Three rollers and a ruler

25.12.2008 11:04

I've seen a lot of discussion recently (e.g. BoingBoing, xkcd) on the Internet about the possibility of a vehicle moving directly downwind faster than the wind. I don't want to go into that debate, but what did caught my attention was the following cute video that demonstrates the behavior of a cart constructed out of three rollers under a moving ruler:

Under the ruler faster than the ruler

(Click to watch Under the ruler faster than the ruler video)

Theatrical skills of the author aside, what I found interesting is that very few people had any doubts about the validity of the experiment and the explanation given in this video. When I watched it for the first time, I was pretty sure the little plush monkey got it right. And after some back-of-the-envelope calculations I still thought he was tricked. The cart shouldn't be able to move at all.

So I did an experiment of my own and it just confirmed my thoughts. The experiment is easily duplicated and I encourage you to try it yourself.

However, just before posting my notes I realized there is one minor, but important difference between the geometry of the cart I analyzed and the geometry used in the video. I'm now confident that the video is genuine and I'll be posting the corrected calculations after I get back from Berlin.

Meanwhile, below is my original analytical solution followed by a video recording of my version of the same experiment (minus the furry spectators). Mind that it's still correct, it just provides an answer to a slightly different question (see if you can spot the difference).

Let's start with a simpler case of a single roller on a flat surface:

Single roller

Here vc is the velocity of the center of the roller relative to the ground, while vr1 and vr2 are the velocities relative to the center at different locations along the roller's surface. Remember, the magnitude of vr is constant around the circumference while it's direction changes around the circumference and is always tangential to the surface.

Obviously, the velocity of the roller's surface at the point where it touches the ground must be 0 relative to the ground or the roller would be slipping. So for that point the following equation is true (subtracting magnitudes, since vectors are parallel and pointed in opposite directions):

v_r - v_c = 0
v_r = v_c

Now that we know vr, we can calculate the velocity of the top point of the roller (adding magnitudes, since vectors are again parallel, but pointed in the same direction):

v_t = v_r + v_c = 2 \cdot v_c

Single roller with a ruler

So, in this case the top point is moving in the same direction as the center and at twice its speed relative to the ground. And this is of course also the speed of any non-slipping ruler that rests on the top of the roller.

This result is expected: if you're moving a large rock by rolling it on tree trunks, you put trunks in front of the rock and pick them up behind it.

Ok, so let's now go on to the cart. The situation is very similar to the previous example. The centers of all rollers are moving with the same velocity, vc. At the points where the top roller touches the bottom two rollers surfaces must have identical velocities as there is no slipping. From here it follows that the magnitude of vr is equal for all rollers.

Cart with a ruler

Again, we can write equations for the points where the cart touches to top and the bottom surfaces:

v_r + v_c = v_t
v_r + v_c = 0

And therefore:

v_t = 0

So the ruler at the top can not move relative to the ground as long as it is not slipping. The crucial difference here was that the second roller rotates in the opposite direction to the top one. This changed the sign in the second equation, since vectors vc and vr were now pointed in the same direction at the bottom.

As you can see, the radii of the rollers don't even come into play in this calculation. So the final result is identical with arbitrary roller dimensions.

The conclusion therefore is that the ruler is either stationary in respect to the ground or two surfaces are slipping somewhere. It's impossible to move the cart by applying a horizontal force only to the ruler since the bottom rollers will apply exactly the same torque to the top roller, but in the opposite direction.

I've made a series of simple experiments that confirm the theory above. You can see them on video below:

Experiments with a three-roller cart

(Click to watch Experiments with a three-roller cart video)

You can see that moving the ruler in the 4th experiment didn't move the cart - it only caused the ruler to slip along the top wheel.

The only way to move the cart is to apply the force to it directly as in the 3rd experiment, or as the last experiment in the video shows, by resting the ruler on the cart at an angle, so that the force of the ruler is no longer parallel to the force of the ground. The force and torque diagram in that case is left as an exercise for the reader.

Again, you don't have to believe everything I said, but do try it yourself if you have any doubts. Experiments are fun and this one really just takes some cardboard and a couple of minutes (or seconds if you have Legos handy).

Posted by Tomaž | Categories: Ideas


The missing element in Tomaz's analysis the nature of the bottom two rollers. In the orginal video, two thread spools which vary in diameter are used. They have a smaller interior diameter (on which the upper roller rests) and a larger exterior radius which is in contact with the ground. This difference in inner and outer diameter of these two rollers enabled the machine to move exactly as shown in the orginal video. On the other hand, if the two rollers were simple cylinders of uniform diameter, the machine would not move without slippage - as your video shows.

Posted by Frank

Frank's comment is correct. In order for the machine to work, the point of the top wheel that touches the ruler must be moving forward, but *slower* than the cart as a whole. This is accomplished in my video by using cotton reels, where the diameter at their centre parts is less than that at the ends: the top wheel touches the cotton reels at their centre parts, whereas the end parts touch the tabletop. In my case, the ratio of end diameter to centre diameter is about 2 to 1. It's worth pointing out that the size of the top wheel is unimportant: the cart would move at the same speed if the top wheel were twice as big, or indeed any size you choose.

If you don't have old-fashioned cotton reels handy, you should be able to duplicate the effect by putting different-sized Lego wheels on the same axle.

Posted by coolaun

Yes, that's exactly the one detail I missed. Next time I should give more attention to the machine and less to the surroundings.

I'll post an updated experiment soon.

Thanks for stopping by Coolaun and congratulations on the nicely done video.

Posted by Tomaž

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