The ghost rad unit in angular motion

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SUMMARY

The discussion focuses on the concept of the radian as a unit of measurement in angular motion, emphasizing its dimensionless nature. Participants clarify that while radians appear in formulas such as angular frequency (ω = √(k/m)), they are fundamentally ratios of lengths and thus dimensionless. The conversation highlights the importance of distinguishing between radians and degrees, noting that radians can be omitted in calculations without loss of meaning, unlike degrees. The confusion surrounding the radian's role in angular velocity and other dimensionless units is also addressed, particularly in relation to practical examples like rotational motion.

PREREQUISITES
  • Understanding of angular frequency and its calculation using the formula ω = √(k/m).
  • Familiarity with the concept of dimensionless units in physics.
  • Knowledge of trigonometric functions and their relationship to angles in radians and degrees.
  • Basic principles of rotational motion and how they relate to linear velocity.
NEXT STEPS
  • Explore the mathematical derivation of angular frequency in the context of harmonic motion.
  • Study the implications of dimensionless units in various physical contexts, such as Hertz and Becquerel.
  • Investigate the differences between radians and degrees in trigonometric applications.
  • Learn about the practical applications of angular velocity in engineering and physics.
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Physics educators, students learning about angular motion, and professionals in engineering fields who require a clear understanding of angular measurements and their applications.

stefano77
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unit of measurement '' rad '' appears and disappears as in calculating angular frequency as square root of k/m in spring-mass model.how can l explain to students?not as a dogma? you can say unit of measuremnt is the inverse of second, but what about the final formula of rad/ s ? rad appears in final result as a ghost
 
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A radian is the ratio of two lengths, so is dimensionless. Like the ratio of two masses, or the ratio of two velocities.
 
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To put it slightly differently: The unit of rad is equal to 1, dimensionless. Angles are dimensionless and therefore must be measured in dimensionless units. Another unit of angle is the degree, which is ##\pi/180## - still dimensionless.

We write out these units for convenience and for showing that we are dealing with an angle.
 
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stefano77 said:
unit of measurement '' rad '' appears and disappears as in calculating angular frequency as square root of k/m in spring-mass model.how can l explain to students?not as a dogma? you can say unit of measuremnt is the inverse of second, but what about the final formula of rad/ s ? rad appears in final result as a ghost
You can omit specifying radians, but you can not omit specifying degrees.

## \sin(2\pi\,rad) ## has the same value as ## \sin(2\pi) ## while ## \sin60\,^\circ ## does not have the same value as ## \sin60 ##

for angular velocity
## 2\pi\,rad/s ## has the same value as ## 2\pi\,1/s ## while ## 60\,^\circ/s ## does not have the same value as ## 60\,1/s ##
 
The radian is a unit, but it's not a dimension. Compare this to something like the meter, which is both a unit and a dimension.

The same issue that arises with the radian also arises with all other dimensionless units. Take for example the revolution. A disc of circumference 2.0 m spins at a rate of 3.0 rotations per second. How fast is a point on its rim moving? You multiply 2.0 m by 3.0 rotations per second and get a result of 6.0 m/s. How did the unit rotation disappear?
 
Mister T said:
A disc of circumference 2.0 m spins at a rate of 3.0 rotations per second.
Confusingly the dimensionless something affects what unit is used for something/second: Hertz vs. Becquerel.
 
Last edited:
A.T. said:
Confusingly the dimensionless something affects what unit is used for something/second: Hertz vs. Becquerel.
I never thought about it before, but perhaps that confusion is another layer that conflates the confusion over the use of the radian.
 

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