It shouldnt matter when you animate the object but once the simulation kicks in your objects will all be keyed by reactor. unless you set an object to unyielding. make sure you have a reactor rigid body collection in the scene and make sure your objects have mass. Select the objects go to properties under reactor in the utilities panel or use the properties dialogue on the reactor toolbar. That is also where you can make an object unyielding ie: not be affected by gravity or other objects but have an effect in the simulation. if you key an object moving then start simulating from say frame 5 the object will inherit its initial velocity, like throwing a ball for instance...

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