MYTHBUSTERS 10/5: "Newton's Crane Cradle"

[Christopher]

Wrecking-ball Newton's cradle: Well, the viral video was blatantly computer-animated, but it was still an interesting concept to test. Newton's cradles are very cool. It was an interesting illustration of how physics doesn't always scale up or down; the larger the rigs got, the more opportunity there was for the forces to be absorbed by the distortion/movement of the balls or the cables or the frame.

I knew right away that the single-cable suspension in the video wouldn't work, that the V-shaped suspension is needed to cancel out lateral movements that would ruin the effect. Though in the full-scale Mythbusters rig, it turned out the problem was more with rotation than side-to-side shifting of the wrecking balls. So I was basically right but I still learned something new. Best of both worlds.

Another thing I learned is that wrecking balls are steel shells filled with concrete. I always figured they were solid metal.


Bird tipping a car off a cliff: Another straightforward physics principle that illustrated how many complicating factors there can be in a large-scale application. In principle, the car was precariously enough balanced that it should've been easy to tip it over with a slight addition of weight on the hood, but it wasn't just a question of balance; there was also the friction of the car's underside against the edge to consider, and maybe a bit of bending of the metal, further helping to hold it against the edge.

The most surprising thing about this is that they did a myth about a car going over a cliff and didn't actually wreck the car. In fact, I think I saw the same car being used in the same bomb range in the promo for next week's episode (and it looks like it definitely won't survive that one). In fact, I wondered why they were at the bomb range for this one, since nothing was blowing up; maybe they were working on two myths at once?

But it's nice to see an effective episode that's built around cool principles of physics and doesn't involve explosions or crashes.


In related news, I checked out the new Penn and Teller Tell a Lie show which came on after this. It's a similar sort of premise, testing the veracity of implausible claims, and they actually overlapped something the Mythbusters tested last season (whether profanity can reduce pain), but different enough that it doesn't feel like a ripoff. The gimmick is that they present seven unbelievable claims, one of which is a lie, and you have to guess which one. I guessed wrong. It's a pretty funny show, though not as family-friendly as Mythbusters. The sort of thing you'd expect from Penn & Teller.

 

[Mr. Adventure]

Another thing I learned is that wrecking balls are steel shells filled with concrete. I always figured they were solid metal.

I think they said older wrecking balls were made with concrete and I can't find anything online that suggests they are made of anything ohter than steel. However, at least with the search terms I was using, I wasn't able to find much information on them.

 

[Roger Wilco]

Why didn't they just use full steel balls instead of rigging up something so complicated? The original video was obviously fake, so using real wrecking balls would have been pointless anyway. Was it a matter of price tag?

 

[Trekker4747]

It seems to me physics should scale, I mean otherwise things like constants are meaningless since there's always an "x-factor" of scale.

Probably the biggest problem I perceive in the large-scale Newton's Cradle is that the "balls" were more prone to vibration as they began to scale up from solid balls and had to improvise.

Large buoys filled with concrete probably aren't going to deliver all of the energy some of it is going to get absorbed in the vibrations in the gap between the separate pieces and probably in the porous concrete.

Now I'm sure making five, real, solid-steel Newton's Cradle balls would be impractically hard, expensive and just crazy to do but I sort-of think that's why this didn't work. The "wrecking balls" they made at the end weren't solid pieces of material that'd distribute all of the energy of an impact.

I'd also argue that the type of cord used to move the ball(s) probably could play a role to in how much twisting and vibration they allowed.

Still an interesting "myth" for to experiment with and a cool result in the large-sized Netwon's Cradle. It would've been awesome if it had worked to some larger degree.

 

[Roger Wilco]

It seems to me physics should scale, I mean otherwise things like constants are meaningless since there's always an "x-factor" of scale.

Probably the biggest problem I perceive in the large-scale Newton's Cradle is that the "balls" were more prone to vibration as they began to scale up from solid balls and had to improvise.

