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Final Answers
© 2000-2015   Gérard P. Michon, Ph.D.    

Physics of Slingshots

The poor man's shooting sport.
Jack H. Koehler

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Related Links (Outside this Site)

Wikipedia Portal  |  PhysLink.com  |  Physics Central
Speed of a Screaming Slingshot Monkey  by  Dmitriy Gekhman (2007).

Slingshots  &  Supplies :

Books :


Slingshots, Germans and Slow Motion  |  The Physics of Slingshots, with Jörg
Jörg Sprave's best slingshots
Rufus Hussey (1919-1994)  "The Beanshooter Man"
Fast Draw Shots on Day One of 2013  by  PFShooter  (Pickle Fork Shooter).

The Physics of Slingshots

(2012-07-21)   Elasticity
The stress-strain curve.

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 still working on this one...

Rubber elasticity   |   Hyperelastic material   |   Cauchy elastic material

(2013-01-04)   Characteristics of a rubber compound.
Strict relationships between bands made from the same rubber recipe.

Manufacturers may not always use the same type of rubber for all their bands.  Their nominal formulas may evolve over time and various coloring additives can substantially influence the physical properties of rubber.  There may also be some slight variations from one batch to the other.

However, long bands made from strips cut from same wide band spool have consistent physical properties which can be predicted from their geometry alone.  (The flat bands supplied by  Thera-Band are 128 mm  wide.)

Thera-Tube: New recipe by  Jörg Sprave   ("The Slingshot Channel").

(2012-07-21)   Straight Cuts   vs.   Tapered Bands
Applied forces are inversely proportional to their virtual displacements.

Tapered bands are easily fabricated from wide flat rubber bands, using a cutting mat and rotary cutter.  The only widely available tapered  tubular  rubber bands are the  red bands  (RRT model)  from Trumark.

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 still working on this one...

Slingshot Forum
Videos :   Chrony Test   |   Flatband banding   by  John "Gamekeeper" Webb.
Squeezing 5/16" balls inside tubular bands makes them ready for a quick-banding system.

(2013-01-07)   Temperature
How the characteristics of rubber vary with temperature.

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 still working on this one...

Video :   Richard Feynman on rubber bands 
Slingshot with heated bands  by  Jörg Sprave   ("The Slingshot Channel").

(2013-01-03)   Calibrated Straight Rubber Bands
Characteristics of commercial color-coded rubber bands  (tubular or flat).

The  Thera-Band  company sells high-quality  rubber  tubing  manufactured for them by  Hygenic  in the  7  color-coded characteristics tabulated below.

They also provide,  8  different types of  flat  rubber bands.  Those used to be  138 mm wide; they are now  128 mm  wide, with more efficient rubber  (reduced width and thickness makes their new bands lighter but a given length of any of their color grades retains virtually the same elastic propertiesthe same dynamical properties).

The thickest and stiffest grade for flat bands is dubbed  gold  (that strength isn't available for tubular bands).  The improved formula is also used for the newer tubular rubber bands, whose outer diameter has been reduced to produce the same pull as before  (the inner diameters remain the same).

All those bands are primarily intended for the  Thera-Band  progressive  exercise systems but are also extensively used by slingshot enthusiasts  (who commonly call the tubular bands  thera-tube  for short).

Straight Tubular Rubber Bands
BrandModelStretchDraw Linear densityInner ÆDate
Thera TubeSilver 167   N0.535  g/cm0.2 "2012
Thera TubeBlack 73.5  N0.43  g/cm0.2 "2012
Thera TubeBlue   0.2 " 
Thera TubeGreen   0.2 " 
Thera TubeRed   0.2 " 
Thera TubeYellow   0.2 " 
Thera TubeTan   0.062 " 

The small inside diameter of tan tubing makes it unsuitable for traditional slingshot rods.  All of the others are perfectly suited for standard  ¼"  round-rod frames, except  Thera-tube yellow  which doesn't provide sufficient grip around a  ¼"  (either secure it with a constricting knot, or use  5/16"  rods).

Straight Flat Rubber Bands
BrandModelStretchDrawLinear densityDate
Thera BandGold    
Thera BandSilver    
Thera BandBlack    
Thera BandBlue    
Thera BandGreen    
Thera BandRed    
Thera BandYellow    
Thera BandTan    

Isothermic Calibration :  Draw (force in newtons) vs. stretch (strain in %).
Strain Tan Yellow Red Green Blue Black Silver Gold
0 % 0000 0000
25 % 12.516.022.235.1
50 %  8.011.814.0 
75 %    
100 %    
125 %    
150 %    
175 %    
200 %    
225 %    
250 %    

Office Rubber :

Size #107 rubber bands  (office supply)  are 7" long and 0.625" wide  (5/8).  Here are the product numbers for a one-pound box at the  Alliance Rubber Company :

  • 27075: 40 per pound.  Alliance Advantage  grade  (max. 700%).
  • 25075: 50 per pound.  Alliance Sterling  grade  (max. 750%).
  • 21075: 60 per pound.  Alliance Pale Crepe Gold  grade  (max. 775%).

