Inertia for Christmas

I made this doorstop for Mrs B for Christmas. Hidden inside is a large pebble surrounded by bubble wrap. The pebble has a large mass and by Newton's 2nd Law F=ma, large mass means small acceleration ie large masses don't move very easily. The m in F=ma is called INERTIA. The doorstop needs a lot of inertia so that the door doesn't move. The other use of m is as GRAVITATIONAL MASS in Newton's Law of Gravitation. Both versions of mass have the same measurement in kg. As far as I am aware, people are still trying to figure out why. But in this case we can say that the attraction of the Earth on the gravitational mass gives the doorstop WEIGHT. The normal reaction force (N) is equal in size to the weight. Frictional force = coefficient of friction x N. I think there will be a large coefficient of friction between the carpet and the felt thus making the doorstop even harder to move.