Add a hinge joint and apply a one time force and if the door exceeds a certain angle stop applying the force.
Unity hinge joint door example.
But this joint can be used to do the same.
This joint simulate pendulum.
A gameobject s functionality is defined by the components attached to it.
You do not need to assign a connected body to.
The hinge joint 2d component allows a gameobject the fundamental object in unity scenes which can represent characters props scenery cameras waypoints and more.
More info see in glossary the hinge will rotate at the point specified by the anchor property moving around the specified axis property.
A single hinge joint should be applied to a gameobject the fundamental object in unity scenes which can represent characters props scenery cameras waypoints and more.
More info see in glossary controlled by rigidbody a component that allows a gameobject to be affected by simulated gravity and other forces.
Multiple hinge joints can also be strung together to create a chain.
The main door should be assigned as the connected body so the doggy door s hinge is dependent on the main door s rigidbody.
This joint restricts the movement of a rigid body to follow the movement of the rigid body it is attached to.
How to open doors on a hinge like a real door.
A gameobject s functionality is defined by the components attached to it.
In unity you can relate two bodies to transform by parenting one to the other.
Playing with the same concept more or less this week experimented some with 2d unity joints i think the issue you are having is identical to one i had yesterday the force you are applying breaks joints for some frames hence that almost spring joint effect make sure the mass and or force applied are not too heavy also maybe increase the chain parts mass makes the joints more solid too.
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Unity 4 6 hinge joint broken in asset bundles.