Allow one object to pass through another whilst keeping collisions.

I want to create an envirionment where one object is able to fall through another, however when it does fall through it I still need to activate a function, so I need to test for collisions still (unless there is another way of doing this) whilst still letting it pass through the object in question. If possible I would like to do this in javascript.

Thanks, Tom.

I think what you need is a trigger.
A trigger is ignoring the physics engine but still can detect collisions.

To activate the trigger function you need a Collider on your GameObject and check the trigger box.
Then you can ask for collisions using the OnTriggerEnter function.