Work and Energy
Energy conservation for an isolated system is a fundamental principle of
physics. Energy for an isolated system is always conserved. It may change
forms, but the total amount of energy in an isolated system is constant.
Energy can, however, be converted from one form to another form. Work is the conversion of one form of energy into another.
Energy comes in different forms, kinetic energy, potential energy, chemical
energy, thermal energy, etc. If an object has energy, it has the potential
to do work.
There are several forms of potential energy. Kinetic and potential energy
are called mechanical energy or
Thermal energy is disordered energy. Friction converts mechanical
energy into disordered energy.
When no disordered energy is produced, then
mechanical energy is conserved.
Today we will track the mechanical energy in various
systems and explore the relationship between work and energy.
- Track with bumper and plunger cart
- Force sensor
- Motion sensor
- Ruler and metal block
- 50 g mass
How does the motion sensor work?
transducer in the face of the Motion Sensor transmits a
burst of 16 ultrasonic pulses with a frequency of about
49 kHz. The ultrasonic pulses reflect off a target
and return to the face of the sensor. The target
indicator flashes when the transducer detects an echo.
The sensor measures the time between the trigger
rising edge and the echo rising edge. It uses this time
and the speed of sound to calculate the distance to the
object. To determine velocity, it used consecutive
position measurements to calculate the rate of change of
position. Similarly, determines acceleration by
using consecutive velocity measurements.
Note: The motion sensor must face the target.
Open a Microsoft Word document to keep a log of your
experimental procedures and your results. This log will form the basis of
your studio session report. Address the points highlighted in blue.
Answer all questions. Include the information that your answers are based
In this experiment you will do work compressing a spring. You will then
let the spring do work converting elastic potential energy into gravitational
- Set up a track with a motion sensor attached to one end and a bumper to
the other end. Set the range switch on top of the motion sensor to
short range ().
Note: You will use the motion sensor for experiment 2 only.
- Place the feet of the track at ~10 cm and ~120 cm and make sure the
leveling screws are turned all the way in.
- Plug the Pasco motion sensor into digital channels 1 and 2 of the Pasco 850 interface
and the USB/Bluetooth force sensor into a USB port of the computer. Open the Capstone program.
- In the Hardware Setup window add a motion sensor and check that the force
sensor appears in the window. Select only the force measurement and
deselect all other measurements. If you want to connect via Bluetooth,
unplug the sensor from the USB port. If more than one sensor appears in
the hardware setup window, select the one that matches the device ID XXX-XXXX number found on the
front of the sensor. Click the properties gear for the force sensor check
"Zero sensor measurement at start." Close the Hardware setup window.
- Choose a sample rate of 40 Hz for both instruments.
- Place a plunger cart on the track and level the track. The cart
should remain stationary and the track should rest on all four feet.
- The cart has a three-position spring plunger, activated
by a trigger located on the front end cap. Use the force sensor to
measure the force as you compress the plunger spring all the way to the
third position. Make sure that, while you compress the spring, you hold
the force sensor horizontally. Monitor the force on a graph.
Repeat a few times to get a value for the maximum force
Fmax required to compress the
spring. Record Fmax.
- Use a ruler to measure the distance d the
plunger moved. Record it.
- The work you do in compressing the spring is W = Faverage
*d. What is Faverage? Why? Calculate the
work you do to compress the plunger spring to the third position and record it.
- Take the cart off the track, put the plunger into position 3 and put the
cart vertically on the table. Place a 50 g mass on top of the plunger.
- Release the trigger and measure with a ruler the maximum height above
its starting position to which the mass jumps. Calculate and record the maximum change in
potential energy of the mass. Repeat a few times to get a reliable
measurement of the maximum height.
- Compare the work done to compress the plunger spring to the maximum change in potential
energy of the mass.
- Was some energy "lost" in the process? If so, where did it go? Elaborate!
In this experiment you will lift one end of the track. You will then
measure the conversion of gravitational potential energy into kinetic energy.
- Lift the end of the track with the motion sensor and place the feet on
the metal block.
- Place the cart near the motion sensor and let it accelerate towards the bumper. You will
measure the cart's speed when it is between 0.2 m and 0.7 m from the motion sensor.
- In Capstone, drag two graphs, one for position versus time and one for
velocity versus time onto the main display.
- Click "Recording Conditions" below the main display.
Choose Start Condition, Measurement Based, Position, is above 0.2 m.
- Choose Stop Condition, Measurement Based, Position, is above 0.7 m.
- Start taking data. Let the cart accelerate. Determine its
speed when it is at 0.2 m and when it is at 0.7 m from the motion sensor.
- Measure the difference in the height of the track at positions 0.5 m apart.
(Attach your calculations)
- Compare the change in the gravitational potential energy of the cart to
the change in its kinetic energy when its distance from the motion sensor
changes from 0.2 m to 0.7 m. Attach your calculations. Discuss your number.
- Can you jump keeping your legs completely straight? Does the amount of bending of your legs have any
relation to how far up you can jump? Try this out and describe your results.
- From a physics point of view, why does bending your legs help you jump?
Is energy stored in your legs when bent? How do you know?
Use an on-line simulation from the University of Colorado PhET
group to track mechanical energy in a skate park.
Link to the simulation:
- You can build tracks, ramps and jumps and view graphs of
kinetic energy, potential energy and friction as the skater moves.
- You can also take the skater to different planets or into free space.
(a) Explore the interface!
- You can resize the windows.
- You can Pause the simulation and then put the Skater anywhere.
Return Skater returns the Skater to this spot
and you can rerun the scenario.
- You can use the Save feature in the File
menu to save a track and Skater position.
- The Energy Position Graph has a few subtle features. It erases as the
simulation plays, but you can Pause the simulation and the graph will
- The Copy button will let you freeze the graph to compare
different scenarios, but it cannot be saved as a file. If you Zoom,
the graph clears; you can make a new graph by rerunning your scenario.
- If you use the Show Path feature, you can click on the purple dots and show
quantitative information. Height refers to height from the Potential Energy
Reference line. Click again to hide.
- Step is a good way to incrementally analyze the motion. Use
the step button in the large window and in the Energy Time window.
- When the Skater lands on the track, some of the kinetic energy will be
dissipated and the skater will subsequently move along the track.
(b) As a group, design your own
frictionless track. You session instructor will give you some design
guidelines that you should follow.
- Design a track that is fun, challenging and relatively safe.
- Use the Energy Graphs to track the Skater's mechanical energy. Decide which graphs or charts best help you understand what makes your track
- Explain why your track is successful in terms of conservation of mechanical
energy. Refer to Charts or Graphs to help explain your reasoning.
- Using conservation of mechanical energy, explain what things need
to be considered when designing any successful track.
(c) Add friction to your track.
- Explain what changes in the simulation when you add friction. How
does the energy distribution change?
(d) Optional: Move the skater to a
different planet or to free outer space.
- Explain what changes in the simulation when move the skater.
Convert your log into a session report, certify with you signature that
you have actively participated, and hand it to your instructor.