THIS PROCEDURE WILL HELP YOU BUILD A DEVICE THAT INTERFACES WITH YOUR COMPUTER TO EASILY MEASURE PROJECTILE VELOCITIES.  CONSTRUCTED EASILY AND INEXPENSIVELY WITH AN OLD COMPUTER MOUSE.

 

 
 

DESCRIPTION

A BALLISTIC PENDULUM IS A CLASSIC PHYSICS TOOL USED TO CALCULATE PROJECTILE VELOCITIES.  A PROJECTILE IS FIRED INTO A LARGE MASS SUSPENDED FROM HINGES FORMING A PENDULUM.  BY MEASURING THE HEIGHT TO WHICH THE PENDULUM RISES AFTER THE IMPACT, ONE CAN CALCULATE THE ENERGY OF THE PROJECTILE, AND THEREBY THE VELOCITY.  NOT ONLY IS THE BALLISTIC PENDULUM A USEFUL TOOL FOR CHEAPLY MEASURING VELOCITY, BUT IT IS A POPULAR CONSERVATION OF ENERGY DEMONSTRATION IN PHYSICS CLASSROOMS.

PRESENTED HERE IS A MODERN REVIVAL OF THIS TOOL.  IN THE TRADITIONAL CONFIGURATION THE EXPERIMENTER MUST MARK THE MAXIMUM ACHIEVED HEIGHT OF THE PENDULUM, WHICH CAN BE DIFFICULT IN CERTAIN CASES.  THIS SMART BALLISTIC PENDULUM WORKS IN A SIMILAR FASHION, HOWEVER USING A COMPUTER ENABLES THE HEIGHT MEASUREMENT TO BE MADE AT MULTIPLE POINTS IN THE PENDULUM'S SWING, AND WITH MUCH GREATER ACCURACY.  THE RESULT IS A MEASUREMENT OF INCREASED ACCURACY AND PRECISION, WHILE EXPOUNDING ON THE ORIGINAL DESIGNS GREATEST FEATURES:  LOW COST, AND EASE OF USE.

A BALLISTIC PENDULUM SUCH AS THIS MAY BE PARTICULARLY USEFUL FOR PERSONS WITHOUT THE MONEY OR KNOW-HOW TO BUY OR BUILD A MICROCONTROLLER BASED PHOTOSENSOR SPEED TRAP.  MOREOVER, IT WOULD BE USEFUL IN SITUATIONS WHERE THE TEST PROJECTILE CREATES A LARGE AMOUNT OF MUZZLE FLASH THAT CAN BLIND SPEED TRAP SENSORS.

SPECIAL THANKS TO JASON ROLLETTE FOR SUGGESTING THIS EXPERIMENT!
VISIT HIM AT HTTP://WWW.ROLLETTE.COM
 

MATERIALS        

 -FLOWER ARRANGEMENT FOAM (OR SUITABLE SUBSTITUTE)
-SCROLL WHEEL MOUSE
-MISC. HARDWARE (COAT HANGER ETC)
-EPOXY

EQUIPMENT        

-COMPUTER
-CUTTING TOOL (DREMEL)

PROCEDURE

  

WHY?

1.  LOCATE A MOUSE WITH A SCROLL WHEEL FOR DESTRUCTION.  IT SHOULD BE A 'NEWER' MOUSE, I GOT THIS ONE AT WALMART FOR $18.00

