1J30.25 - Forces on a String - Three Students and a Rope

Code Number:
1J30.25
Demo Title:
Forces on a String - Three Students and a Rope
Condition:
Good
Principle:
Vector Addition of Forces
Area of Study:
Mechanics

Disclaimer

Equipment:
2 - 100 Newton Spring Scales, 5kg Weight, and Cord String.
Procedure:

Stretch the string between the two scales until a force of 70 Newtons shows on each scale.  Add the 5 kg weight to the center of the string and observe the angle displacement and the force changes.  Observe that no matter how much force is applied to the ends of the string a relatively small amount of weight applied to the center of the string will give an angular displacement.

Another version of this involves a large rope and three students.  Have two large student grab the ends of the rope and by pulling hard, stretch it as straight as possible.  The third, much smaller student then goes to the center of the rope and pulls at 90 degrees to the rope, easily pulling the two larger students toward each other.

The instructor may do this demonstration by themselves if they attach one end of the rope to a wall, and run the other end of the rope over a pulley and attach it to a very large weight.  The instructor can pull at the center of the rope and easily lift the large weight.

References:
  • Paul Hewitt, "Answer to May 2019 Figuring Physics", TPT, Vol. 57, #6, Sept. 2019, p. 423.
  • Paul Hewitt, "Answer to February Figuring Physics", TPT, Vol. 57, #3, Mar. 2019, p. 195.
  • Thomas B. Greenslade Jr., "Erie Canal Technology: Stump Pullers", TPT, Vol. 51, #7, Oct. 2013, p. 437.
  • Wojciech Dindorf, "A Remarkable Vector Addition Exercise", TPT, Vol. 49, #8, Nov. 2011, p. 517.
  • Michael Bernstein, "Motivation for the Force Vector", TPT, Vol. 9, #3, Mar. 1971, p. 148, reprinted in TPT, Vol. 40, #6, Sept 2002, p. 379.
  • Paul Hewitt, "Figuring Physics: Tension on a Clothesline", TPT, Vol. 37, #6, Sept. 1999, p. 361.
  • James E. Court, "Free-Body Diagrams", TPT, Vol. 31, #2, Feb. 1993, p. 104.
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  • D. Easton, "Hooke's Law and Deformation", TPT, Vol. 25, #8, Nov. 1987, p. 494.
  • Father John Milton and Ellen Springer, "Walkway Forces", TPT, Vol. 20, #5, May 1982, p. 328.
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  • J. Higbie, "The Farmer and the Fence Post", TPT, Vol. 18, #7, Oct. 1980, p. 535.
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  • Herbert H. Gottlieb and The AAPT Committee on Apparatus, "A Vector Resolving Device", TPT, Vol. 5, #5, May 1967, p. 242.
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  • "M-268. Secretary Beats Two Professors",  DICK and RAE Physics Demo Notebook.
  • "M-264. Rope Tension", DICK and RAE Physics Demo Notebook.
  • Freier and Anderson, "Mj-1", A Demonstration Handbook for Physics.
  • David Kutliroff, "53, A Demonstration of Two Forces Required to Suspend One Weight", 101 Classroom Demonstrations and Experiments For Physics Teachers, p. 118.
  • Vicki Cobb and Kathy Darling, "Bet You Can't Out - Pull A Book!", Bet You Can't!, p. 24.
  • Borislaw Bilash II and David Maiullo, "Move It!", A Demo a Day: A Year of Physics Demonstrations, p. 63.
  • Robert Ehrlich, "C. 3. Chain with a Suspended Weight", Turning the World Inside Out and 174 Other Simple Physics Demonstrations, p. 22 - 23.
  • "Tug of War", Physics From the Junk Drawer, 3rd Edition, The Science House, North Carolina State University, p. 13.
  • Julius Sumner Miller, Q4 & A4, Millergrams I – Some Enchanting Questions for Enquiring Minds, p. 17 & 76.

Disclaimer: These demonstrations are provided only for illustrative use by persons affiliated with The University of Iowa and only under the direction of a trained instructor or physicist.  The University of Iowa is not responsible for demonstrations performed by those using their own equipment or who choose to use this reference material for their own purpose.  The demonstrations included here are within the public domain and can be found in materials contained in libraries, bookstores, and through electronic sources.  Performing all or any portion of any of these demonstrations, with or without revisions not depicted here entails inherent risks.  These risks include, without limitation, bodily injury (and possibly death), including risks to health that may be temporary or permanent and that may exacerbate a pre-existing medical condition; and property loss or damage.  Anyone performing any part of these demonstrations, even with revisions, knowingly and voluntarily assumes all risks associated with them.