# The Physics of the Star Wars BB-8 Droid: How Does It Work?

Introducing the BB-8!

Fans of Star Wars are rejoicing with the release of the seventh episode in the series, The Force Awakens.  With it comes a new robot, the BB-8, that has immediately captivated viewers at first sight.  It is an astromech droid in the Star Wars Universe. An awesome new toy version is available, made by Sphero. It’s a remote controlled robot that rolls without the use of external wheels.  Regardless of how the robot functions in the Star Wars universe, it’s probably not too different from the way the toy version works. Rumors from production claim Sphero’s technology helped with the development of the actual BB-8 and there was also some puppetry involved in the film version of the droid.

How do you make a sphere roll without pushing it?

An article in Wired recently explained that you need a moveable mass inside the sphere. The mass could be any number of things, like a tiny car with wheels, or a hamster.  We’ve probably all seen how a hamster rolls one of those plastic see-through spheres from running on the inside of it.  Whatever the mass is, when it moves up the wall of the sphere a little bit, the center of mass for the whole sphere shifts.

And now for some science stuff:

When the center of mass shifts away from being vertically over the contact point, there is an external torque on the sphere.  The contact point is where the movable mass lies within the sphere. Torque is the tendency of a force to rotate an object about an axis. This torque then increases the angular momentum of the sphere and causes it to roll.  In physics, angular momentum is the rotational analog of linear momentum, meaning the momentum of the spinning sphere will keep spinning until a force acts upon it.  This is similar to an object moving in a linear line – it will keep moving in a straight line until a force acts upon it. The angular momentum of a system remains constant unless acted on by an external torque.

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.  The external friction is the floor surface on which the sphere is rolling on.  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.

Magnets keep it together:

As far as the head of the robot goes, it uses a magnet to stay on. There is probably 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. The inside magnet would have to be movable so that you could make BB-8 do fun head moves.

This is truly an adorable, awesome addition to the Star Wars Universe. I for one would love to see a buddy movie spin off featuring BB-8 and R2-D2.

Media

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##### Lesson Plan

Lesson Objective: After this lesson, students will have gained a better understanding of the mechanics of toys like the BB-8 remote controlled robot. They will get a chance to explore how a sphere-like object can rotate without being pushed.