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Whats making it stay in place/attached?
Setting up a physics based crane is no simple task, but one easy way to sort of do it is... you can disable the rigidbody, parent the box to the crane and when you want to drop it just unparent it and turn on the rigidbody, gravity will take over.
All you have to do to make it fall with physics is add a Rigidbody Component and turn on gravity in the component. You shouldn't need Add Force down unless there is some reason you need to disable gravity. Which is fine, gravity is equivalent to roughly (9.81 * mass) if you wanted to do it explicitly in an action.
The hard part with doing it with physics is setting up the physics joints properly. Faking this with parenting and disabling rigidbody parts is easier than using the joints I think.