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October 1991

Grams of Force

Author Affiliations

Cleveland, Ohio

Arch Ophthalmol. 1991;109(10):1346. doi:10.1001/archopht.1991.01080100026016

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To the Editor.  —I compliment Britt et al1 for their excellent photographs and interpretation of the clinical significance of microaerosol formation. In their "Comment" section, I counted five instances in which they used the phrase "g of force." The authors may recall that gram is a unit of mass, not of force.2 Force is measured in dynes (g·cm·s-2) or in newtons (kg·m·s-2), and is the product of mass and acceleration, according to Sir Isaac Newton's second law. For example, a 1-g mass at the surface of the earth is acted on by 1 g times 981 cm·s-2, or 981 dynes, of force, where 981 cm·s-2 is the acceleration due to gravity. To a physicist, the phrase "grams of force" is meaningless. Britt et al may reply that two textbooks on glaucoma3,4 described the scale of Goldmann's applanation tonometer as being calibrated in "grams

Britt JM, Clifton BC, Barnebey HS, Mills RP.  Microaerosol formation in noncontact 'air-puff tonometry . Arch Ophthalmol . 1991;109:225-228.Article
Shortley G, Williams D. Elements of Physics . 5th ed. Englewood Cliffs, NJ: Prentice-Hall International Inc; 1971:9.
Epstein DL. Chandler and Grant's Glaucoma . 3rd ed. Philadelphia, Pa: Lea & Febiger; 1986:17.
Hoskins HD Jr, Kass MA. Becker-Schaffer's Diagnosis and Therapy of the Glaucomas . 6th ed. St Louis, Mo: Mosby—Year Book; 1989:67.