012-06379B
Photogate Timer
15
®
Experiment 6: Conservation of Momentum
EQUIPMENT NEEDED:
-Air track system with two gliders
-Two Photogate Timers.
Introduction
When objects collide, whether locomotives, shopping carts, or your foot and the sidewalk, the
results can be complicated. Yet even in the most chaotic of collisions, as long as there are no ex-
ternal forces acting on the colliding objects, one principle always holds and provides an excellent
tool for understanding the dynamics of the collision. That principle is called the conservation of
momentum. For a two-object collision, momentum conservation is easily stated mathematically
by the equation:
p
i
= m
1
v
1i
+ m
2
v
2i
= m
1
v
1f
+ m
2
v
2f
= p
f
;
where m
1
and m
2
are the masses of the two objects, v
1i
and v
2i
are the initial velocities of the ob-
jects (before the collision), v
1f
and v
2f
are the final velocities of the objects, and p
i
and p
f
are the
combined momentums of the objects, before and after the collision. In this experiment, you will
verify the conservation of momentum in a collision of two airtrack gliders.
Procedure
c
Set up the air track and
photogates as shown in
Figure 6.1, using bumpers
on the gliders to provide an
elastic collision. Carefully
level the track.
d
Measure m
1
and m
2
, the
masses of the two gliders to be used in the collision. Record your results in Table 6.1.
e
Measure and record L
1
and L
2
, the length of the gliders. (e.g., push glider
1
through photogate
1
and
measure the distance it travels from where the LED comes on to where it goes off again.)
f
Set both Photogate Timers to GATE mode, and press the RESET buttons.
g
Place glider
2
at rest between the photogates. Give glider
1
a push toward it. Record four time mea-
surements in Table 6.1 as follows:
t
1i
=
the time that glider
1
blocks photogate
1
before the collision.
t
2i
=
the time that glider
2
blocks photogate
2
before the collision.
(In this case, there is no t
2i
since glider
2
begins at rest.)
t
1f
=
the time that glider
1
blocks photogate
1
after the collision.
t
2f
=
the time that glider
2
blocks photogate
2
after the collision.
➤
IMPORTANT:
The collision must occur after glider
1
has passed completely through
photogate
1
and, after the collision, the gliders must be fully separated before either glider
interrupts a photogate.
➤
➤
➤
➤
➤
NOTE:
Use the memory function to store the initial times while the final times are being
measured. Immediately after the final times are recorded, the gliders must be stopped to prevent
them from triggering the photogate again due to rebounds.
Photogate
1
Glider
2
Glider
1
Photogate
2
m
2
m
1
Figure 6.1: Equipment Setup
