```Demostrations of Law of Conservation of Linear MomentumTisdale, Robert W.                  Percy L. Julian High School                                    821-2571                           Objectives:

My objectives are to - (1) give a graphic demonstration of what the
momemtum of an object really is; (2) review some of the more commonly
available laboratroy equipment used to study momentum; (3) and produce
as well as use some momentum equipment that is very simple, cheap, and
easy to use.

Apparatus needed:

The apparatus consisted of an air track, air table, momentum machine,
billiard balls, small rubber ball, rope, steel pipe, seltzer water,
rubber stopper, two wheel chairs, and small springs.

Recommended strategy:

The first step is to establish what momentum is.  This is done with a
little twist of humor by telling a story of how three little boys
attempt to kill a big ferocious bear by using projectiles of different
size mass and different velocities.  Of course only the projectile with
sufficient mass and velocity is able to do the job.

From their we move on to establish what it means to conserve momentum.
We first observe a stationary pipe sealed on one end and with a rubber
stopper on the opposite end.  The students are asked to tell what the
momentum of this system is and with no exception all of them say the
momentum is zero.  After seltzer water is placed in the pipe the rubber
stopper flies out one way and the pipe another way.  The students are
once again asked to give the momentum of this new system and most will
say that the momentum has changed, changed to some nonzero or positive
value.

We then proceed with the concept of positive and negative velocity and
how the sum of the two momentums resulting from this reaction add up to
zero.  Hence, momentum is conserved and the law of conservation of
momentum is supported.

From this point we examine some of the equipment that is traditionally
used to study comservation of linear momentum: air tables, air tracks,
and the momentum device.  A small demonstration is performed with this
equipment.  I also bring out that the equipment is expensive and easily
damaged.  I introduce the idea of using simpler materials to illustrate
conservation of momentum.

One piece of equipment consists of two balls with a spring glued to at
least one of the balls.  You use both balls to compress the spring
about one meter directly above a point on the floor.  Very quickly you
release the balls.  Both balls should have equal momentum.  If they are
of equal mass then they should have equal velocities which will lead to
to them landing equal horizontal distances from from the point on the
floor.  If they are of unequal mass they will still have the same
momentum but the more massive ball will have a smaller velocity the
less massive ball.  This results in the more massive ball traveling a
smaller horizontal distance than the less massive ball.  The results
for this experiment don't give good quantitative data but the results
do produce qualitative data that is good enough to support the point.

Another demonstration involved three momentum carts of equal size.  One
of the carts is securely taped on top of another cart.  The result is
two carts, one which is roughly twice the mass of the other cart.  A
spring is placed between these two carts to propel them in opposite
directions.  Because of the law of conservation of momentum, we can
assume that both carts will be given equal momentum.  We can also
assume that the velocity of the lighter cart will be exactly twice the
velocity of the heavier cart.  This is the only way two masses, one
which is half the mass of the other, can have equal linear momentum. If
the velocity of small cart is twice that of the larger cart then the
small cart should be able to cover 2 times more distance in any given
period of time.  We place a block of wood some arbitrary distance, say
2 meters, from the smaller cart and another block of wood one half of
former distance, 1 meter in this case, from the larger cart.  Both
carts should arrive at their finish lines at the same time.  It will be
clearly observable to the students when the carts crash at the same
time.  This experiment works very well to be so simple in design.

A third experiment involves the use of wheel chairs.  Two students, one
much larger than the other, face each other seated in the wheel chairs
about about 15 to 20 feet apart while while holding the opposite ends
of rope.  Before they start pulling they have a total momentum of zero.
To maintain a state of zero momentum after they start pulling the
larger student will move at a smaller velocity and, therefore, not
travel as far.  The smaller student will move at a much higher velocity
and, consequently, roll for a greater distance.  It is important that
the wheels of the wheel chair be aligned correctly before the pulling
starts and that some one be there to catch the students before they
crash in to each other.  There are a lot of variations that one can do
with this.  Two students of similar size can pull.  Students can push
off of each others hands instead of pull.  The results will probally
not be good enough to make good quantitative measurements but they will
be quite sufficient for making a qualitative estimate of what should
take place.
```