CONSERVATION OF MOMENTUM
Ann M.W. Brandon Thornwood High School
17100 South Park Avenue
South Holland, IL 60473
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.
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.)
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
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
Velocity = Distance/Time
So, if you can time the tennis ball over some measurable distance, you have it
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.
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
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
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.
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.
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