Measuring the Acceleration of Free Fall with a Light Gate and Datalogger

A datalogger is a device which collects and processes raw data from sensors such as light gates. This method involves dropping a double interrupt card through a single light gate connected to the logger, as you can see. People sometimes ask about the advantages of light gates. All they are is a start/stop timer – a reduction in light intensity turns them on, a restoration of normal intensity turns them off again and software does all the rest to calculate velocities, accelerations and so on depending on the configuration. They have a very rapid response time so they start or stop in a matter of microseconds. Also, repeat determinations can be done quickly and efficiently.

we needn’t worry about dropping it from the same height each time, since a=constant

The logger is programmed with the length of each interrupt card ( I used 40mm), weighted a little bit to minimise air resistance and dropped vertically between the light gates.

The internal clock starts when the gate is interrupted and stops when it is not. It then calculates the average speed of each part of the double card as it falls. One speed is greater than the other because the card is accelerating. The internal clock in the logger records the time between the interrupt cards’ fall. Acceleration = change in speed / time.

The logger software can be used to display all speeds and times, or the calculated acceleration directly, which is simpler. Results were as shown.

These eight determinations can be output to a spreadsheet and their average calculated

As you see, all readings (should all be 9.8) are in error by approximately the same amount. This indicates a systematic error in the readings, perhaps because the weight of the card is unevenly distributed or the card lengths are not quite precise enough.

This is the calculation:


7 thoughts on “Measuring the Acceleration of Free Fall with a Light Gate and Datalogger

  1. can u give me the description about the card thing
    and can u expln y do we use the card to get the acceleration speed
    im confused


  2. Karan, sorry I should have replied earlier
    The datalogger is a clock. If we tell it the length of card it starts the clock when the beam is interrupted and stops it when the beam is restored. It can then calculate the average speed of the card. This is the first speed. It does this again as the second piece of card interrupts the beam, this is the last speed. It knows how long between the measurements on average – the time taken. It then uses (last speed – first speed) / time to find the acceleration.


  3. I know this is really old. But can light gates be used in pairs to start and stop a timer (electronic) . For eg when a bead goes through the first light gat it stops the timer and when it goes through the second light gate it stops the timer ?


  4. Sure they can – a pair acts as two start/stop gates to measure a pair of average velocities for example.


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