Physics Java Labs

Before you start your lab report, it might be useful to look at a sample report.

Lab: Determination of g

  • Purpose of experiment:
    Recording the trajectory of free-falling objects enables us to extract their equations of motion and, in particular, determine experimentally the value of the gravitational acceleration, g, near the surface of the Earth.
    It is claimed that Isaac Newton was sitting under an apple tree and was hit on the head by a falling apple, which triggered the insight needed to come up with Newton's universal law of gravitation. So we also drop an apple for this experiment. But just for fun we used an Apple computer. It makes a better smack when it hits the ground.
  • Video clip of experiment:
    Larger size, (1.2 MB, with sound) or smaller size (0.4 MB, no sound) versions are available.
  • Instructions for performing the experiment:
    1. Run the java applet by clicking this button:

      (It will open in a separate window).
    2. Digitize the motion of the falling object by clicking on it. After each click, the mouse position is recorded and the movie is advanced one frame. You should always try to click on the same point of the computer while it falls. If you make a mistake, click on the "Undo Pt." button in the applet, and the last point will be erased.
      Please Note: If you are using a phone line to perform this experiment, then please keep in mind that it can take a couple of seconds between two successive frames of the video you are digitizing. Please do not get impatient. You can see when the next frame is ready for you from observing the text area: as soon as the result of your previous click is displayed, the next video frame is ready for processing.
    3. You can display the points that you have already digitized before by clicking on the "Plot y(x)" button. It will turn red and show the points overlaid on the video. Clicking the same button again will turn this feature off.
    4. You can also plot the trajectory as a function of time directly within the applet by clicking on the "Plot y(t)" button. Clicking on the "Digitize" button will bring you back to the data input mode.
    5. The numerical data you have collected are displayed in the text area on the right side of the applet. After you are done digitizing all frames, please copy your data into your clipboard and export them into your favorite spreadsheet or graphing program. There, you can fit a parabola
      y(t) = a + b·t + c·t2

      to your data. The coefficient c will be g/2, where g is the gravitational acceleration near the surface of the Earth. (Why?)

    6. Now produce a plot with your graphing program or by screen-capture of the applet.
    7. Finally, write the report with your favorite word processor or text editor and email it (as email attachment) back to the instructor.

© W. Bauer, 1999