Joints as constraints¶
A joint doesn't add motion — it removes some of a body's 6 free degrees of freedom and leaves the rest. What's left over is the motion the joint allows:
| Joint | DOF removed | DOF left | The motion it allows |
|---|---|---|---|
| Revolute (hinge) | 5 | 1 rotation | spins about one axis only — a door hinge |
| Prismatic (slider) | 5 | 1 translation | slides along one line only — a drawer runner |
| Spherical (ball) | 3 | 3 rotations | rotates freely about a fixed point — a hip joint |
A revolute joint pins a body to a fixed point and allows only one rotation about the hinge axis — that single remaining DOF, driven by gravity, is a pendulum. Two details matter in practice:
- The hinge axis has to be perpendicular to gravity to swing. A vertical hinge axis produces zero moment from a downward force, so the "pendulum" just hangs there.
- No damping means no decay. With nothing to dissipate energy, the swing amplitude stays constant — energy is conserved.
BriskFyr's other joint types (Cylindrical, Fixed/Weld, Planar, Axial Rotation…) are combinations of the same idea — each removes a specific, named set of freedoms and leaves the rest. The full catalogue, with input fields and viewport behaviour for each, is in the component reference.
Build your first joint in Tutorial 2 — a pendulum Compare Revolute / Prismatic / Spherical side by side in Tutorial 3