Actually, I think it does scale in this case. I admit I didn't listen very closely, so maybe they did the math, but you have to realise those those balls weren't just bigger, they were a shitload bigger; I think they said the steel/concrete balls weighed 1000 pounds each? The original small balls probably weigh like 0,1 pounds at most, that's a factor of 10000 and with an according increase in loss. I also think the bigger mount is a problem when you consider the weight increase of the ropes and the not quite as perfect alignment.
What doesn't scale just is the nice looking effect apparently.

 

[Christopher]

Why didn't they just use full steel balls instead of rigging up something so complicated? The original video was obviously fake, so using real wrecking balls would have been pointless anyway. Was it a matter of price tag?

Even when a myth is "obviously" unreal (at least to the perceptive of educated viewer), the Mythbusters' standard approach is to test the alleged circumstances of the myth, to actually show that it won't work rather than just asserting it. After all, not every viewer is going to be able to see up front that it's phony, and they need to have it proven to them. That's always the way it's been since the beginning: first test the conditions of the myth, then if that doesn't work, alter the conditions in order to find out what's needed to replicate the results.

It seems to me physics should scale, I mean otherwise things like constants are meaningless since there's always an "x-factor" of scale.

Yes, but there's more than one physical principle at work here. You've got not just the transfer of energy from one object to another, but the internal structure of the balls, the flexibility of the strings/cables, the strength of the frame, etc. In a small Newton's cradle, there's little enough material that relatively little energy is dissipated through deformation and the transfer of kinetic energy is the dominant effect. But in the larger cradles, there was a lot more material that could absorb energy. No matter how securely they bolted the frame down, the beams were long enough that they would've had some sway in them; indeed, they're no doubt designed to be able to bend a little to absorb structural stresses. So they're not exactly analogous to the structure of the small cradle's frame.

There's also the fact that the impacts between the balls were so much more forceful, strong enough to overwhelm the crystalline structure of the metal and cause the steel plates to flatten out at the point of impact -- something that doesn't happen with the much gentler impact energies of the small cradle.

So the same physical laws do apply on both scales. It's just that, since this is real life and not a math problem, there are multiple physical processes interacting. On the small scale, the processes that absorb energy are trivial in proportion to the processes that transfer it. But on a larger scale, those energy-absorbing processes become more pronounced and the balance is different. Same physics, different ratios. This is always the problem with scaling.

 

[Forbin]

I imagine that even full-sized completely steel giant balls, even if they were radically hardened, would deform (dent) under the impact of another 1000-pound steel ball, rather than transfer as much energy as the tiny balls.

 

[Roger Wilco]

Even when a myth is "obviously" unreal (at least to the perceptive of educated viewer), the Mythbusters' standard approach is to test the alleged circumstances of the myth, to actually show that it won't work rather than just asserting it. After all, not every viewer is going to be able to see up front that it's phony, and they need to have it proven to them. That's always the way it's been since the beginning: first test the conditions of the myth, then if that doesn't work, alter the conditions in order to find out what's needed to replicate the results.

Yes, I understand that. But I meant after they had already decided to not use the metal buoy filled with concrete idea and instead inserted that steel ring in the middle. My point is, that after seeing that just concrete won't work, it would have been more interesting to use solid steel balls, although I don't believe it affected the test results.

 

[Christopher]

Maybe they just couldn't find a source for five identical 1-ton balls of solid steel.

 

[Trekker4747]

By the way, I hate the new theme and tweaked incidental music.

I've no idea why after some 10 years of doing this show the decided to change the freakin' theme.

 

[Christopher]

I've no idea why after some 10 years of doing this show the decided to change the freakin' theme.

Because they felt it was time to try something new? Lots of shows change their themes after a few years, often a lot sooner than this. Heck, Mythbusters redid the title graphics style a couple of seasons ago, so it's not like they've never been willing to change things.

 

[Trekker4747]

Yeah, but I suspect few shows change their actual main theme, at least so drastically. (Plenty of shows usually change it to the same theme just with a different melody or tempo or something; here it's pretty much a whole new theme.)