Additional Info

• ID Code: E1-11
• Purpose: Demonstrates motion of planets or satellites in an inverse square gravitational field
• Description:
  Giving a small ball a tangential velocity near the outer radius of the well, one can create elliptical orbits which demonstrate conservation of angular momentum as the ball rolls around the well.

  Invite students to predict how changing the ball’s starting velocity (in magnitude or direction) will affect its path. This is a good opportunity for one or more student volunteers to participate.

  Background

  The surface of this “potential well" is shaped so as to model an inverse square gravitational force. When a ball enters the well enters the well, it is attracted to the center; if it has no initial velocity, it will fall directly to the center. But if it enters with some velocity tangential to the center, it will fall into an elliptical orbit that gradually decays to the center as the ball rolls around the well.

  When you roll the ball across the surface, you use some initial force to start it moving. Once it is rolling on its own, though, the only forces acting on it are the force of gravity, pulling downwards, and the normal force and frictional force of the surface holding it up. So the ball accelerates as it rolls down the surface, exchanging potential energy for kinetic energy, until it falls into the hole.

• Availability: Available
• Loc codes: FS1, E1
• Videos-1: Demonstration video on YouTube