ENERGY CONSERVATION
Component list
Laptop computer with AC adapter & mouse
N3-PR,M0-PR, or T0-L
Data logger with I/O cabling
Y4-PR
Photogate with I/O cabling
Scale, digital with ac adapter
Air Supply
Y2-PR
DD4-PR
PP1-L
with hose @ corner of N5-PR
Air Track
BB4-L
Air Track Kit
PP2-L
Glider, Air Track
Lab Jack
PP2-L
or fixed width/height masses 500g, 200g, 100g,
G2-L
location @ H2-L
Meter Stick
5N, 1N
location @ H1-L
H5-L (in window)
Ringstand, Miniature
F5-L
Typical setup
Fig 1A partial equipment setup version A of an elevated air track using masses for elevating one end
Fig 1B equipment setup version B using a lab jack to elevate one end
Terminology
gravitational potential energy: U = mgh.
Kinetic energy: K  21 mv 2
total energy: E = K + U
Overview
As an object slides down a frictionless surface its total energy remains constant. However, its potential
and kinetic energies change. You will make a plot of K, U, and E for a glider sliding down an air-track
to test this idea.
Introduction
When the non-conservative (frictional) work on an object is zero, its total energy remains constant. This is a
fundamental tenet of science and has extremely broad application in all technical fields.
The two types of energy we will consider are the gravitational potential energy U, and the kinetic energy K,
both for our air-track glider. Since there are no other energies relevant for the glider, the sum of these two
energies will be its total energy E.
You will make measurements of U and K for five different locations on the air-track that the glider will pass
over. U  mgh is simply found by measuring the height at the five locations on the track that we will consider
and multiplying by mg, where m is the mass of the glider.
The values for h x
(where x = 1, 2, 3, or 4)
start at a minimum of h0 and progress to a maximum at h4 . You can see
that the h is what we are looking for. (Hint: hx  h0  h )
h x . This should give you corresponding values for the horizontal positions.
You’ll need these values when you do your graph. Remember the ℎ𝑥 ′𝑠 are located at 0 cm, 30 cm, 60 cm, 90 cm, and 120 cm.
You will need to do this operation for all five locations of
(See Figure 2B.)
The velocity for calculating K will be measured from five locations on the track. (It is at rest for the first
location, h0 , so the value K at this position will be zero.) K is found by measuring the speed v, at h0
corresponding to the different release points and then calculating from:
K  21 mv 2
where
m is the mass of the glider.
v is the velocity at h0
The flag will also be centered on the glider so that in effect we are considering all the mass to be located at the
center of the glider. As the flag passes through the photogate, LoggerPro can calculate its speed for the relevant
travel distances Remember the h x ’s are located at 0 cm, 30 cm, 60 cm, 90 cm, 120 cm, 150cm, & 180cm.
Once this data is collected you will plot the three quantities U, K, and (K + U) versus the position of the glider
on the track. This plot will allow you to test the idea of energy conservation as it applies to the glider air-track
system.
Fig 2 A elevated by lab jack
150cm
120 cm
90 cm
60 cm
Photogate
~30 cm
h4
Figure 2B.
h3
h2
h1
h0
Air-track, glider, and geometry. Measure the h-values from the tabletop surface to a reference point
on the air track of your choosing.
Accuracy

All of your observations should be recorded to 3 significant figures. You should carry 3 significant figures
in all of your calculations as well.
Procedure

Set up the air-track with an inclination roughly similar to that shown in Figure 1A or B or 2A or B.
Measure h0 through h4 as shown in Figure 2B. Adjust them to the h ’s and record in data table.

Using the large flag mounted at the center of the glider, slide the glider through the photogate to see that it
triggers the photogate properly. The computer should indicate a velocity (v) on the screen when collecting
data. The flag should not strike anything as it passes through. All reference measurements should be
measured at the center of the glider. By using the large flag, the computer is defaulted to that size and
calculates the velocity for you.

When you are ready to collect data, turn on the air source and let the glider accelerate from rest. Record the
velocity through the gate.

Release the glider 30.0 cm from the photogate. This is so you can find the terminal velocity that is
generated from h1 . Record relevant data in Table 1.

Repeat the measurement of time with a similar procedure for h2, h3, h4, and h5 & record relevant data in
Table 1.

Measure the mass of your glider using the digital scale. Your data acquisition is now complete.
Data and Analysis
Mass of the glider
air track
reference
mark
(cm)
travel
distance
d
along the
air track
(m)
m = __________________
height
above table
Δh
change in
height
(hx - h0)
where x=1, 2, 3, 4, or
5
U = m g Δh
(J)
calculated
velocity
v=
(flag width)/(photogate
time)
(m/s)
as measured by the logger
pro software
K = 1/2 m
v^2
(J)
calculated
total
Energy
E=K+U
(J)
reference
height
30
location
of
photogate
ho =
60
0.3
90
0.6
120
0.9
150
1.2
180
1.5
h1=
h2=
h3=
h4=
h5=
Table 1
After filling out this entire table you are ready to make your graph. The vertical axis will be energy in joules and
the horizontal axis should be positions in meters e.g. 0.0, 0.30, 0.60, 0.90 m, etc. (it should be in SI units).
E
K
energy
(J)
U
horizontal pos. m
Figure 3. Graph of U, K and E for the sliding glider.
Graph best-fit lines for each quantity, U, K, and E for your graph.
Questions
1. Is the best-fit line for the total energy E horizontal?
2. Should the best-fit line for E be horizontal?
Explain why or why not.