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Physics 251 – Fall 2021 Make-up Lab: Conservation of Mechanical Energy

Make-Up Lab: Conservation of Mechanical Energy

Purpose of the Lab: To better understand the conservation of mechanical energy.

Reference: The video links listed below under Background.

Materials and Equipment: The simulation link embedded in this write-up and a cell phone for taking pictures.

Collaboration with Classmates: Feel free to discuss and compare your results with other members of the class, but the report you submit must be your own work. You are also free to email questions to [email protected]. Be sure to show your work when appropriate, do not simply list an answer. You will be graded on your method of calculation in addition to your result.

The Write-up: Use regular paper for your lab write-up. You do not need to rewrite the steps, or questions. However, you should maintain the numeral designations for the steps, activities, and the questions answered.

Background:

View the following short video first: https://www.youtube.com/watch?v=9Fw2jY1sHhU

View the following short video next: https://www.youtube.com/watch?v=sYvDK8FZ8f8

In this lab the only form of potential energy we will consider is gravitational potential energy (GPE). We will ignore spring potential energy and other forms of potential energy. Thus, we define mechanical energy as:

ME = KE + GPE = ½ mv2 + mgh

Where m = mass in kg, v is velocity or speed in m/s, g = 9.81 m/s2 and h is the height above the zero point of GPE, usually taken as ground level.

Throughout this lab, we will use the following simulation “Energy Skate Park: Basics”

https://phet.colorado.edu/sims/html/energy-skate-park-basics/latest/energy-skate-park-basics_en.html

Investigation 1 will use the “Intro” simulation, Investigation 2 will use the “Friction” simulation. Explore the “Playground” simulation on your own if you like, outside of the lab period.

Investigation 1: Conservation of Mechanical Energy without friction

Go to the following link and select “Intro”

https://phet.colorado.edu/sims/html/energy-skate-park-basics/latest/energy-skate-park-basics_en.html

You can play with the setup to get used to the various features and when you are ready, configure the skate park as shown with the following initial settings:

· Select Bar Graph, Grid and Speed

· Set mass to Large, and select Slow Motion

· Set the large blue button to pause

· The small blue button is used when you stop the skater’s motion and want to move the skater a small amount further along the path

· You can drag and drop the skater with your mouse to any location on the grid

· The Orange Reset button should be avoided since it will reset the mass without you being aware of it

Activity 1-1: Conservation of ME in Free Fall

Step 1: Push the pause button and place the skater anywhere on the grid at a height of 8 meters. Now use your cell phone to take Picture #1 of the Energy Bar graph for future reference.

Step 2: While observing the Energy Bar graph closely, release the skater. Once the skater is on the ground, use your cell phone to take Picture #2 of the Energy Bar graph for future reference.

Question 1-1: Describe how the Kinetic Energy changed during free fall.

Question 1-2: Describe how the Potential Energy changed during free fall.

Question 1-3: Describe how the Total Mechanical Energy changed during free fall.

Question 1-4: Describe how the Thermal Energy changed during free fall.

Question 1-5: How are the Thermal Energy and the Total Mechanical Energy related?

Activity 1-2: Conservation of ME while skating on a frictionless ramp

Step 1: Place the skater on the skating ramp at a height of 6 m with the Pause button on. NOTE: The red dot centered under the skateboard is the point of reference for positioning the skater.

Step 2: Release the pause button and observe the skater go through one or more cycles on the ramp. A full cycle will be defined as the time it takes to return to the point where the skater was first released.

Question 1-6: Describe the skater’s motion through one cycle.

Question 1-7: At what location or locations in a cycle does the skaters speed drop to zero?

Question 1-8: At what location or locations in a cycle is the skater’s speed a maximum?

Step 3: Once again place the skater on the skating ramp at a height of 6 m with the Pause button on. Take Picture #3 of the Energy Bar graph for future reference. Release the Pause button and watch the Energy Bar graph while the skater goes through one or two cycles on the ramp.

Question 1-9: Describe how the Kinetic Energy changed during one cycle

Question 1-10: Describe how the Potential Energy changed during one cycle.

Question 1-11: Describe how the Total Mechanical Energy changed during one cycle.

Question 1-12: Describe how the Thermal Energy changed during one cycle.

Step 4: Once again place the skater on the skating ramp at a height of 6 m with the Pause button on. Release the Pause button and allow the skater to get close to the bottom of ramp but hit the Pause before the skater gets there. Use the small blue button to slowly step the skater to the point where the red dot under the skateboard is as close to ground level as possible. Stop the skater there and take Picture #4 of the Energy Bar graph for future reference.

Question 1-13: Compare and contrast the Energy Bar graph in Picture #1 and Picture #3. How and why are they similar and/or how and why are they dissimilar?

Question 1-14: Compare and contrast the Energy Bar graph in Picture #2 and Picture #4. How and why are they similar and/or how and why are they dissimilar?

Question 1-15: For the free fall situation, when was the mechanical energy conserved and when was it not conserved?

Question 1-16: For the frictionless ramp situation, when was the mechanical energy conserved and when was it not conserved?

Investigation 2: Conservation of Mechanical Energy with friction

Switch over to the “Friction” simulation and configure the setup as follows:

· Select Bar Graph, Grid and Speed

· Set mass to Large, and select Slow Motion

· Leave the friction at the default position, in the middle of the slider panel, half way between “None” and “Lots”

· The Orange Reset button should be avoided.

Step 1: Place the skater on the skating ramp at a height of 6 m with the Pause button on.

Step 2: Release the pause button and observe the skater move until the skater comes to rest

Question 2-1: Describe the skater’s motion from his release at 6 m to the point where the skater comes to rest.

Step 3: Once again place the skater on the skating ramp at a height of 6 m with the Pause button on. Take Picture #5 for future reference. Release the Pause button and watch the Energy Bar graph while the skater moves and until the skater comes to rest. Take Picture #6 of the Energy Bar graph after the skater has come to rest.

Question 2-2: Describe how the Energy Bar graph changes while the skater is moving and until the skater comes to rest.

Question 2-3: Compare and contrast Picture 5 and Picture 6. How and why are they similar and/or how and why are they dissimilar?

Question 2-4: For the case of the ramp with friction, when was the mechanical energy conserved and when was it not conserved?

Question 2-5: Compare and contrast Picture #4 (frictionless and at the bottom of the ramp) with Picture #6 (friction and at the bottom of the ramp). How and why are they similar and/or how and why are they dissimilar?

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