Hey, it's me, Destin.
Welcome back to Smarter Every Day.
It's time for the Weed Eater episode.
And the way-- I wanted to shut the door.
The way you can tell that I've staged all this
is that this Weed Eater's going to crank up immediately.
But here's the deal.
When it comes to operating a Weed Eater,
I have a mental model in my head of how the Weed Eater works.
Like, as it goes faster, it slings out these lines
in a disk, right?
But is the disk like a solid line
pointing straight out from the Weed Eater head?
Or I guess you could call it the trimmer head.
Or is it more like a lagging wave?
I don't know.
Check this out.
[REVVING MOTOR]
Let's look.
So if I'm going to weed eat this whole spot right here,
I rev up the Weed Eater.
But when I don't rev up the Weed Eater,
you can see that the line is lagging.
It's all curled up.
So at some point in there, it straightens out.
Then I can go in.
I can trim.
And for some reason, I feel like
I should trim from right to left.
I don't know why, but it just feels like that's
something I should do.
Let's go over here to this.
Watch this.
If I come down, should I plunge?
You see that lays the grass down in a certain way,
but if I go like this, it doesn't?
I don't know.
My point is there's a lot of physics
in how Weed Eaters work.
And there's these quantitative things
that we should be able to measure
with a high-speed camera.
But there's these qualitative things
that you just know if you spend time running a Weed Eater.
So today on Smarter Every Day, I want
to take that internal mental model of how Weed Eaters work
that I've developed over years and years
of running Weed Eaters.
And I want to test it with a high-speed camera
to see if we can understand the physics
of how a Weed Eater works.
Let's go get "Smarter Every Day."
[THEME MUSIC]
What up?
TRENT: What's up, man?
DESTIN: Trent is going to be the trigger man
on the slow-mo camera for us.
So I'm going to be running the Weed Eater.
And I'm going to get that Weed Eater head positioned
over this X right here.
And we're going to use this Phantom v2511.
We'll record it at about I'd say 12,000 frames per second.
And then we're going to see if the Weed Eater line slings out
or, more accurately, see how it slings out.
Because if you think about it, as the line is whipping
through the air, you have aerodynamic drag
acting on the Weed Eater line.
But you also have the angular acceleration
slinging the line out.
So you've got this balance of forces.
That being said, the question is,
does the line ever truly get straight?
Because in my head, I'm operating with a disk
with straight lines out of it.
12,000 frames per second.
OK, ready on trigger?
TRENT: Yup.
DESTIN: Here we go.
Ha!
[REVVING MOTOR]
Ready?
TRENT: Yup.
DESTIN: It's a simple test,
but it should show us exactly how that straightens out.
All right, so as we speed faster and faster,
you can see that the line is lagging behind,
probably due to that drag that we talked about.
But there's also still a little curl at the end of the line,
probably because it remembers being wrapped around a spool.
My question is, if we make the line even longer,
will this drag be even more pronounced?
Meaning, would it lag behind even further?
There's only one way to find out.
We got to test it.
If we were to remove this guard-- don't do this at home,
all that stuff--
I think what would happen is we would
see that the aerodynamics would start
to win, like the drag on the line, right?
Here we go.
Let's do this.
[LAUGHING]
It's hot in this helmet.
[REVVING MOTOR]
It's going to hit my knee.
So I'm learning all kinds of stuff.
Like, the torque demand on the motor
is a function of the length of the line
and the aerodynamic drag.
So for example, while he's saving that high-speed file,
listen to this.
[REVVING MOTOR]
It doesn't rev all the way up.
Right?
But if I cut that line off...
[REVVING MOTOR]
I never thought about that.
Huh.
TRENT: This looks so Alabama.
DESTIN: That's a good thing.
It's like The Stig.
This is the Alabama version of The Stig.
Roll tide, baby.
So once the Weed Eater line gets to a certain length,
the aerodynamic drag is just too much,
and the whole system lags significantly.
But Weed Eater designers were clever,
and they integrated a really interesting tool
to make sure the line both stays balanced
and at the optimum length.
There's a feature in a Weed Eater
where you can tap the head down on the ground.
You hit the button at the end, and it will release more line
from both sides of the spool.
There's also an integrated blade in the guard itself.
So when the line gets long enough,
it will trim the weed eater line to the right size.
But I've always wondered how that cut takes place.
This is an older Weed Eater I've had for a while.
And I don't expect it to make a clean-cut.
But I want to use the slow-mo camera here.
And Trent was pretty smart and came up with a mirror
to bounce light up in there so we can see it.
So let's see what exactly happens when we bump the line
and get more.
We'll see what that cut looks like.
[REVVING MOTOR]
TRENT: Good?
DESTIN: Yeah, got it.
There's so much to see.
So it's not a single cut like I've always thought.
It's like a continuous whipping until it pulverizes it
into the right length.
The torque on the engine is a function
of the length of the line.
The line is determined by the position of the knife blade.
The balance is determined by both sides of the line
being equal.
It is time to cut grass.
Let's go cut grass.
TRENT: Ready?
DESTIN: Yup.
[REVVING MOTOR]
OK, I've got a theory that there are different types of grasses
that behave differently.
Like, that was a blade of grass, and it kind of de-laminated.
When it started getting hit by the thing,
it started ripping apart.
But if we go over here to something like this
that's more of a stalk-y type of grass--
I don't know if you can see this.
I wonder what's going to happen there,
because it's going to rip differently.
