When I was kid, I had Star Wars toys—but they weren’t “smart” and most of them weren’t even battery powered. But kids these days have access to awesome toys like this remote controlled BB-8 (by Sphero) from the Star Wars VII movie. It looks pretty cool. Gizmodo has lots of details if you are looking for a review.

But how does it work?

### How do you roll a ball from the inside?

First, let’s look at the bottom of BB-8. It’s a sphere that can roll. So, how do you make a sphere roll without pushing it? This one isn’t so difficult. All you need is a moveable mass inside the sphere. Maybe this mass is a tiny car with wheels (on the inside of the sphere) or maybe it’s a hamster. Either way, when the mass moves up the wall of the sphere a little bit, the center of mass for the whole sphere shifts.

When the center of mass shifts away from being vertically over the contact point, there is an external torque on the sphere. This torque then increases the angular momentum of the sphere and causes it to roll. Once the sphere starts rolling, the inside mass could just stay at the lowest point—except there is some external friction which will require the inside mass to continue to ride up the side and provide some torque.

But wait? Can you really make the ball roll from the inside? Wouldn’t that be like lifting yourself up by pulling on your belt? It might sort of seem that way, but it’s actually different. If you wanted to lift yourself up from your belt, there would be no external force on you except for the gravitational force pulling you down. When the inside mass moves up the wall, there is an external force to accelerate the sphere horizontally—friction. Here is a more detailed image showing all the forces on the sphere when the mass moves up the wall.

But why is there a frictional force? Well, there are two possibilities. First, if there is no friction (smooth surfaces) then this torque from the gravitational force will cause the sphere to rotate—and rotate it will, but in place. If there is friction, then this frictional force will push the sphere forward and make it roll. I assume the BB-8 toy has enough friction to get it moving.

All I know for sure is that the head uses a magnet to stay on. My guess is that there is some type of magnet inside the sphere to attract a magnet inside of the head. The head then has rollers so that it can roll along the top of the sphere. Of course the inside magnet would have to be movable so that you could make BB-8 do fun head moves.

But is that the only way to make it work? No, of course not. Check out these cool instructions for building a different kind of BB-8. This other version has the motor only in the head and sits on top of a plain ball. By moving the head, you can also exert a torque on the ball to make it move. It’s an interesting idea (even if the head for this version is huge).