Workout: The conservation of mechanical energy


the web page: The conservation of mechanical energy.


the PhET simulation Energy Skate Park: Basics.

Set up

Three different tracks are available by clicking on the image windows under the option box at the upper right of the screen. (There’s no friction on the track in this tab.)

Turn on all the square checkboxes in the options box and set the radio button next to the start button to "Slow Motion" so you can watch what is happening. Put the skateboarder on the track so the red dot beneath the wheels is on the track and at a height of 4 meters. Your screen should look like the image below. Watch her go through a few cycles and watch how her energy and speed changes.

Answer these questions

  1. Stop the sim and put her so that she is starting at a height of 4 meters. At what height do you expect that her KE and PE will be equal? Explain why you think so. Then use the "step" button (the smaller bar-arrow one to the right of the "play" arrow button) to find the point where the KE and PE are equal. Holding the step button down moves her continually but slowly. Pressing once takes a small step. Is the KE=PE point where you thought it should be? 
  2. Start her at 4 m again and find her speed at the bottom of the track. (You can read it off the speed meter. The first mark represents a speed of 0.) At what height do you expect her speed to be half of that speed? Check it. Is it where you expected it to be? Why is it there? 
  3. Now switch to the middle track and start her at 4 m again. How fast is she going when she gets to the end of the track (off the track)? How does this compare to how fast she was going at the bottom in the previous question? Why?
  4. Now go to the friction tab (in the black bar at the bottom). Turn on all the check boxes in the option box and set the friction to maximum. Turn on slow motion and watch a few cycles and watch the components of the energy change. Describe what you see. They don't show you the friction coefficient. Do you think that there is enough information in the sim for you to find the scale that they use? 

Joe Redish, Fall 2016

Article 764
Last Modified: November 3, 2019