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 ordered energy.  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.

Equipment needed:

• Track
• Smart Cart
• Ruler and metal blocks
• 50 g mass

Open a Microsoft Word document to keep a live journal of your experimental procedures and your results.  Include all deliverables, (data, graphs, analysis, outcome).  Write a 'mini-reflection' immediately after finishing each investigation, experiment or activity, while the logic is fresh in your mind. on.

Experiment 1

A spring‑loaded foam toy gun is a little physics machine hiding in plain sight.  Before performing this experiment, ask an AI how a spring‑loaded foam toy gun works.

In this experiment you will do work compressing a spring.  You will then let the spring do work converting elastic potential energy into gravitational potential energy.

• Set up a track with a bumper to the other end.  Place the feet of the track at ~10 cm and ~110 cm and make sure the leveling screws are turned all the way in.
  Note:  You will use the track for experiment 2 only.
• Press the power button on the side of the Smart Cart to turn it on.
• In Capstone, click Hardware Setup.  Under available devices, you should see your Smart Cart.  If you see more than one cart, refer to the label your cart to add the correct one.  Choose to measure Force and Position.
• Create a Graph Display by dragging the Graph icon onto the main display.  Select Force, (N) for the y-axis and Time (s) for the x-axis.
• Choose a common sample rate of 50 Hz.
• Place the Smart Cart on its wheels on a level surface and zero the force sensor.  Then set the cart upright on the table on its magnetic bumper.  The cart has a three-position spring plunger, activated by a trigger located on the front-end cap.  Release the plunger.
• For a short time record the force sensor reading with the cart just sitting there.  The force sensor reads the weight w of the cart.  Record the weight w.  (Take the mean value.)
  Note:  Because we are pushing on the force sensor, we read a negative value for the force.  Ignore the negative sign and take the absolute value or magnitude.
• Practice exerting a force on the plunger and very slowly pushing it to the third position without it catching.  Then start recording the force as you slowly push the plunger to the third position in without it catching.
  As you compress the spring, the force increases linearly.  Monitor the force on a graph.  Repeat a few times to get a value for the maximum force F_max required to compress the spring to the third position.  Record F_max.
  Note:  You have to subtract the weight w of the cart from the maximum value you read of the graph to determine the maximum force F_max required to compress the spring.
• Use a ruler to measure the distance d the plunger moved from being released to the third position.  Record it.
• The work you do in compressing the spring is W = F_average *d.  What is F_average?  Why? 
• Calculate the work you do to compress the plunger spring all the way in and record it.

• Now 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!

Experiment 1 Deliverables: (to be included in the your journal)

• Data:  Your measurements of F_max, d, F_average, and the height to which the mass jumps.  Your calculations of the he work you do to compress the plunger spring and the the maximum change in the potential energy of the mass.

• Analysis:  Compare the work done to compress the plunger spring to the maximum change in potential energy of the mass.  Determine what percentage of energy is 'missing' and identify the culprit.

• Outcome:  The percentage of elastic potential energy stored in ste spring converted into maximum potential energy of the mass.

Discussion

• 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?

Experiment 2

In this experiment, you will lift one end of the track.  You will then measure the conversion of gravitational potential energy into kinetic energy.

Before performing the experiment, ask the AI: 'I am dropping a 500 g cart down a 30-degree ramp.  What are three real-world factors that will prevent my final Kinetic Energy from equaling my initial Potential Energy?  Rank them by how much energy I will likely lose to each.'

Perform the experiment

• Lift the end of the track with without bumper and place the feet on two stacked metal blocks.
  
  
  
• Place the cart near the elevated end and let it accelerate towards the bumper.
• Delete all your previous data runs.
• Change the axes of your graph to measure velocity (y-axis) versus position (x-axis).
  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 0.8 m.
• Start taking data.  Let the cart accelerate.  Use the coordinate tool to determine the cart's speed when it has traveled 0.2 m and 0.7 m.   Record those speeds.
• Measure the difference in the height of the track at positions 0.5 m apart.  Record your measurements.
• 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.  Record your calculations.

Experiment 2 Deliverables: (to be included in the your journal)

• Data:  Your measurements of height and speed and your calculation of the change in the gravitational potential energy and kinetic energy of the cart.

• Analysis:  Discuss your numbers in light of the response you got from the AI.

Exploration

Use an on-line simulation from the University of Colorado PhET group to track mechanical energy in a skate park.
Link to the simulation: https://phet.colorado.edu/en/simulations/energy-skate-park-basics

• You can build tracks, ramps and jumps and view graphs of kinetic energy, potential energy and friction as the skater moves.

(a)  Click the Playground image.  Explore the interface!
Note:

• You can Pause the simulation and then put the Skater anywhere.  Restart Skater returns the Skater to this spot and you can rerun the scenario.
• You can fix the skater to the track or let him loose contact with the track.

(b)  Design your own frictionless track.  You can ask for some design guidelines in the discussion forum.

• 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 successful.
• Ask an AI: 'I am designing a PhET Skate Park track with a loop.  What is the minimum height the skater must start from to complete the loop without falling off, and why?'   Use the simulation to test the AI's claim.  Was the AI was right?  Why?

(c)  Add friction to your track.

• Explain what changes in the simulation when you add friction.  Paste you explanation of friction into an AI with this prompt: 'I am a physics student. Here is my explanation of how friction affects energy in a skate park: [Insert Explanation].  Play the role of a skeptical physics professor and point out any gaps in my logic or ask me two challenging follow-up questions.  Include the A's "critique" and your rebuttal in your journal.

Exploration Deliverables: (to be included in the your journal)

• Analysis:  What makes a successful track in terms of conservation of mechanical energy?  Refer to your conversation with the AI and Charts or Graphs to explain your reasoning.

• Outcome:   A picture of a successful track with a loop that you have designed.

Convert your log into a lab report.

Name:
E-mail address:

Laboratory 4 Report

• Make sure you completed the entire lab and answered all parts.  Make sure you show your work and inserted and properly labeled relevant tables and plots in your journal.
• Add a summary reflection at the end of your report in a short essay format.

Save your Word document (your name_lab4.docx), go to Canvas, Assignments, Lab 4, and submit your document.