Speed of Two Falling Ladders (Veritasium Video)

[Drakkith]

TL;DR Summary

See rest of thread.

Just saw this Youtube Short by Veritasium. The idea is that two identical chain ladders are dropped at the same time.
One hits a table shortly after release, while the other continues to free fall.
Curiously, after the bottom of the first ladder hits the table, the remainder still in free fall appears to accelerate faster than 'free fall', as you can clearly see from the video.
Any ideas on why this might happen?

 

[Lnewqban]

Could it be progressively less air drag on the left chain ladder?

 

[hmmm27]

Magnets, of course.

Wonder if the angle was a calculation or just fiddling around.

 

[Ibix]

Not sure if this is meant as a puzzle, so I've spoiler tagged my thinking.

Spoiler

In the ladder that hits the table the center of mass of the bit in free-fall is rising up the ladder. I suspect the centers of mass fall at equal rates, so the collapsing ladder does, in a sense, move faster.

I haven't watched the video yet. Prediction: the top of the collapsing ladder hits the table at the same time as the middle of the other one passes the table, plus or minus experimental messiness.

 

[jbriggs444]

It is pretty obvious when you consider what happens when a slanted rung hits the ground. And is pretty clearly confirmed when you look at which strings become taut when that happens.

So... what happens to the free end when a not-quite-horizontal rung hits the ground on one end first?

This is similar to the "chain fountain" trick, but much easier to analyze.

 

[hmmm27]

Not sure if this is meant as a puzzle, so I've spoiler tagged my thinking.

Spoiler

In the ladder that hits the table the center of mass of the bit in free-fall is rising up the ladder. I suspect the centers of mass fall at equal rates, so the collapsing ladder does, in a sense, move faster.

I haven't watched the video yet. Prediction: the top of the collapsing ladder hits the table at the same time as the middle of the other one passes the table, plus or minus experimental messiness.

Umm... is that the same as...

Spoiler

Lower side of a rung hits the table, which spins the rung, which pulls the other side down. Rinse, alternate and repeat

 

[Baluncore]

Any ideas on why this might happen?

I think it must be something to do with the waves travelling up the rope. Notice the kinks that form in the rope above a rung as it lands, those S-wave bends are also forming in the still falling ladder, which is reducing the distance between rungs, and so pulling the ladder down.

The heap of rungs builds up, until it tumbles forwards or backwards, which is when the top of the ladder really seems to get ahead. It could be that the sideways deflection is placing tension on the ropes.

 

[Ibix]

Umm... is that the same as...

Spoiler

Lower side of a rung hits the table, which spins the rung, which pulls the other side down. Rinse, alternate and repeat

I don't think so. Like I say, that was my thinking and I haven't watched the video yet.

 

[OmCheeto]

Umm... is that the same as...

Spoiler

Lower side of a rung hits the table, which spins the rung, which pulls the other side down. Rinse, alternate and repeat

Agreed.

Now where's Andy Resnick? Didn't he say a while back that he bought a high speed camera and needed subject matter ideas? I propose he drop a drumstick at an angle and plot the speed of the non-bouncing end of the stick before and after the bouncing end of the stick makes contact with the ground. Similar angle as the rungs on the cable ladder in the OP video, of course.

I have googled to find videos of such a thing but have had no luck. The mathematical analysis of what's happening is way beyond my skill level. But it's kind of obvious to me that

Spoiler

the bouncing end of the stick is imparting a torque on the rest of the stick, levering the non-bouncing end of the stick downward, adding to the force of gravity.

 

[erobz]

I think when the slanted rungs hit the table the free end is trying to accelerate at greater than $g$. It’s tugging on remaining the ladder above it.

Here is a hw problem that caught my attention for this idea.

https://www.physicsforums.com/threads/rotation-and-torque-about-an-accelerating-point.1049120/

 

[AndyRuina]

Magnets, of course.

Wonder if the angle was a calculation or just fiddling around.

Definitive answer about the choice of angle. A single random guess, no calculation and no fiddling.

