Reality Composer

You can compute it:

• if the sphere is full, the weight is Volume * specific weight, where Volume = 4/3 * pi * RRR
• if sphere is empty with d thickness (d smal compared to R): weight is Surface * d * specific weight, where Surface = 4* pi * R*R

You can find specific weight in many places on Internet.

Assuming you set up a physics body for your entity in Reality Composer, you can actually directly get the mass of the physics body:

        // Load the scene from your .rcproject.
        let sceneAnchor = try! MyRealityComposerProject.loadScene()
        
        // Get a reference to the entity that you configured a physics body for.
        let box = sceneAnchor.box
        
        // Get the physics body from the entity's component set.
        guard let physicsBody = box?.components[PhysicsBodyComponent.self] as? PhysicsBodyComponent else {
            fatalError("box has no PhysicsBodyComponent.")
        }
        
        // Access the mass properties of the physics body.
        print(physicsBody.massProperties.mass)