# What happens if BB-8 rolls the wrong way?

BB-8 is a spherical droid that rolls around with a stationary head. While an interesting idea, I feel there's a fatal flaw with the design of the character. Given the utility that he as a droid has with lighters, zappers, hologram projectors, and other things of that nature located along his body, he is incredibly useful.

But there's a problem. What happens if BB-8 rolls incorrectly, and the utility item ends up on the bottom? It really does make it hard to use, considering the little droid can't jump and spin.

How does BB-8 always end up with his "tools" facing in the correct direction?

• I feel like this question might as well be "What happens if a mechanic is facing away from the car he's supposed to be working on?". The answer is "He turns around and starts working on the car". Commented Nov 18, 2016 at 21:40
• Given imperfect friction, it's possible that he goes one direction, then reverses to skid enough that he can compensate by adjust the length of the tool. It is also possible that he does have a jump option (even a small one by shifting some internal weight) and we just don't see it. Commented Nov 18, 2016 at 21:45
• @Anoplexian BB-8 isn't a battle droid though.... Commented Nov 18, 2016 at 21:49
• is there any accounting for the possibility that perhaps the internal tools themselves can rotate to be accessible via the most applicable port? Also - it's a droid - it can probably account for rotation / speed, etc...in it's approach to whatever it needs to interact with Commented Nov 18, 2016 at 22:00
• After skimming some Youtube cliips of BB-8 I noticed that the orange circles on his main sphere can actually rotate independently of the main sphere, meaning he can easily reorient any tool coming out of that circle. Assuming he can also rotate his main sphere in place, that means as long as the tool is somewhere around his equator when he comes to a stop, no ridiculous acrobatics are required. There's also the possibility that he has more than one of each tool; we know he has several tethers(?) for instance. Commented Nov 18, 2016 at 22:01

When BB-8's attachment don't line up correctly, he simply rolls around until they're facing the correct way. You can also see from the clip below that several of his circular panels seem to rotate, allowing him an even greater range of movement and reach with his manipulator and welding arms.

In defiance of physics, he can also pivot around a central point. This strongly suggests he's either got internal flywheels or some kind of inbuilt repulsor technology. This means that if an attachment is on the wrong side, he can simply rotate his body until it's facing the right way.

• That's literally the exact clip I was basing my earlier comment on. Too bad I was too lazy to turn it into an actual gif and post an answer. +1 to you. Commented Nov 18, 2016 at 22:56
• Changing orientation without a flywheel does not violate physics for a non rigid body. Ending up facing a different direction doesn't violate conservation of angular momentum. See this answer physics.stackexchange.com/q/88322/47565. Note he turns his head first to achieve this. Commented Nov 19, 2016 at 2:07
• This happens: 8-BB. Commented Nov 19, 2016 at 2:16
• Keep in mind the BB-8 used for filming was a real-life radio controlled prop. No physics violations were used. Commented Nov 19, 2016 at 18:52
• @whatsisname - They actually had a considerable number of BB-8 props; In this case what you're looking at is "the wiggler" which is described as being able to "twist and turn on the spot" but lacks the ability to actually roll anywhere. Commented Nov 19, 2016 at 18:58

The first part of this answer is clearly true, but the second part is in fact incorrect. As @JohnMeacham says, rotating on the spot is not in defiance of physics. Note that when he twists his head at the start, his body jerks a little in the opposite direction. When he stops turning his head suddenly, his body is 'pulled' along. You can do it on yourself sitting on a rotating chair; with practice you can rotate a fair amount with a single jerk (asymmetric of course; slow start and fast stop).

I also want to point out that even in the complete absence of friction between a non-rigid entity and its surroundings, it is possible for it to move its parts in such a way that they all end up in the same relative orientation as before but such that the whole entity has a different orientation relative to the surroundings. This is exactly how a cat can right itself in midair. Since we do not know what is inside BB-8, it might well be that he has internal weights that he can use for this purpose. This might explain the last part of the clip (reproduced below) where the head slows down and then suddenly both head and body accelerate around.

Finally, it is a mathematically provable fact that a sphere can be rolled without slipping on a plane to achieve any orientation at the initial location. A simpler version of this fact is exploited in many games such as Tatham's Cube and Clarke's Bloxorz.

• There are other examples where he moves, then his head turns. Commented Nov 19, 2016 at 10:41
• @Valorum: The second paragraph of my answer does provide an explanation even in that case, and in fact I have already said that the clip is better explained by that rather than the asymmetric jerk technique. If you are the downvoter, point out which sentence in my answer is wrong and I will amend it. Commented Nov 19, 2016 at 15:32
• I'm not the downvoter although I'd suggest that this answer would be dramatically improved if it dwelled less on my answer and concentrated on the second part of the answer which points out that it's trivially easy to rotate a moving body to the right position. Commented Nov 19, 2016 at 16:27
• @Valorum: I had tried to keep the part about your answer to the minimum, and I'll remove it if you delete "in defiance of physics" in your answer. =) Commented Nov 20, 2016 at 6:56
• +1 for the link to Simon Tatham's puzzles. They're a great collection and deserve to be much better known than they are. Commented Nov 20, 2016 at 18:45

As a roboticist, I just wanted to quietly throw this example into the mix:

just to give a simple example of how robotics and physics are intersecting in this sort of fashion.

When I first saw the movie I thought that his various tool 'openings' were most likely just positioned in an evenly-spaced fashion, with the actual tool mounts on a free-rotating frame inside, like a gyro, however Valorum's gif clearly shows he has a dedicated opening for each tool, and that his locomotion system and tool kinematics have to work in tandem.

From a design perspective that feels like quite a compromise! However given the reduced number of external parts and the inherent strength of a well-built spherical frame it's quite a smart way to tackle robustness IMO.

• My pleasure (seriously, I really liked that video) Commented Nov 20, 2016 at 18:00
• That's really awesome, although I'm super tripped out by it. +1 Commented Nov 21, 2016 at 15:16