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A 2 m bar stretches 1 mm. What are the units of its strain?

None: strain is a ratio of lengths, so it is dimensionless.

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  1. Millimeters
  2. Pascals
  3. None; it is dimensionless ✔

Why: Strain is the change in length divided by the original length: 0.001 m / 2 m = 0.0005. Meters cancel, leaving a pure number.

Huh, didn't know that: Engineers often write small strains as microstrain; 0.0005 is 500 microstrain.

The dad joke
I asked the strain gauge how it felt. It said, no units of complaint.
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References

Answer: None; it is dimensionless. Strain is the change in length divided by the original length: 0.001 m / 2 m = 0.0005. Meters cancel, leaving a pure number.

  1. OpenStax University Physics Volume 1: 12.3 Stress, Strain, and Elastic Modulus openstax.org/books/university-physics-volume-1/pages/12-3-stress-strain-and-elastic-modulus
    Therefore, strain is a dimensionless number.
    Checked 2026-10-10.

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