Apparatus Competition

2006 AAPT Summer Meeting

Syracuse, NY


The Conical Pendulum with a LEGO MINDSTORMS® Robotics Invention System Stepper


John J. Lynch

Department of Physics

Wheeling Jesuit University

316 Washington Ave.

Wheeling, WV  26003



jlynch ‘at’ wju ‘dot’ edu



            This apparatus is used to study uniform circular motion using a LEGO MINDSTORMS® Robotics Invention System (RIS) stepper motor with a steel ball attached to a string. It is an alternative to the variable-speed rotator/centripetal force apparatus. The equipment needed is inexpensive and the resulting motion takes place over longer time and length scales. Periods and distances can be measured with a stopwatch and a meter stick. In addition, students get a brief introduction to robotics.

Construction of Apparatus: 

Metal ball


Meter stick

1x8 board


Computer with USB port

From the LEGO MINDSTORMS® Robotics Invention System:

   RCX Programmable Brick

   Stepper motor

   Shaft Attachment

   Extra LEGO pieces for bracing

   USB infrared transmitter

   2 leads

   ROBOLAB software

Use of Apparatus: 

(1) Support the board horizontally and clamp the stepper motor to the side using the C-clamp.

(2) Attach the metal ball to the string and the string to the shaft of the stepper motor.

(3) Connect the yellow RCX programmable brick to the stepper motor and line it up with the USB infrared transmitter. Connect the transmitter to the computer.

(4) Write a simple ROBOLAB program on the computer that turns the shaft of the stepper motor at a constant speed.

(5) Turn on the RCX brick and upload the program.

(6) Measure L the distance between the pivot and the center of the ball.

(7) Press the Run button on the RCX and give the ball a tangential boost to get it moving in sync with the stepper motor.

(8) When the ball is moving in a steady horizontal orbit use the stopwatch to measure the time for ten orbits, 10T.

(9) Position the meter stick horizontally so it passes directly under the shaft. Use it to obtain the orbital diameter of the ball, 2r. A sheet of paper resting vertically on the meter stick can be used to determine the position of the outermost edge of the ball. Gradually move the paper radially inward until it is grazed by the orbiting ball.

(10) Calculate r from Newton's second law.

(11) Compare the two r's from steps (9) and (10).

(12) Change the motor speed and return to step (7).