Hey @Cestarian!
I think typically this is based on the “Mass” attribute.
In your example, you’re looking at a door with 22,680g (22.68kg) versus a 10.3g key (according to Google). So the key would have to have either infinite force acting upon it to move the door, or around 2,200 times the amount of force it would take to move the key by itself. That being said, digitally simulating physics in any program is difficult to predict at best.
If it’s two actors, you can simply attach the key at a socket on the static/skeletal mesh of the door and disable physics on the key until it’s removed.
Hope that helps!