Create panel > Helpers > reactor > Point-Path
Menu bar > reactor > Create Object > Point-Path Constraint
reactor toolbar > Create Point-Path Constraint button
The Point-Path constraint allows you to constrain two bodies so that the child is free to move along a specified path relative to the parent. Alternatively, you can create a single-bodied version of the constraint, where the constrained body can move along a path in world space. The child body's orientation is not restricted by the constraint.
You could, for example, use a Point-Path constraint to simulate a bead on a wire. In most cases, you don't need to specify a parent rigid body for a Point-Path constraint, unless the motion of the path itself should follow another rigid body.
The constraint is simulated if it has the correct number of rigid bodies attached, a specified path shape, and is included in a valid Constraint Solver. When not selected, invalid constraints are red in the viewport.
To specify objects and a path for the constraint:
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Create a line or curve in the viewport using one of the Shapes tools on the Create panel.
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In the constraint’s Properties rollout, click the Child pick button, and then select the object you want to use as the child object in one of the viewports.
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If you do not want to assign a second body to the constraint, turn off the Parent check box if necessary.
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If the constraint is to be two-bodied, repeat step 2, using the Parent pick button to set the parent for the constraint.
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Use the Path pick button to designate the path.
The constraint icon moves to the designated path. By default, this constraint type is aligned so that the parent constraint space is aligned with the path shape's local space, while the child space is aligned with the child body's local space.
To change the path position and orientation for the bodies:
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In the modifier stack open the constraint’s sub-object list.
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Access the sub-object level corresponding to the object whose constraint space you want to move: Child Space or Parent Space. If the Point-Path constraint is single-bodied, Parent Space represents the world position and rotation of the path.
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The space, represented by a set of axes and the path, becomes active in the viewport and can be moved or rotated using the Move or Rotate tool, respectively. The space maintains its position and rotation relative to its corresponding object. For example, if you move or rotate the child body, Child Space moves with it.
Parent Space—At this sub-object level, you can move and rotate the representation of the constraint for the parent body. The axes and path are defined in the parent body's space, which means that if you alter the position or rotation of the parent, the space moves as well and maintains its offset rotation and translation with respect to the parent. When you assign a parent body to the constraint, Parent Space is aligned with the path's local space. If the constraint is single-bodied, then Parent Space represents the world orientation and position of the path.Child Space—At this sub-object level, you can move and rotate the representation of the constraint for the child body. The axes and path are defined in the child body's space, which means that if you alter the position or rotation of the child, the space moves as well and maintains its offset rotation and translation with respect to the child. When you assign a child body to the constraint, the Child Space is aligned with the child body's local space.
Parent—When on, the constraint has two bodies and you can designate an object to be the parent body. When off, the constraint is single-bodied and the point occupied by the Parent Space sub-object is the parent.
You assign the parent object by clicking this button and then selecting a rigid body from any viewport. Thereafter the button displays the name of the parent body.
Child—Displays the name of the second rigid body attached to the constraint. You assign the child object by clicking this button and then selecting a rigid body from any viewport.
Align Spaces To—Use these options to align the bodies' local constraint spaces. You can find out more about each option in Working With Constraint Spaces.
Lock Relative Transform—When on, the relative transform between the child and parent constraint spaces is locked: If you move either space in the viewport, the other space moves and rotates along with it, and vice-versa.
Strength/Tau—Govern the impulses applied to the constraint's bodies in order to maintain the constraint, and so how strongly the constraint works to restrict their movement. You can find out more about these parameters in Strength and Tau.
Breakable—When on, the constraint is breakable. If its breakable limits are exceeded during simulation, it ceases to exert impulses on the attached objects. You can find out more about breakable constraints in Breakable Constraints.