 NEWER MICE HAVE HIGHER RESOLUTION SCROLL WHEELS.  THIS IS ESSENTIAL IN THIS CASE.
2.  DISCARD THE PRETTY PLASTIC.  YOU MAY HAVE TO PEEL OFF SOME LABELS AND PADS ON THE BOTTOM TO FIND THE SCREWS. 'CUZ
3.  REMOVE THE CIRCUIT BOARD AND CUT OUT THE PIECE OF THE BASE WHERE THE VERTICAL SCROLL WHEEL SUPPORTS ATTACH.
THESE WILL COME IN HANDY LATER WHEN THEY'RE USED TO SUPPORT THE PENDULUM.
4.  REMOVE THE SPRING THAT APPLIES TENSION TO THE SCROLL WHEEL.  IT CAN BE FOUND ON THE SCROLL WHEEL ASSEMBLY.  ALSO, PREPARE THE MOUNT FOR GLUING BY SETTING IT BACK INSIDE THE CIRCUIT BARD AND TESTING THE SCROLL WHEEL IN IT TO ENSURE CLEARANCE.
THE SPRING IS WHAT PRODUCES THE 'BUMPY' FEELING WHEN YOU TURN A MOUSE WHEEL.  LEAVING IT IN WOULD MAKE THE PENDULUM DIFFICULT TO MOVE AND RUIN THE DATA.
5.  USE EPOXY TO SECURE THE MOUNT DIRECTLY TO THE CIRCUIT BOARD.  
6.  REMOVE THE WHEEL FROM ITS HOUSING AND ATTACH A PENDULUM ARM TO THE FREE END OF THE WHEEL.  NOTE THAT THIS ARM SHOULD BE MADE OF A SEMI RIGID MATERIAL, AND ITS MASS SHOULD BE MUCH LESS THAN THAT OF THE PENDULUM ITSELF.  THE ARM LENGTH WILL DEPEND ON THE MAGNITUDE OF ENERGIES YOU'RE MEASURING.  8" IS A GOOD START. IF THE MASS OF THE ARM IS COMPARABLE TO THE MASS OF THE PENDULUM ITSELF, CONSERVATION OF ENERGY IS NOT LONGER VALID (AS IMPLEMENTED IN THE SOFTWARE).  DATA WILL BE VERY POOR IN THIS CASE.
7.  PUT THE SCROLL WHEEL BACK IN ITS HOUSING AND SLIDE THEN ASSEMBLY INTO THE MOUNT.  GLUE IT IN PLACE, CHECKING FOR ADEQUATE CLEARANCE.  IF YOU DO NOT CHECK FOR AMPLE CLEARANCE ALL AROUND THE SCROLL WHEEL THE WHEEL WILL DEFLECT AND RUB AGAINST THE MOUNTING WHEN YOU HAVE A LARGE PENDULUM ATTACHED.
8.  SINCE I AM GOING TO USE BLOCKS OF FOAM AS MY TEST MASS, I NEEDED SOME WAY TO ATTACH THE FOAM TO THE PENDULUM ARM.  A PIECE OF WOOD EPOXIED TO THE ARM WORKS WELL.
FOR LARGER ENERGIES, THIS WOULD BE THE PERFECT PLACE TO ATTACH A LARGER PENDULUM MASS.
9.  THE PENDULUM WILL NEED TO BE SUPPORTED SO IT MAY SWING FREELY.  I MADE AN IMPROVISED STAND WITH A COAT HANGER AND SOME WOOD.  YOU MAY EXPERIENCE SUCCESS BY SIMPLY CLAMPING THE CIRCUIT BARD TO A TABLE AND LETTING THE PENDULUM DANGLE.
I KNOW WHAT YOU'RE THINKING: "THATS A HIGH QUALITY STAND."
10.  FLOWER FOAM IS SIMPLY A VERY SOFT FOAM THAT WILL TRAP LIGHTER PROJECTILES VERY WELL. THE FOAM IS IMPORTANT BECAUSE IT WILL TRAP THE PROJECTILE INSIDE IT.   IF THE PROJECTILE BOUNCES OFF, IT WILL CARRY SOME ENERGY AWAY WITH IT.  ON THE OTHER HAND, USING A TOO-HARD FOAM WILL CAUSE ENERGY LOS DUE TO HEATING.  THE IDEAL SITUATION IS A SOFT FOAM THAT WILL ALLOW THE PROJECTILE TO PENETRATE AND HIT THE HARD MASS AT THE END OF THE PENDULUM ARM WITHOUT SLOWING IT.
11.  HACK OFF A CHUNK WITH A KNIFE.  THIS FOAM IS A LOT OF FUN.  I CAN'T STOP STICKING MY FINGERS IN IT. 
12.  CUT A HOLE THAT WILL FIT THE PENDULUM TIGHTLY  
13.  ATTACH THE FOAM TO THE PENDULUM AND YOU'RE DONE!  


RESULTS

USE OF THIS DYNO IS SIMPLE.  SIMPLY CONNECT TO YOUR COMPUTER, FIRE UP THE SOFTWARE (BELOW) AND THEN SHOOT WHEN THE LIGHT IS GREEN. 



A SMALL VIDEO OF THIS TEST SHOT IS AVAILABLE HERE (MPG, 112KB)

 

SOFTWARE

THIS DEVICE REQUIRES CUSTOM SOFTWARE TO ACCESS AND ANALYZE THE DATA:

 

DOWNLOAD SCI-SPOT.COM SMART PENDULUM CONTROLLER
DOWNLOAD SOURCE

 

TO USE, SIMPLY INPUT ALL OF THE PARAMETERS OF YOUR DEVICE AS SHOWN ON THE MAIN SCREEN:

PENDULUM MASS - THE MASS OF THE WEIGHT AND FOAM AT THE END OF YOUR PENDULUM.

PROJECTILE MASS - THE MASS OF WHATEVER YOU'RE SHOOTING

PENDULUM LENGTH - THE DISTANCE FROM THE AXLE OF THE SCROLL WHEEL TO THE CENTER OF THE TEST MASS.

STEPS PER ROTATION - HOW MANY IMPULSES DOES THE SCROLL WHEEL SENSOR GIVE PER ROTATION.  CLICK THE MEASURE BUTTON TO FIND THIS VALUE AUTOMATICLY.

NEXT TO THE VELOCITY DISPLAY IS A RED AND GREEN LIGHT.  WHENEVER THE LIGHT IS GREEN, THE SOFTWARE IS READY AND WAITING FOR A SHOT.  WHEN THE LIGHT IS RED, DO NOT SHOOT.


READER QUESTIONS

NONE YET, BE THE FIRST!

GOT A QUESTION OF YOUR OWN?  CLICK HERE TO ASK IT!