for the water to escape.  The water escapes not due to a force, but due to inertia.

 Centripetal means “center-seeking”  This is a force that keeps the object in a circular motion  Centrifugal means “center-fleeing”  This is the tendency of the objects to want to break away from the circular motion due to inertia.

Consider a ladybug in a can that you are spinning around your head. There is only one force acting on the ladybug. – Centripetal force holding the bug in circular motion. The lady bug pushes against the edge of the can due to inertia or centrifugal tendency. The outward push is NOT due to a force, it is due to the lady-bug’s MASS (inertia)

 Describe what happens to a person as they are riding a roller coaster that goes upside-down.  Why don’t the fall out of the coaster when they are upside down?  What keeps the person in the circular motion and not leaving the circle?