you can also set the 'disable motion' and 'activate on physcannon grab' properties so that it only comes off when the player tries to grab it. This works good for those handy radiators.
You can also use a phys_constraint, or other constraints like phys_hinge (the physics object is then able to rotate on one axis) and phys_lengthconstraint (allows a physics object to move around a central point with a maximum distance).