[:PiraScheme#Mechanics: Table of Mechanics Demonstration]

[:MEEquipmentList: List of Mechanics Equipment & Supplies]

[:Demonstrations:Lecture Demonstrations]

Cut the String, 1D55.15

Topic and Concept:



Using a homemade apparatus, a ball is put into a circular trajectory. A string connecting the ball to a rotating arm is quickly cut, and the ball flies off tangent to the original trajectory.



ID Number

Apparatus with Spool

ME, Bay A9, Shelf #1


Exacto Knife with Adjustable Stand

ME, Bay A9, Shelf #1



Found with Exacto Knife, Spares in ME, Bay A13, Shelf #3


Lead Block

Rod and Clamp Cabinet


Important Setup Notes:

Setup and Procedure:

  1. Place the apparatus on the lecture bench in such a way that the rotating arm w/ball just over hangs the front of the lecture bench.
  2. Place lead block on top of apparatus to keep it in place throughout the demo.
  3. Plug in the apparatus making sure the switch is in the off position beforehand.
  4. Unwind the appropriate amount of thread off the spool so that the ball can be tied as pictured above. The spool is set up as a cam. There are notches in the bottom of the spool that line up with a pin so that the length of the thread is fixed throughout the demo.
  5. To attach the ball, run the thread through both holes of the D-shaped portion of the rotating arm and tie the thread securely around the eye-hook. Lock the thread in place by pushing a small matchstick (the flammable portion removed)into one of the holes as pictured below.
  6. Place the Exacto knife in its stand on the table. Align the knife so that it cuts the thread in the D-shaped portion of the rotating arm. The stand should be placed such that when the thread is cut, the ball flies off to the right or left of the audience.
  7. The speed and direction of rotation can be adjust using the silver knob located below the center of rotation on the apparatus.
  8. Before turning the apparatus on, (by pushing the switch into the on position), make sure the rotating arm w/ball is over the knife. This way there is enough time for the ball to get into orbit before its to the knife.
  9. The ball will be placed into a circular trajectory. Cut the string at the desired time by raising the Exacto knife. This is done by turning the lower of the two knobs on the stand. The ball will assume a trajectory tangent to its the original circular trajectory. The point of tangency being determined by the placement of the Exacto knife.

Cautions, Warnings, or Safety Concerns:


Using our homemade apparatus, a ball is put into a circular trajectory. The centripetal force is provided by a circularly-rotating rod. A string that connects the ball to the rod is quickly cut using an Exacto knife. The ball immediately assumes a linear trajectory tangent to the original trajectory in accordance with Newton's first law. This also demonstrates two-dimensional, or more specifically projectile, parabolic motion.