Conservation of Energy at the Skate Park
Part A: Thermal Energy (Internal Energy)
Learning Objective:
Describe how a change in thermal energy of the system affects the motion of molecules at the
microscopic level, and the motion of the skater at the macroscopic level.
PhET Friction Simulation:http://phet.colorado.edu/sims/html/friction/latest/friction_en.html
PhET Energy Skate Park Simulation:
http://phet.colorado.edu/sims/html/energy-skate-park-basics/latest/energy-skate-park-basics_en.html
Directions:
1. In the microscopic model boxes, draw and describe the motion of the skater’s molecules at her
two different positions. Hint: Think about molecules in the friction simulation.
2. In the macroscopic model boxes, draw and describe the energy of the skater at the two
positions.
Microscopic Model (Left Image)
Microscopic Model (Right Image)
●
●
The molecules (skateboard wheels and
track) are mostly organized and still
(vibrating in place).
The molecules (skateboard wheels and
track) are not organized and are moving with
no discernable pattern. The molecules are
also moving faster.
Macroscopic Model (Left Image)
Macroscopic Model (Right Image)
●
●
The gravitational potential energy is close to
being maxed out. The kinetic energy is
almost gone. The total energy is constant.
The kinetic energy is almost full while there
is very little amount of thermal energy. The
total energy is constant.
Part B: Energy Changes in the Skate Park System
Learning Objectives:
1. Differentiate between total energy and various forms of energy in a system.
2. Explain how each model (bar graph and pie chart) shows the total energy of the system, and
draw each model for a situation with a different amounts of initial energy.
3. Describe energy changes in a system over time using both words and graphical
representations.
Directions: Use the simulation investigate energy in the skate park. Use different tracks on the
"Introduction" and "Friction" pages.
1. Describe the system represented by the bar graph and pie chart. Explain your
reasoning.
System description
Objects in system
Without Friction:
The objects in the
system include
gravitational
Reasoning
●
The total energy in the system remained constant during the
skater’s run. The total energy was conserved and only changed
forms from one object to another. The total energy was
represented by a bar along with bars for specific forms of energy.
potential energy
and kinetic energy.
There is no thermal
energy due to
friction.
With Friction: The
objects in the
system include
gravitational
potential energy,
kinetic energy and
thermal energy.
When there is no friction, the total energy is the sum of
gravitational potential energy and kinetic energy. On a pie chart,
the total energy is represented by a full pie and is equal to the
sum of the gravitational potential energy and kinetic energy when
friction is not involved.
●
When friction is involved, the total energy in the system remains
constant. Energy may change in different forms but is conserved
as shown in the bar graph and pie chart of the total energy of the
system. Friction causes energy to change forms as molecules
gain thermal energy. As molecules gain speed after gaining
thermal energy, the skater's gravitational potential energy or
kinetic energy is converted to thermal energy as shown in the bar
graph and pie charts.
2. Investigate the meaning of “total energy”.
“Total energy” observations from bar graph
At different positions
With or Without Friction: The total
energy always remain constant during the
skater’s run as observed from the bar
graph even at different positions on the
track.
In relationship to other energy forms
The sum of the other forms of energy
(gravitational potential energy, kinetic
energy, and thermal energy) is always
equal to the total energy which is constant
and conserved. The total energy is
represented by a bar along with bars for the
three different forms of energy. The y-axis
on the bar graph does not have a specific
quantity (e.g., percentage or numbers). As
the skater is at different positions along the
track, the bar height of the different forms
of energy changes but the bar height of the
total energy remains the same.
“Total energy” observations from pie chart
At different positions
In relationship to other energy forms
With or Without Friction: The total
energy always remains constant during the
skater’s run as observed from the pie chart
even at different positions on the
track.
On a pie chart, the total energy is
represented by the full pie. Like the bar
chart, the pie chart does not have a specific
quantity (e.g., percentage, fractions, or
numbers). As the skater is at different
positions, the different forms of energy are
represented as different “slices” of the pie.
The size of the pie slices represent the
energy and changes depending on the
position of the skater on the track. The sum
of all the slices however is still equal to the
total energy and remains conserved and
constant.
3. Find a way to increase the “total energy”. Draw diagrams of the bar graph and pie
chart before and after you increase the “total energy”.
Energy diagrams
Before increasing the total energy
After increasing the total energy
Bar graph
Pie chart
What did you do to increase the total energy?
●
To increase the total energy, the mass of the skater was increased and by lifting and dropping
the skater from a greater height.
4. Define “total energy” of the system based on your observations.
●
Based on my observations, the total energy of the system represents the sum of the different
forms of energy (gravitational potential energy, kinetic energy, thermal energy). The total
energy of the system in the skate park scenario may be increased by increasing the mass of
the skater, and by lifting and dropping the skater from a greater height.
5. Use “friction” page. Investigate the different forms of energy, and their relative
amounts, when the skater is at different positions.
Position of
skater
Forms and relative amounts of energy at this position
Top of hill
●
At the top of the hill, the total energy consists of gravitational
potential energy as shown in the bar graph and pie chart (~100%).
Middle of hill
●
At the middle of the hill, the total energy consists of gravitational
potential energy (~50%), kinetic energy (~47%), and thermal
energy (~3%) as depicted in the bar graph and pie chart.
Bottom of hill
●
At the bottom of the hill, the total energy consists of kinetic energy
(~92%) and thermal energy (~8%) as depicted in the bar graph and
pie chart.
6. Use trends in the table above to write a “rule” about the changes in energy,
including total energy, of the system as the skater moves along the track. Use your own
words.
●
As the skater moves along the track, the total energy remains constant. Further, the total
energy is conserved meaning the type of energy changed form from one to the other. For
instance, at the top of the hill, the total energy is entirely gravitational potential energy but at
the bottom of the track, the total energy is the sum of kinetic energy and thermal energy. The
gravitational potential energy has been converted to kinetic and thermal energies.
7. Use your experience with the skater system to complete the table below:
Skater’s
Position
Describe skater’s
speed
Describe energy
forms
A
At position A, the skater
is at the top of the track
and about to begin the
run, so the speed is
initially zero.
The total energy of the
system consists
primarily of gravitational
potential energy
(~100%).
B
At position B, the skater
has gained speed.
The total energy of the
system consists of
gravitational potential
energy (~80%), kinetic
energy (~18%), and
thermal energy (~2%).
Graphical
representation of
energy (bar graph/pie
chart/line graph)
C
At position C, the
skater’s speed has
decreased slightly
compared to position B.
The total energy of the
system consists of
gravitational potential
energy (~60%), kinetic
energy (~12%), and
thermal energy (~28%).
D
At position D, the skater
has increased its speed,
which is greater than at
position B.
The total energy of the
system consists of
gravitational potential
energy (~10%), kinetic
energy (~50%), and
thermal energy (~40%).