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Constraint forces
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Forces that enforce geometric restrictions such as strings and surfaces.
Constraint forces (normal, tension) arise to enforce a geometric restriction and adjust their size to whatever is needed. They have no fixed formula and are found from the equations of motion.
A bead on a wire exerts whatever force is needed to keep it on the wire.
Constraint forces do no work if the constraint doesn't move.
In more detail
A constraint force keeps an object moving along a given path or surface. Constraint forces do no work when the constraint is ideal, such as a smooth surface or a rigid rod.
Real-world uses
- Rails constrain trains to their tracks, providing sideways forces on the wheels.
- Bearings constrain rotating shafts to their axis.
- A pendulum bob held by a rigid rod, or a bead sliding on a wire, is constrained, and the constraint force does no work.
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Frequently asked questions
Does a constraint force do work?
For an ideal constraint, no, because it acts perpendicular to the motion.
Is tension a constraint force?
It can be, when a string keeps objects at a fixed distance.
Do constraint forces ever do work?
In an ideal constraint like a smooth wire or a fixed rod, they do no net work, because they are perpendicular to the motion.
Related topics in Newton's Laws & Forces
Center of mass · Motion of center of mass · Normal force on curves · Newton's laws and inertial frames · Dimensions and limits of Newtonian mechanics