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Spring force
Beginner
A spring pulls back in proportion to its stretch.
F = −kxA spring exerts F = −kx, a restoring force proportional to its extension from the natural length, pointing opposite to the displacement. k (N/m) measures stiffness.
k = 200 N/m stretched 5 cm: force 10 N.
x is the stretch from the natural length, not the total length.
In more detail
A spring exerts a restoring force proportional to its extension, F = −kx, where k is the spring constant. The negative sign shows the force opposes the displacement.
Real-world uses
- Car suspensions use springs to absorb bumps.
- Mechanical scales measure weight by the extension of a spring.
- A car's suspension spring compresses over bumps, and the force that pushes back is proportional to how far it is squeezed.
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Frequently asked questions
What is the spring constant?
The force per unit extension, in newtons per metre.
Does the spring force always oppose displacement?
Yes. It always pushes or pulls back towards the natural length.
Why is the minus sign in F = -kx?
It shows the force opposes the displacement, always pulling the spring back toward its natural length.
Related topics in Newton's Laws & Forces
Drag force · Stokes' law · Terminal velocity of a sphere · Springs in series and parallel · Hooke's law limits · Equilibrium of concurrent forces