What sign convention?

F and x are often taken positive for extension; use consistent signs for your convention.

Online Hooke's Law Calculator

Calculate spring force F = kx (spring constant × displacement).

Spring constant k

Displacement x

What is k?: Spring constant (stiffness), in force per unit length (e.g. N/m).
What is x?: Displacement from the spring's equilibrium position (extension or compression).
What sign convention?: F and x are often taken positive for extension; use consistent signs for your convention.

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Calculate C(n,r) — the number of ways to choose r items from n without order. Uses n!/(r!(n-r)!).

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Count how many permutations of n objects are even (expressible as an even number of swaps). For n ≥ 2, equals n!/2.