012-06379B
Photogate Timer
17
®
Experiment 7: Conservation of Kinetic Energy
EQUIPMENT NEEDED:
-Two Photogate Timers
-Air Track System with two gliders.
Introduction
Momentum is always conserved in collisions that are isolated from external forces. Energy is also
always conserved, but energy conservation is much harder to demonstrate since the energy can
change forms: energy of motion (kinetic energy) may be changed into heat energy, gravitational
potential energy, or even chemical potential energy. In the air track glider collisions you’ll be
investigating, the total energy before the collision is simply the kinetic energy of the gliders:
E
k
= (1/2)mv
1
2
+ (1/2)mv
2
2
.
In this experiment you’ll examine the kinetic energy before and after a collision to determine if
kinetic energy is conserved in air track collisions.
Procedure
c
Set up the air track and
photogates as shown in
Figure 7.1, 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 7.1.
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 7.1 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.
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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.
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➤
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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
Bumpers
Figure 7.1: Equipment Setup
