Vernier Tech Info Library TIL #1938

Question

Should I use a photogate or a Motion Detector?

Answer

Motion Detectors and photogates can both measure motion, but both have advantages for certain experiments.

Photogates are sometimes capable of higher accuracy and function more like a spark timer in that the only information collected is the time at which a specific event occurred. Because of the sparse amount of data, students need to have a deeper understanding of the math behind an event in order to analyze the data. The age and ability of the student determines whether this is a pedagogical advantage or a disadvantage. The apparent precision of photogate measurements can be misleading, as parallax error can swamp all timing errors. For more information on parallax error, see:
TIL 1467: What is the timing precision of photogate measurements in LabPro or LabQuest?

Motion Detectors present a more comprehensive record of the experiment since they can produce real-time graphs of an event. For example, a student can walk back and forth in front of a motion detector and see instantly that moving away from the sensor produces an increase in a position vs. time graph. This added feedback is especially powerful for visually-oriented learners, and those with fewer math skills. However, motion detectors can give unexpected results due to sound reflecting off of something other than the intended target.

The area of detection for a photogate is much smaller than that of a photogate, which can help to focus to data on only the event of interest. As a result, there are some events that can only be measured with photogates such as smart pulley (rotational data)and projectile motion.

Our starter package for physics includes only the motion detector, while our deluxe package includes both a motion detector and two photogates. For a list of the types of experiments and innovative uses that can be performed weth each sensor, see below:

Motion Detector uses:
http://www.vernier.com/publishers/probecorrelation.html?sensor=MD-BTD

Photogate uses:
http://www.vernier.com/publishers/probecorrelation.html?sensor=VPG-BTD

Created by: jhopkins on January 12 2009
Last updated by: jhopkins on January 14 2009