CONSERVATION OF MOMENTUM

Ann M.W. Brandon               Thornwood High School
                               17100 South Park Avenue
                               South Holland, IL 60473
                               1-708-339-7800 

Objectives:         
     1) To use a memorable example of an explosion to impress the fact of 
        Conservation of Momentum. 
     2) Using the Conservation of Momentum, determine the velocity of the tennis 
        ball as it comes out of the tennis ball cannon. 

Materials:
 1) One tennis ball, new ones work better than old ones.
 2) One tennis ball cannon. Directions for construction appear at the end.  It 
    must be made out of steel cans, aluminum ones may blow up in your face! 
 3) Lighter fluid
 4) Some matches, and some wooden splints.
 5) A photocell timer to time the recoil of the cannon. (optional, but probably 
    the most accurate means of determining the times and velocities.) 

Strategy: 
 1) What?
    Get their attention! With some fluid in the cannon, shake the cannon,place the 
    ball in the front and fire. ( Be sure you aim at a solid object,not the 
    windows etc.)   
 2) Propose the problem:
    What is the velocity of the tennis ball as it leaves the cannon?  How are we 
    going to find out?  What kinds of measurements can we make to give us the 
    information we need to determine this velocity?  
                   
    At this point in the year your students should have several methods to 
    propose: 

    Velocity = Distance/Time

    So, if you can time the tennis ball over some measurable distance, you have it 
    made. 
 3) How:
    Demonstrate the timer: Show that it times in thousandths of a second, while 
    the beam is obstructed.  

    Fortunately for the point here, for a memorable shot the tennis ball moves too 
    quickly for an accurate timing. ( Somewhere between one and two thousandths 
    of a second for its passage past the photocell.) In order to use the timer we 
    must time the recoil of the cannon and then use Conservation of Momentum to 
    determine the velocity of the tennis ball. 

    On the board write the equation for the Momentum in this explosion: 
                                                                                
    0 =(Cannon Mass)(cannon velocity) + (Ball Mass)(velocity of ball) 

    Ask your students
     "What information will we need to find the velocity of the ball?"  
    They should mention the masses,and the length of the cannon, and 
      the time that the cannon takes to go past the photocell. 

2
    Assign each of these tasks to a different student.  Record this data in its 
    location in the formula. 

    Set up the timer in a convenient location. I go out in the hall, this allows 
    you a long range for firing, and lets all the other science students to notice 
    your existence!. 
 
    Warn your fellow teachers, if you wish, and send a student down the hall to 
    retrieve the ball. 

    The cannon should be loaded with fuel, shaken up, then loaded with the ball 
    and placed so that it will roll backwards through the photogate as it fires.  
    Be sure to zero the timer before firing! 

    Light a splint, and hold it to the hole at the back.  With any luck, it will 
    fire! 
 
    Record the time.  Return to class, place it in the equation and calculate the 
    velocity of the tennis ball.  

    If you have recorded the length of the cannon in meters, this will give you a 
    velocity in meters per second.  Have someone convert this to miles per hour -- 
    the students have a much better feel for this, and should be impressed. 60 mph 
    is fairly typical of a good shot, the Thornwood HS record is 109 mph. 

    Advice:
      Be sure that you discuss the safety involved here.
    The cannon must be steel!!!!
    Impress your students that aluminum will not work, and will probably send 
    bits of aluminum shrapnel into their face. 

    If the ball does not fire, remove the ball, add some more fluid, and shake.  
    Concentrate on forcing air into the cannon as you shake. This is especially 
    important if you have had a previous shot.  

    Your students will probably want to fire the cannon again. I usually do, if 
    you choose to do so, assign the task of remembering the time for the first 
    firing to a nearby student.  Remember to zero the timer before firing again. 

Making a cannon of your own:
     You need three or four cans made of steel.  The cans should have an inside 
diameter of 2 and 9/16 ths of an inch. It is possible that this could be 1/32 in 
larger, but a reasonably tight fit is important. 
     With the tops out of two cans, use a bottle opener to punch holes in the 
bottom of one of these cans. With a nail, punch a hole in the side wall near the 
bottom of the other. Tape the open ends of these two cans together. These compose 
the fuel chamber.  
     Cut the tops and bottoms out of the other two cans.  Tape these cans 
together, and tape them to the bottle openered end of the fuel chamber.  These 
make up the ball chamber.  Tape all four to a set of wheels. Be sure the nail hole 
faces up.  It may be useful to tape a stiffening bar to the top of all four cans 
to keep them from sagging.