Unit 7: Force, Torque, and Pressure Measurements Force, torque, and pressure measurements can be related by temporal and geometric coupling. Consider the schematic of a see-saw balance. The relative masses of objects M1 and M2 can be determined by the torques they exert about point P at different distances (L1 and L2) from that point under the acceleration of gravity g. Many more sophisticated geometries and sensing arrangements can be coupled to allow measurements of related quantities. In this unit, you will review some of the common configurations for such measurements.
Unit 7 Time Advisory
This unit should take you 8 hours to complete.
☐ Subunit 7.1: 2 hours
☐ Subunit 7.2: 2 hours
☐ Subunit 7.3: 1 hour
☐ Assignment: 2 hours
☐ End of Unit Self-Assessment: 1 hour
Unit7 Learning Outcomes
Upon successful completion of this unit, the student will be able to:
- Demonstrate proficiency with use and conversion of units of pressure, force, and torque.
- Demonstrate understanding of absolute, gauge, static and dynamic pressures.
- Research, understand, and communicate the capabilities and operation of modern torque measurements.
7.1 Force Measurements
7.1.1 Units and Standards
- Reading: National Physics Laboratory (UK): “SI Unit of Force”
Link: National Physics Laboratory (UK): “SI Unit of
Force”
(HTML)
Instructions: Read the linked section above and familiarize
yourself with commonly used units of force. For example, how is a
dyne related to an ounce of force?
Terms of Use: Please respect the copyright and terms of use
displayed on the webpage above.
7.1.2 Inference of Mass from Weight
- Reading: Wikibooks: The Free High School Science Texts: A Textbook
for High School Students Studying Physics: “Newtonian
Gravitation/Mass and Weight”
Link: Wikibooks: The Free High School Science Texts: A Textbook for
High School Students Studying Physics: “Newtonian Gravitation/Mass
and
Weight”
(PDF)
Instructions: Read the linked section above. Calculate your mass
and weight in kg and lb_{m} and Newtons and lb_{f}
on Earth.
Terms of Use: The article above is released under a Creative
Commons Attribution-Share-Alike License
3.0 (HTML). You
can find the original Wikipedia version of this
article here (HTML).
7.1.3 Strain or Deflection Measurements
- Reading: All About Circuits: “Volume 1, Chapter 9: Strain
Gauges”
Link: All About Circuits: “Volume 1, Chapter 9: Strain
Gauges”
(PDF)
Instructions: Read this section and consider the following issues:
Why does the resistance of the strain gauge depicted in the resource
cartoon increase under tension? How might strain measurements be
confounded by changes in temperature? Might you design a strain
gauge to work by measuring changes in capacitance? To view as a
PDF, click the PDF link in the top right corner.
Terms of Use: This material has been released under the terms of
the Design Science
License.
7.2 Pressure Measurements
7.2.1 Units and Standards
- Reading: Simon-Fraser University: Stephen Lower’s Chem1 General
Chemistry Virtual Textbook: “Observable Properties of Gases”
Link: Simon-Fraser University: Stephen Lower’s Chem1 General
Chemistry Virtual Textbook: “Observable Properties of
Gases”
(PDF)
Instructions: Review this first chapter. Calculate atmospheric
pressure in units of atm, bar, mm Hg, and ft. of water. What might
be meant by the term “negative pressure?”
Terms of Use: This work is licensed under a Creative Commons
Attribution-NonCommercial 2.5 Generic
License. It is
attributed to Stephen Lower and can be found in its original
form here.
7.2.2 Static Versus Dynamic Pressure Note: These concepts arise from Bernoulli’s equation. They are not to be confused with Gauge and Absolute pressures. Gauge pressure is the system pressure minus some reference (atmospheric pressure).
- Reading: NASA Glenn Research Center’s “Bernoulli’s Equation” and
“Pitot-Static Tube”
Link: NASA Glenn Research Center’s “Bernoulli’s
Equation”
(PDF) and “Pitot-Static
Tube”
(PDF)
Instructions: Read the two web pages linked above and consider the following issues: Do static and dynamic pressures have the same units? What is the origin of the terminology? How would you use measurements of both to determine the speed of an airplane?
Terms of Use: This material is in the public domain.
7.2.3 Barometers and Manometers
- Reading: Georgia State University Hyperphysics Pages: “Fluid
Pressure Measurement”
Link: Georgia State University Hyperphysics Pages: “Fluid Pressure
Measurement”
(HTML)
Instructions: Read the linked page above and those following as
interested. Consider the following questions during your reading:
What is the difference between a barometer and a manometer? Why is
mercury often used as the fluid in a manometer? Why might one use
another fluid? You may wish to play with the applet to consider the
effects of fluid properties on the observed measurements.
Terms of Use: Please respect the copyright and terms of use
displayed on the webpage above.
7.2.4 Pressure Transducers
- Reading: National Instruments’ Guide for Pressure Measurements:
“Measuring Pressure with Pressure Sensors”
Link: National Instruments’ Guide for Pressure Measurements:
“Measuring Pressure with Pressure
Sensors” (HTML)
Instructions: Read the first three sections in the linked material
above, entitled “What is Pressure?”, “The Pressure Sensor,” and
“Pressure Measurement.” What factors influence the time-response of
a pressure transducer?
Terms of Use: Please respect the copyright and terms of use
displayed on the webpage above.
7.3 Torque Measurements
- Web Media: Khan Academy’s “Introduction to Torque”
Link: Khan Academy’s “Introduction to
Torque”
(YouTube)
Also available in:
iTunes
U
Instructions: This video should be a review of concepts you learned
in physics coursework. You may refer to previous or subsequent
videos in the series if you need additional exposure. Make sure you
understand appropriate units for torque.
Terms of Use: This video is licensed under a Creative Commons
Attribution-NonCommercial-NoDerivs 3.0 Unported
License. It is
attributed to the Khan Academy.
Activity: The Saylor Foundation’s Dynamometers Activity Link: The Saylor Foundation’s Dynamometers Activity
Instructions: Perform the same exercise on YouTube for dynamometers as you did for length measurement devices in Section 5.4 of this course.
Unit 7 Assessment
- Assessment: The Saylor Foundation’s “ME301: Unit 7 Quiz”
Link: The Saylor Foundation’s “ME301: Unit 7
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.