The Physics of a Rolling Rubber Band

sciencehabit writes "Modern physics can get complicated. Sure, researchers know exactly what forces act on a ball rolling down an incline — an experiment that helped Galileo develop universal laws for movement and acceleration. But what happens when a deformable shape like a rubber band rolls around? A new study reveals that the faster it goes, the more squashed it gets (video included)."

Comments

Wow, interesting!

[rotide]

If you would have asked me how it would react as it rolled faster and faster, I would have just assumed it would have gotten "rounder" and possibly larger (elastic) due to centrifugal force.

Always amazes me how things don't always work as expected. Nature, physics, etc, are truly interesting... no, fascinating. Now if only I had a better grasp of higher level maths and wasn't a Network Engineer (data plumber).

Re:Wow, interesting!

[silentcoder]

>>...due to centrifugal force.

>My high school physics teacher used to electrocute (With a handheld generator made from a rotary pencil sharpener) people for saying that; also for misspelling accelerate or satellite.

Yes a common reaction of overzealous highschool teachers - and wrong. Do the math. Forces per se is an oversimplification as it is, useful to model certain aspects of physics, and not for others. Gravitons attract one another, thus is created what we PERCEIVE as the force of gravity if you go down to the quantum level for example.

At the level of Newtonian physics - centrifugal force is as real a force effect as any of the others. True it's a consequence of other forces working together and against each other but it's there nonetheless and it's impact is crucial to predicting how many things will behave.
When two forces work at an angle - the object moves along a (predictable) path created by both forces - it is both correct and useful to model that path as a force in it's own right. At least it OFTEN is, always remember that no scientific model is ideal for EVERY scenario - that's why we don't just HAVE one scientific model.
You want to know how gas will fill a chamber - gravity has so little influence that it's not worth thinking about, quantum mechanics is your friend there. You want to know how it will fill a universe - gravity becomes very important suddenly and this is why the universe does not have it's gases distributed as quantum mechanics would predict, instead of entropy, the universe got decidedly clumpier over time.

For some problems - it's incredibly useful to model centrifugal effects as a force, for others it's much more crucial to work on the shaping forces that create it in the first place - but it's not "wrong" to talk about a centrifugal force if what you want to describe is the effect it causes.

Re:Wow, interesting!

[ledow]

Yes, but then people "weigh" themselves in kg's by standing on a scale that is affected by gravity.

There are certain things that, although "visible" by ordinary people and named, don't actually exist or exist only because we *perceive* them to exist, like that optical illusion with the white triangle that isn't ACTUALLY there.

Centrifugal force may be misnamed (i.e. not a force), it may be incorrect, but it's generally accepted that "a force" exists that has an effect on your when you're spun in a circle. Just because the direction / origin / name of that force is incorrect is no reason to tell people that they're stupid for having felt it and knowing what it is before you explain its origins.

Back in the 60's there was an advertising campaign by scientists working on the behalf of government to target heat loss in elderly people's properties. It encouraged old people to "keep the heat in". It didn't go down well and it took them years to discover why. Eventually it was changed to "keep the cold out" and more elderly people understood that. "Cold" doesn't actually exist, it's just the absence of heat, but old people didn't think that way as easily (and who can blame them? "Shut the door, you're letting the cold in" is a common cry in my family - despite the fact that you're neither letting cold in nor arranging for some mystical "cold" entity to enter your property rather than, say, air with slightly less heat).

There's 100% pedantic accuracy. There's complete bollocks. And somewhere in the middle is how *everybody* thinks, even if they know both extremes in detail.

Re:Wow, interesting!

[electrostatic]

"Spinning faster = more velocity perpendicular to slope on the leading edge of the loop. It makes sense that it would flatten out."

Very good point. The back edge of the loop is being accelerated perpendicularly upward. IOW, the small length of rubber that's breaking contact with the table is yanked -- accelerated -- upward to a high vertical velocity. Therefore, it will rise higher that it does at a slower rolling speed. Like throwing a ball upward with a high velocity against gravity, it reaches a higher distance. In the case of the rubber loop the restoring force is tension in the rubber just ahead of the peak.

