AP Physics 1‎ > ‎Unit 1‎ > ‎Force Details‎ > ‎

PVC Freefall

PVC Freefall


In this activity you will be learning how to use a photogate to determine how the speed of an object in freefall is related to the distance it has fallen.


  1. A photogate is a device that creates a beam of infrared light that is picked up on the other side of the photogate.  A computer will track when the beam is broken and when it intact.
  2. You will be finding the average speed of the cylinder as it passes through the photogate.  This is found by dividing the length of the cylinder by the time the photogate beam is broken.
  3. More accurate results will ensue if you add half of the cylinder's height to the length of PVC through which the cylinder has fallen.  This will assume the average speed occurs when you are halfway through the photogate instead of as you are entering the photogate.  So to get the height (or distance fallen) you add the PVC distance to half of the cylinder length.

  4. The computer will tell you the time that gate is first blocked and then the time at which the beam is unblocked.  The difference between these two times gives the time the object was passing through the photogate.
  5. Lab setup

Part 1:  Mastering the Procedure (This part you are to do individually)

  1. For the first part of this lab you will be doing this procedure in a virtual environment to make sure you understand what you are doing and to demonstrate your ability to get reasonable data.

  2. Open the program found here and set your location to the moon, Mars or Vesta.  You can use a cylinder made of whichever material you choose.

  3. The lines on the PVC pipe are 5 cm apart and the cylinder you are dropping through the pipe are 5 cm tall.

  4. Set up a data table that gives columns for nail position, distance fallen, blocked time, unblocked time, pass time, and speed.  It would be helpful to do this on a secondary device in a google spreadsheet.

  5. Collect data for 8 different nail positions and then graph the speed of the cylinder vs. the height from which it fell.  If you are graphing on a computer without Logger Pro, you can use the program found here

  6. Transfer your graph and equation into your lab book and get your teacher's approval to start working on the live portion of this lab.

Part 2:  Live portion of Lab (Work in groups of 3 or 4)

  1. Get a photogate, metal cylinder, and other materials and set up the equipment as shown above.

  2. Measure and record the length of the metal cylinder in your lab book.

  3. Make sure your LabQuest is hooked to your computer's USB port and place the plug from the photo gate into the "DIG 1" port of the LabQuest.

  4. Open LoggerPro, single click on the column in the data table for "Distance" and then hit the big delete key below the "F11" and "F12" keys on your keyboard.  Finally, remove the three graphs so all that you have left in Logger Pro is a data set with "Time" and "State".
  5. In your lab book and if possible on a spreadsheet on your Chromebook or phone, set up a data table to record the information you will need for this lab.  You will not be able to do this in the Logger Pro that is being used to collect data until you are done collecting all your data and you remove the photogate from "Dig 1.

  6. Measure the distance the cylinder will fall by measuring the distance from the hole where you are placing the nail and the bottom of the PVC pipe.

  7. Place your photogate just below the bottom of the PVC pipe with the eye of the photogate centered so the cylinder will pass through the eye immediately after leaving the PVC pipe.

  8. Hit play the play button on Logger Pro to begin collecting data.  A few seconds after you hit play you should quickly remove the nail from the PVC pipe and allow the cylinder to fall through the photogate.

  9. Record the time the cylinder first entered the photogate and the time when the cylinder exited the photogate.  Subtract these two times to get the time it took to pass through the photogates (∆t).  This should be a positive number.

  10. You will find the speed of the cylinder as it emerges from the PVC pipe by diving the length of the metal cylinder by time the cylinder is in the gate (∆t).

  11. Repeat this procedure using a total of 10 different starting heights

  12. Make a graph of speed (y-axis) vs. distance (x-axis) fallen and then develop an equation that will relate the speed of an object and the distance it has fallen.

Part 3:  Changing Mass

  1. Repeat the procedure with a different type of metal.

  2. Put your graph for the new metal on the same axes as your original metal.

  3. Answer the questions found here.

Stuff to have in your lab book

  • Purpose of the lab.

  • Labeled picture of your lab set up. 

  • Data table and one sample calculation of ∆t, speed and height for each part of the lab.

  • A graph with all the things a good graph should contain for each part of the lab.

  • The equation that relates the variables being studied in this lab.

  • A few sentences of conclusion talking about the results of the lab, sources of error in the lab and how get rid of the errors.