Unit 6: Time Measurements Throughout history, we have marked time by the motion of objects in the sky that indicate the passage of hours, days, months, and years. More accurate measures of time gradually emerged in response to the demands of navigation, commerce, communications, and curiosity. Today, atomic clocks operate with a time resolution of one part in 1015.
In this unit, you will learn about standards of time measurement, the limits of human reaction times, and the practical limits of precise time measurement via readily available, computer-based sensors.
Unit 6 Time Advisory
This unit should take you 5 hours to complete.
☐ Subunit 6.1: 2 hours
☐ Subunit 6.2: 2 hours
☐ End of Unit Self-Assessment: 1 hour
Unit6 Learning Outcomes
Upon successful completion of this unit, the student will be able to:
- Demonstrate an understanding of the limitations of human- and machine-based time measurements.
Demonstrate knowledge of the time scales of common physical and chemical processes.
Reading: Simon-Fraser University: Stephen Lower’s Chem1 General Chemistry Virtual Textbook: “Understanding the Units of Scientific Measurement” Link: Simon-Fraser University: Stephen Lower’s Chem1 General Chemistry Virtual Textbook: “Understanding the Units of Scientific Measurement” (PDF)
Instructions: Review this chapter, focusing on the short section on time. To view as a PDF file, scroll to the bottom right corner and click on the “download” link.
6.1 Human Reaction Times Note: When performing any physical measurement, you must know the limits of the measurement system you are using. Here, we will briefly explore the limits of human reaction times.
6.1.1 Dropping Meter Stick Exercise Instructions: Estimate the time required for a person to let go of a meter stick and grasp it again by measuring the distance that the meter stick falls under the acceleration of gravity. You will need to use Newton’s laws and some simple calculations to determine that d= at2/2 where d = the distance dropped, t= the reaction time, and a= the acceleration of gravity. Repeat the measurement several times for several different individuals. Calculate statistics (e.g. means, standard deviations).
6.1.2 Flashing Light Exercise Instructions: Estimate human reaction time by performing a computer- based reaction time test (i.e. press a mouse button when the light flashes). Compute the mean reaction time for an individual and the standard deviation of reaction times for that individual. Compare that reaction time with the one determined from the meter-stick experiment.
- Web Media: Human Benchmark’s Reaction Time Test
Link: Human Benchmark’s Reaction Time
Test (HTML and
Instructions: Perform the reaction time test at least five times. Calculate the mean and standard deviation. How does this time differ from the one calculated in 5.1.1.
6.2 Clock History and Mechanisms
- Reading: NIST’s History of Time and Frequency: “History of
Link: NIST’s History of Time and Frequency: “History of Timekeeping
Instructions: Read the linked section above and the links contained therein. Use the Internet Time Service link to explore how the time is set on your computer.
Unit 6 Assessment - Assessment: The Saylor Foundation's "ME301: Unit 6 Quiz" Link: The Saylor Foundation's "ME301: Unit 6 Quiz"
Instructions: Please complete the linked assessment. You must be logged into your Saylor Foundation School account in order to access this exam. If you do not yet have an account, you will be able to create one, free of charge, after clicking the link.