Physics Demo Number: 032

Approximate Run Time:10 min

Inertia Shown by Breaking Strings Attached to a Hanging Mass

Demo Description

Two suture threads are attached to a 1 kg mass. One above supporting the mass, the other beneath it for the demonstrator to pull on. A slow steady pull with increasing force breaks the top thread. A quick jerk breaks the bottom thread.

 

Scientific Principles

  • Newton's 1st Law - law of inertia

 

 

 

Equipment

  • 1 kg Hooked Mass

  • Suture Thread

  • Floor Stand and Support Rod

Equipment Location

  • Kit (032) on [E-4-5] contains the Hooked Mass, Thread, and Rod with Hook Collar for using with floor stand

  • Floor Stand Next to 4327 Door (Beside the Barber Chair)

Instructions

The photos show a 1 kg hooked mass hanging from a hook collar by a cotton suture thread. Moreover there are cotton suture threads attached to the bottom of the hooked mass.

One may pull on a suture attached to the bottom of the mass and break only it, if one jerks the bottom suture quite crisply, or one may pull slowly with a steadily increasing force on the bottom suture and break the upper suture only.

The key element is the correct tying of the knots.

One can cut about 80 cm lengths of suture and then construct loops on one end of a given length by the following procedure:

     1)   Form a bite on the end of a 80 cm length of suture and tie a figure eight knot on that bite, leaving a loop.

     2)  See the third photo where an example figure-eight-on-a-bite knot , leaving a loop , is tied in much bigger cord for clarity.

     3) One may pass the free end of the suture in 1) through the bottom metal bar on the hooked mass and back through  the suture 

         loop.

This procedure gives a knot attaching the suture line at the bottom  of the  mass with no give  and no slipping possible.

Repeating the above first two steps at both ends of a suture  gives  a suture for hanging the hook  mass from the top hook collar, with no give or slipping possible in the knots.

These sutures, tied as specified, will not allow slippage in the suture knots.

Slippage in knots can lead to unreliable results.
  
Writeup created by David A. Burba
Copyright © 2013, Vanderbilt University.   All Rights Reserved.