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SESK: Stuff Everyone Should Know · sesk.info/eng-statics-sum-of-forces-zero

For an object at rest, Newton's second law F = ma becomes what?

The sum of forces equals zero, because the acceleration is zero.

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  1. The sum of forces equals zero ✔
  2. Force equals mass
  3. Force equals mass times g

Why: Statics studies things that do not accelerate. With a = 0, the net force must be zero, and the net moment must be zero too.

Huh, didn't know that: Zero net force can mean constant velocity, not only rest.

The dad joke
Statics is the only class where doing nothing is the right answer.
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References

Answer: The sum of forces equals zero. Statics studies things that do not accelerate. With a = 0, the net force must be zero, and the net moment must be zero too.

  1. Engineering Statics (Baker and Haynes): 1.1 Newton's Laws of Motion engineeringstatics.org/newtons-laws.html
    Newton's Second Law for translation simplifies to
    Checked 2026-10-11.
  2. OpenStax University Physics Volume 1: 12.1 Conditions for Static Equilibrium openstax.org/books/university-physics-volume-1/pages/12-1-conditions-for-static-equilibrium
    The second equilibrium condition for the static equilibrium of a rigid body expresses rotational equilibrium
    Checked 2026-10-11.

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