Trajectory From Circular Orbit


To show that the instantaneous velocity of an object executing uniform circular motion is tangent to the circle.


Circular band and pool ball.


Suggest three possibilities: (1) the ball will tend to move away from the center of the circle, (2) the ball will tend to continue to move around in a circle, or(3) the ball will move away tangent to the circle. Let your students vote before doing the exp




A pool ball is rolled around the inside of the circular band. When the ball leaves the end of the circular segment it will travel in the direction tangent to the circle at the point where it leaves.


Jonathan Mitschele, A New Use for an Old Pickle Bucket, TPT 32, 252-253 (1994). Harry Rheam, Tricks of the Trade: Centripetal Force on an Overhead Projector, TPT 33, 182 (1995).