Mover bands:  27" long, 0.75" wide  (3/4).

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 still working on this one...

Thera-Band Elongation Chart   (Slingshot Forum).
Attaching Thera-bands to the pouch and the catapult   by  John Webb  =  "Gamekeeper John"  (Llandudno, Wales).
Bike Inner Tubing   vs.   Office Rubber   vs. Thera-Band   by  Jö Sprave   *The Slingshot Channel).

(2013-01-10)   The Right Stuff
Rubber, polymer, resin, protectant...

303 protectant.

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 still working on this one...

(2013-01-09)   Tools of the Trade
Cutting rubber, punching leather, carving wood...

rotary cutter  is designed to cut fabric on a  cutting mat.  It's what's needed to cut strips from a wide roll of rubber.

Hole punch. Rivet.

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 still working on this one...

(2013-01-12)   Slingshot Ammo and Calibrated Projectiles
Steel ball bearings (BB) glass marbles, nylon balls, cast lead or bismuth.

Odd-shaped natural rocks remain the mainstay of slingshot ammunition for casual plinking.  For competitive shooting, however, only  spherical  projectiles are used.

The density of polypropylene (PP) is 0.946 g/cc.  Nylon 6-6  is 1.15 g/cc.  Teflon (PTFE) is 2.20 g/cc.

The density of corundum  (ruby or sapphire)  is 4.02 g/cc. 

The density of chrome steel is 7.85 g/cc.  The density of 316 stainless steel is 7.99 g/cc.  The density of 304 stainless steel is 8.03 g/cc.

The density of bismuth is 9.807 g/cc.  The density of lead is 11.342 g/cc.  The density of tungsten carbide is 15.63 g/cc.

Spherical Projectiles   (diameters are in mm, inches or caliber units of 0.01")
  24Cast lead82 g1265 
100125.4316 Steel68.6 g1058$12.58
753/419.1Stainless Steel29.1 g449 
  16Lead24.3 g375 
  16Bismuth21.0 g325 
501/212.7Tungsten carbide16.8 g259$10.85
  12Lead10.3 g158 
  12Bismuth8.9 g137 
501/212.7316 Stainless Steel8.6 g132$0.96
 125.4Polypropylene (PP)8.1 g125$0.76
 125.4Hi-Bounce (mini)7.3 g113$0.13
383/89.53Tungsten carbide7.1 g109$5.78
447/1611.1Stainless Steel5.7 g 89 
 3/419.1Nylon 6-64.16 g64$0.48
383/8 9.53Stainless Steel3.6 g 56 
  9Stainless Steel3.1 g 47 
  40Ping-pong ball2.4 g 37 
 1/212.7Teflon (PTFE)2.36 g 36$0.32
315/167.94316 Stainless Steel 2.1 g 32$0.15
303/107.62Stainless Steel1.9 g 29 
 1/212.7Nylon 6-61.23 g19$0.22
251/46.35Stainless Steel1.08 g16.6 
227/325.56Stainless Steel0.72 g11.1 
227/325.56Synthetic Ruby0.36 g5.57$8.48
165/323.97Synthetic Ruby0.13 g2.03$6.88
9.3753/322.38Tungsten carbide0.111 g1.71$1.28
  0.5Synthetic Sapphire0.263 mg0.004$3.37

Synthetic ruby, sapphire or tungsten carbide are inadequate for casual plinking.  They are reserved for specialized experiments.

(2013-01-23)   Ballistic Gel  &  Ballistic Pendulum
Calibrating ordinary gelatin (foodstuff) for ballistic purposes.

The old NATO formula for "20% gelatin" (3.2 oz/lb) is no longer used.  Professionals use 10% gelatin (1.6 oz/lb) and some amateurs are satisfied with a less expensive mix (1 oz/lb).

The preparation protocol is critical to achieve consistent results.  In particular, temperature should be maintained below 40°C.

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 still working on this one...

Gelatin   |   Ballistic gelatin   |   Gun chronograph
Simplified 10& Ordnance Gelatin Preparation and Test  (FirearmsTactical.com)
10% gelatin with defoamer:  How-to ballistic gelatin  by  Scott E. Mayer  (Guns & Ammo, July 2011).
How to Make Your Own Ballistic Gel, using food-grade gelatin.

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