So let's see if we can get a slow-mo of that.
TRENT: Ready?
DESTIN: Look at this.
You've got a stob.
That's a grounding rod.
So let's replicate that.
We've got a metal stob right here in the ground,
and it's hammered in.
I've noticed that when I'm weed eating
and I hit something like that, it'll pull my Weed Eater in.
OK, here we go.
I always thought that hitting an object like this
would pull the Weed Eater directly towards the steel rod.
But if you look closely, you can see that
it's a little different than that.
The line is hitting the top-right side
of the steel rod.
And you can tell this by the way the line wraps around it.
It almost makes it look like a pulley.
If you were to draw a line in the direction of that force,
you can see that that's the direction the trimmer head
gets pulled.
Just play it forward, and you can see
that it kind of follows that line.
So it's not being pulled directly in like I thought.
It seems to be reacting to the force that's
being applied to that rod.
All this happens so incredibly fast
that I never thought about what was actually happening.
OK, the next thing we're going to talk about
is what happens when a Weed Eater goes up next to a fence.
Over here, a long time ago I had an idea.
I wanted to create a little gourd garden.
And so I put this wire fencing, if you can see that.
I put this wire fencing up next to this wooden fence.
Now, over time for some reason I came out here,
and I'd weed eat this thing.
And it would break my Weed Eater line.
And I got tired of that.
So I just quit Weed Eating it.
So now we have big 20-foot trees and stuff like that.
So to try to understand why I got frustrated
and quit weed eating that stuff,
we have set up a Weed Eater obstacle course.
We've got chain-link fence here.
We've got what I call "hog wire" here,
which is maybe not quite hog wire.
But it's a pretty stout wire, but it's still not
quite as strong as the chain-link fence.
And then we've got this other one.
It's a smaller diameter fencing here.
So the idea is with the stob, the Weed Eater line
would rotate around that stob, right?
And it would act like a pulley.
If we have a smaller diameter here, does it do that?
Or is the minimum bend radius a lot smaller,
and will it just cut it?
OK, it feels more like science if we take some measurements.
So looks like about 0.110, 0.112 an inch on the chain link.
And let's look at the Weed Eater line itself.
Looks like we're around 0.095 of an inch.
[REVVING MOTOR]
Yeah, it definitely pulls it to the side.
The question is what is the interaction
of that Weed Eater line to the fencing look like?
Aw, that's awesome!
[CHUCKLING] That's awesome.
So it wrapped around it.
And at times, it doesn't break it.
And at other times, it does.
Oh, what happened there?
Oh.
TRENT: [CHUCKLING]
DESTIN: OK, if the Weed Eater line wraps 360 degrees
around the fencing, then it--
is it a tension break?
It rips it.
TRENT: Wow.
DESTIN: Oh!
OK, cool.
So it's like a spaghettification of the Weed Eater line.
It wraps around the fence.
And then if it goes 360, it has to unwind the whole thing.
And sometimes it's easier just to break.
I can't tell if it's a tension break or a sheer break.
Because the chain-link fence is on a diagonal,
it looks like as the Weed Eater line comes in
and hits the fence, it funnels it to the corner.
And because there's another wire going through the corner,
it's like a pinch point.
I bet if we go to the smaller diameters--
yeah, if we go to the smaller diameter wires,
it's going to have a smaller radius of curvature
it has to bend around.
Mm. Mm.
Interesting.
We need to move on to the next one.
What I'm calling hog wire is about 0.085.
[REVVING MOTOR]
It definitely grabbed more.
And you can see that there are some tension issues there.
It's trying to pull.
The brake is very different than I thought.
It's a tension break, at least on this, because it
seems like it's wrapping.
And it's pulling so fast.
It's unwrapping it so fast, like a whip.
And it's a tension break.
So there you go.
We've learned something.
If it goes around 360 degrees, then it
has a really tough time recovering.
Also, this has got to be different with a square line
or a triangular or a polygon-type line.
We're going to do another video following up
on this with different types of Weed Eater line.
OK, here we go, time for the small stuff.
This is 0.060 of an inch.
This is smaller than the Weed Eater line.
Let's do it.
[REVVING MOTOR]
TRENT: Whoa!
DESTIN: Yeah, that's--
TRENT: That was scary.
DESTIN: That's scary, but that's-- that's what
it's like to weed eat-- that's why you always sneak up next
to the fence, because you're worried that it's going to grab
your Weed Eater and go.
OK, it broke at the welds, which makes sense
because when you weld wire like that,
there's a heat-affected zone on that wire.
So right there, I would expect right near the weld
to be the weakest part of the wire.
This didn't break. Wow.
This episode of Smarter Every Day
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[THEME MUSIC]
I hope you enjoyed this video.
I've heard people my entire life say,
oh, triangular Weed Eater line is best.
Square Weed Eater line is best.
Twisted square is best.
Round is best.
We're going to solve that.
We're going to test them with empirical tests.
So we're going to have data.
We're going to answer the question,
"What is the best Weed Eater line?"
If you would like to see that video,
please feel free to subscribe to Smarter Every Day
by clicking the little Subscribe button,
maybe even the bell.
If not, that's no big deal.
I hope you enjoy your summer out weed eating the yard.
I know I am looking forward to it myself.
Go get some vitamin D.
Anyway, that's it.
I'm Destin.
You get "Smarter Every Day."
Thanks for subscribing, maybe, if you feel
like this video earned it.
If not, that's cool.
Have a good one. Bye.