 

[AndyRuina]

Magnets, of course.

Wonder if the angle was a calculation or just fiddling around.

About the angle, a dfinitie answer. Not calculated. No fiddling. A random guess and that was that. Probably anything between about 5 degrees and 30 degrees would work. At least that's a guess about what would work.

 

[Nidhi Patil]

TL;DR Summary: See rest of thread.

Just saw this Youtube Short by Veritasium. The idea is that two identical chain ladders are dropped at the same time.
One hits a table shortly after release, while the other continues to free fall.
Curiously, after the bottom of the first ladder hits the table, the remainder still in free fall appears to accelerate faster than 'free fall', as you can clearly see from the video.
Any ideas on why this might happen?

Maybe because air resistance in left ladder is less as compared to the right one.

 

[jbriggs444]

Maybe because air resistance in left ladder is less as compared to the right one.

Welcome to Physics Forums.

The thread you responded to had been idle for about two months. The answer had already been given (see responses #5, #6 and #10). It is not air resistance.

It was not necessary to quote the entire original post in the response. A quote of "Any ideas why this might happen" would have been adequate.

 

[PaulM]

Next step would seem to be to define how this squares with conservation of energy?

 

[jbriggs444]

Next step would seem to be to define how this squares with conservation of energy?

Why would anyone expect mechanical energy to be conserved in an inelastic collision of a ladder with a countertop? Where is the conundrum that needs to be resolved?

Possibly you are worried that the top of the ladder is accelerated downward more rapidly and attains a greater kinetic energy than gravitational potential energy alone would allow for. So you ask where the extra energy came from to supply this acceleration. It came from the rung(s) that are striking the countertop.

The upper ends of each rung pull down on the ladder above and, as a result, each of the lower rungs strikes the countertop with reduced impact energy.

Obviously, 100% of the starting gravitational potential energy will end up dissipated as heat, sound and permanent deformation of the ladder and counter top.

 

[AndyRuina]

Magnets, of course.

Wonder if the angle was a calculation or just fiddling around.

Angle is not critical, a pretty big range of angles would work, maybe 5 - 30 degrees

 

[AndyRuina]

Angle is not critical, a pretty big range of angles would work, maybe 5 - 30 degrees

Why would anyone expect mechanical energy to be conserved in an inelastic collision of a ladder with a countertop? Where is the conundrum that needs to be resolved?

Possibly you are worried that the top of the ladder is accelerated downward more rapidly and attains a greater kinetic energy than gravitational potential energy alone would allow for. So you ask where the extra energy came from to supply this acceleration. It came from the rung(s) that are striking the countertop.

The upper ends of each rung pull down on the ladder above and, as a result, each of the lower rungs strikes the countertop with reduced impact energy.

Obviously, 100% of the starting gravitational potential energy will end up dissipated as heat, sound and permanent deformation of the ladder and counter top.

TL;DR Summary: See rest of thread.

Just saw this Youtube Short by Veritasium. The idea is that two identical chain ladders are dropped at the same time.
One hits a table shortly after release, while the other continues to free fall.
Curiously, after the bottom of the first ladder hits the table, the remainder still in free fall appears to accelerate faster than 'free fall', as you can clearly see from the video.
Any ideas on why this might happen?

There are more videos, photos and a paper describing the experiment and concepts. Google ruina falling chains. Or go here: http://ruina.tam.cornell.edu/research/topics/fallingchains/index.html.

 

[hmmm27]

Chain Paper v.13 revised

question : in Fig.7, one of the graphlines is uniform rods, which doesn't otherwise appear in the text. Theoretical calculation of the video's ladder ?

Edge of chair stuff : I found questions/ruminations brought on by a paragraph neatly answered/expanded-upon in the next.

Finally a definitive answer as to why whips crack.

 

[Juanda]

I follow a channel that explained the problem in detail.

Here is the conceptual explanation.


And here is a simulation of the problem using Python which I think it is a very great way of learning.


I might not have done the code just like that but it still works to show the overall physics behind the phenomenon.