Physics Demonstrations
Department of Physics and Astronomy
Physics Demonstrations

Black Board Pendulum

Identifier: M: EngMom - 005

Mobile black board with a horizontal line drawn on it; pendulum with a ball on the end is attached via magnet.

Concept Overview

Conservation of Energy

Lecturer Procedure

Step(s) to FollowExpected Result
  1. Pull the ball back so that it is even with the line.
  2. Release the ball.
  3. Note the height that it swings to.
The ball will rise almost to the same height it was released from.
  1. Place a peg beneath the point from which the ball is suspended.
  2. Repeat steps 1-3 above.                                                                                                     
The ball's path will be interrupted by the peg, but the ball will still rise to almost the same height it was released from.

 

Historical References; Taken from original work of Percy H. Carr, 1970 

“If the position of the rod is sufficiently low the ball will loop the loop around the rod. (Remember that in order to loop the loop the ball must pass over the top with considerable speed. If there is insufficient speed the path will be a parabola making and inner tangent with the expected circle, thus putting much slack in the cord. As it falls along this new curve, there comes a time when this slack is taken up suddenly, often breaking the cord.)”

 

Tutorial: Set-up Instructions for M: EngMom - 5

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