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Inquiry: Measuring Time

Purpose: Measuring time intervals and calculating average velocity with a recording spark timer.


A recording spark timer is a device for measuring the time it takes an object to move a short distance. A paper tape is pulled through a spark timer and a train of small burn dots are left on the paper. When the paper tape is pulled through the timer, the distance between two dots is the distance the tape moved during two successive sparks. The time between two sparks is called the period of the timer. There are two settings on the spark timer: 10 Hz and 60 Hz. We will use the 60 Hz setting, which means there will be 60 dots/second. What is the period of the timer? You then will use the timer to measure the average speed of falling objects.

  • Measure the height of fall from the bottom of the wood support.

  • Cut off a piece of tape about 25 cm longer than this height to make sure the tape will still be running through the timer when the mass hits the floor.

  • Insert the tape up through the slot in the timer, pull it almost all the way through and tape on one of the three weights (100.0, 200.0 or 500.0 grams).

  • Hold the other end of the tape so that the bottom of the weight is even with the bottom of the wooden support.

  • Start the timer and drop the mass. Stop the timer when the mass hits.

  • On the tape, measure the height from the starting dot and draw a stop line.

  • Count the intervals and multiply it by the period. This is the time of fall.

  • Divide the distance (m) by the time (s). This will give you the average speed of the mass, which you should record in your notebook.

  • Answers should still have one significant digit.

  • When your group is finished, each student should cut of a 5 interval section and insert into your notebook. Calculate the average speed for your section.

Inquiry Questions:

  1. What is the formula for average speed?

  2. What was the period of the timer?

  3. What was the order of magnitude of the period of the timer?

  4. How did the average speeds of the masses compare?

  5. Explain in a few sentences why heavier things don't necessarily fall faster than lighter things.

  6. If a spark timer operated at a frequency of 75 Hz, what would be the period of the timer?

  7. Calculate the average velocity of your section of tape.