Physical Science Institute
Summer 2013
Conservation of Energy
PRE-LAB
When you release a cart down an inclined plane, the cart speeds up. In terms of energy, the cart starts with gravitational
potential energy, and gains kinetic energy as it accelerates down the track. In addition, the term mechanical energy
refers to the total kinetic and potential energy all together.
Gravitational potential energy is a function of height, and can be calculated using the following formula:
G.P.E. = mgh
where m is the mass of the object, g is the acceleration due to gravity, and h is the
height of the object.
Kinetic energy is a function of velocity, and can be calculated using the following formula:
K.E. = ½ mv2
where m is the mass of the object, and v is the velocity of the object.
A careful analysis of the units will show that both potential energy and kinetic energy are measured in the same units.
Units of P.E = (kg)(m/s2)(m) = (kg)(m2/s2)
Units of K.E. = (kg)(m2/s2)
The (kg)(m2/s2) is called a Joule in the S.I. and is the standard unit for energy that will be used during this activity.
PRE-ACTIVITY QUESTIONS:
1. What form(s) of energy does the cart have at the top of the incline just before it is released?
2. What form(s) of energy does the cart have near the bottom of the incline?
3. Can the cart have both potential energy and kinetic energy at the same time?
Using your answers to the above questions; predict the general shapes of the position and velocity time graphs of a cart
rolling down an inclined plane. Make your graphs reflect the sensor being positioned at the top of the track and the cart
moving AWAY from it.
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Physical Science Institute
Summer 2013
In this activity, students will observe and calculate the energy transformations of the cart on an inclined plane.
1 Vernier Dynamics Cart
1 Vernier Dynamics track
1 Motion Sensor
MATERIALS
Vernier LabQuest2
Ring Stand
PROCEDURE
1. Create a setup as in the illustration below using the ring stand attachment.
2. Record the mass of the empty dynamics cart in data table one.
3. Set the sampling rate of the LabQuest2 to 100 samples/s.
4. In order to properly collect data for this activity, it will be necessary for both position and velocity graphs to be visible
on the screen. In addition, the sensor will need to be calibrated for this activity. To calibrate the sensor, follow the
instructions below:
a) Hold the edge of the cart that is nearest to the sensor at the 20 cm mark on the track.
b) Press the meter icon.
c) Press the sensors tab.
d) Choose the “zero” option, then select “dig 1: motion detector”.
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Physical Science Institute
Summer 2013
5. Press the start recording button, wait about two seconds and release the cart.
6. How do the corresponding position and velocity time graphs compare with your predictions?
7. Measure the HEIGHT of the track at the following three locations and record in data table 2:
a) The release point
b) The point at which the cart reached its highest velocity

This point is found by touching the velocity graph at its highest point and then reading the corresponding
position graph point.

Then add this number to the distance used when the sensor was zeroed.
In this example notice the position at the highest velocity is 0.734m. This is equal to 73.4 cm. When added to
the 20cm used to zero the sensor this point is 93.4 cm.
c) Any point on the track between the release point and the highest velocity point for the cart.

Choose any point on the graph and read the position point. Then add this number to the distance used when
the sensor was zeroed.
8. Record the velocity of the cart at each of the locations described above and record in data table 3.
9. Repeat the procedure one more time and record the data.
DATA
Data Table 1 – Cart Mass
Trial one
Trial two
Data Table 2
Height at release
point
Height at point between release
point and highest velocity point
Height at which cart
Reaches highest velocity
Trial one
Trial two
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Physical Science Institute
Summer 2013
Data Table 3
Velocity at release
point
Trial one
0
Trial two
0
Velocity at point between release
point and highest velocity point
Highest velocity
CALCULATIONS & RESULTS
Using the formula’s described in the Pre-Lab section; calculate the potential energy, kinetic energy, and
mechanical energy for each data point. Record the results in the table below.
Using the recorded data, calculate the PE, KE, and ME for all data collected.
Energy
Release Point
Point in Between
Highest Velocity Point
Calculations
P.E.
K.E.
M.E.
P.E.
K.E.
M.E.
P.E.
K.E.
M.E.
Trial one
Trial two
Compare the mechanical energies for each trial. Was the energy conserved in your experiment? If not, give a possible
explanation for your observations.
Record your mechanical energy results on the data chart on the board.
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