But there's that difficult-to-model problem of elasticity. I imagine the fast-rising rubber is pulled downward by tension in the bit of rubber just ahead of the highest point. Most of its upward momentum is opposed by the force of that stretched bit. And the kinetic energy, which is proportional to the square of the velocity, is transferred to mechanical energy (force X distance) stored in the stretched bit. Since KE is non-linear (square of velocity) you often get unexpected behavior. Here we get into differential equations!

Regarding the role of elasticity in the transfer of momentum and kinetic energy in the shaping of the rolling loop, it would be interesting to do an experiment using a non-elastic loop. An example would be a metallic chain. I expect the shape would be different.

Re:Physics...

[SpinningCone]

i agree. I always liked physics made the world look different (like "car breaks are kinetic to thermal energy converters"). never could really get into dynamics though. i remember my teacher describing the the problem of rotational inertia of a deformable object (like a jelly disk) faster you spin the more it changes shape which changes its inertia.

props to the people out there with the knack and persistence to solve crap like that.

Re:Physics...

[Seismologist]

...is mind-boggingly awesome.

Actually you can often make a simple assumption and work off of F = m*a or some other well established theorem...

As for the math, now that is some pretty mind boggling stuff. Some of the math that was used to pull string theory together is pretty bleeding edge on top of the physics part of it. PBS had a interesting show on string theory(you can watch in three installments on PBS). What struck me the most was how splintered the physics community was as many researches were doing the math a certain way different from each other, but it was found to be all the same by another physics/math guru when he proposed 11 dimensions instead of 9 like the other researches had inferred.

Flying out of the drum

[azmodean 1]

I was amused by this aside:

(The team couldn't study what happened when the two sides touched: The friction of the two sides moving in different directions sent the rubber bands flying out of the drum.)

What? It seems pretty obvious that they could see exactly what happened when the two sides touched, "The friction of the two sides moving in different directions sent the rubber bands flying out of the drum".

BZZT! WRONG!

[gbutler69]

You, like many of the others above, FAIL! There is Centrifugal Force. If I spin a rock at the end of a rope, the rock experiences a "Centripetal Force" which is a force pulling towards the center of a circle, where my hand is, that is perpendicular to it instantaneous velocity. Note, it is not only experiencing a "Centripetal Force", otherwise it would just accelerate along the radius toward the center. It is in fact also experiencing a force along the tangent (i.e. the diretion of instantaneous velocity) of the same circle. But, here is where it gets interesting. It is actually experiencing neither of those. It is in fact experiencing a force that is at an angle slightly between the direction of the tangent to the circle and along the radius. We can break that actual force (the real direction I'm actually pulling on the string) into the component along the radius and the component along the tangent, but, there really isn't two separate forces acting, just one. So, what you are calling "Centripetal Force" doesn't actually exist either. It's just a a convenient name for the component of the force pulling inward along the radius. Now, according to Newton's Laws, for every force (action) there is an equal an opposite force (re-action). So, if there is a corresponding force pulling inward on the rock (the "Centripetal Force") then there is also a force pulling outward along the radius (the "Centrifugal Force"). In fact, the Rock is pulling on my hand with such an equal and opposite force. So, my hand (and the string) is experiencing "Centrifugal Force". So, "Centrifugal Force DO EXIST!" You FAIL! Go back and re-read your Physics text-book and try again!

Thank You for Playing!

Re:Physics...

[Thing 1]

The other thing you can do is add texture which separates the flow from the object and usually reduces the size of the low pressure bubble behind it, and this is why golf balls are stippled.

I remember a MythBusters episode where they added a bunch of clay to a car (adding quite a bit of body weight), and then carved out dimples like a golf ball would have.

The car ended up getting better fuel efficiency than the original (lighter, smoother) car. I was really impressed, especially because it had to overcome the added weight.