012-06379B
Photogate Timer
19
®
Experiment 8: Conservation of Mechanical Energy
D
L
d
h{=
Table 8.1: Data and Calculations
EQUIPMENT NEEDED:
-Photogate timer and Accessory Photogate
-air track system with one glider
-block of wood of known thickness (approximately 1-2 cm).
Introduction
Though conservation of energy is one of the most powerful laws of physics, it is not an easy prin-
ciple to verify. If a boulder is rolling down a hill, for example, it is constantly converting gravita-
tional potential energy into kinetic energy (linear and rotational), and into heat energy due to the
friction between it and the hillside. It also loses energy as it strikes other objects along the way,
imparting to them a certain portion of its kinetic energy. Measuring all these energy changes is no
simple task.
This kind of difficulty exists throughout physics, and physicists meet this problem by creating
simplified situations in which they can focus on a particular aspect of the problem. In this experi-
ment you will examine the transformation of energy that occurs as an airtrack glider slides down an
inclined track. Since there are no objects to interfere with the motion and there is minimal friction
between the track and glider, the loss in gravitational potential energy as the glider slides down the
track should be very nearly equal to the gain in kinetic energy. Stated mathematically:
Δ
E
k
=
Δ
(mgh)=mg
Δ
h;
where
Ek is the change in kinetic energy of the glider [
Δ
E
k
= (1/2)mv
2
2
- (1/2)mv
1
2
] and
Δ
(mgh)
is the change in its gravitational potential energy (m is the mass of the glider, g is the acceleration
of gravity, and
Δ
h is the change in the vertical position of the glider).
Procedure
c
Level the airtrack as accurately as possible.
d
Measure d, the distance between the air track
support legs. Record this distance in Table
8.1.
e
Place a block of known thickness under the
support leg of the track. For best accuracy,
the thickness of the block should be mea-
sured with calipers. Record the thickness of
the block as h in Table 8.1.
f
Setup the Photogate Timer and Accessory Photogate as shown in Figure 8.1.
g
Measure and record D, the distance the glider moves on the air track from where it first triggers the
first photogate, to where it first triggers the second photogate. (You can tell when the photogates
are triggered by watching the LED on top of each photogate. When the LED lights up, the photo-
gate has been triggered.)
h
Measure and record L, the effective length of the glider. (The best technique is to move the glider
slowly through one of the photogates and measure the distance it travels from where the LED first
lights up to where it just goes off.)
i
Measure and record m, the mass of the glider.
j
Set the Photogate Timer to GATE mode and press the RESET button.
k
Hold the glider steady near the top of the air track, then release it so it glides freely through the
