Hey its me Destin,
Welcome back to Smarter Every Day
If you're like me, you've
spent a ton of your life
on a lawn mower and you've been thinking
about how the blade interacts
with the blades of grass.
So today on Smarter
Every Day we're going to
look at the physics of
what that blade is doing
on the lawn mower.
I'm trying not to hit
stuff, I'm sorry [Laughs]
And then we're gonna
I don't know, whatever, I'm just excited.
Let's do it.
(upbeat rock music)
Okay, I don't know if you know this,
but every time you start
making a video outside
your neighbors always
turn on the lawn mowers.
For this video, I don't
care, so that's pretty cool.
OK, so let's think about
how things are cut.
Think about a piece of paper.
What would you use to cut
that? Scissors, right?
Think about how scissors work.
I've always thought
scissors were ingenious.
You've got two opposing blades
that are pushing against each other
and they're causing shear
stress at a localized area
which causes it to shear.
You fail that very
slight point on the paper
and then you move that
failure front down the line,
right? Pretty cool.
Similar to ripping but
it's way cooler than that.
If you think about how we cut things,
that makes the most sense
because you have two opposing blades
that are causing a localized shear stress.
The thing that's interesting
to me about mowing
is that that doesn't happen.
You don't have things like this.
Like imagine something that
would cut things like this
with opposing blades.
That would be like a
barber shop clipper, right?
The barber shop clipper has
blades that move back and forth
and they create a localized shear stress.
A lawn mower blade is different, though.
There are no opposing blades.
It's like an open ended system, right?
You've got this massive
blade with a lot of inertia
and it swings along really
fast and it hits the grass
but the grass is so lightweight
it kind of moves out of the way, right?
If you think about this,
it's a kind of dull blade
like hitting this paper
it doesn't cut it, right?
I guess you could... I feel
like... see it ripped it.
That was an inertial type tear,
but think about what's going on here.
In order to cut this grass,
you have to really be screaming
unless you could support
the grass at the other end
If you could hold the
grass up, for example,
if you had somebody that
was just holding that grass
just like that, you'd
come along with a blade,
you could swing it really fast,
and it would be a lot easier to cut.
Here's what I want to do, I want to take
a standard push lawn mower and
I want to cut the front off
so that we can look inside
the front of the deck
and see if we can see the grass being cut
and understand more
about what's happening.
(cheerful music)
(saw buzzing)
So lawn mowers are dangerous, obviously,
don't modify your lawn mower.
I've got a buddy Trent
here. What's up Trent?
Trent is an engineering student.
He also helps me with mowing
and high speed cameras
and apparently he's...
You just mowed with a high
speed camera fan dude.
- I was hoping you didn't hear that.
(Destin laughing)
(sounds of lawn mower running)
- It's terrifying.
It just blew it out of the way.
I did the calculations.
The tip of the blade is going
about 200 miles per hour,
but there's gotta be
more we can learn here.
I know it has to do with
cutting and air flow
and waste management
after you cut the grass
and all that stuff and
I tried to figure out
what expert I could go talk to
and the more I thought about
it, I said you know what?
My barber's a smart dude.
He does all those things.
And sure enough, he
gave me the aha moment.
- Lawn mowers suck up the air.
Destin - So there's air flow.
- The vacuum of it sucks it
up which causes it to be even
and it cuts it off even.
Destin - So it pulls the grass up?
- Now if I use these clippers
on you, (clipper buzzes)
there's a vacuum.
Destin - So it'll be more even.
Barber - And then it's going
to pull the hair to the vacuum
and cut it.
- So is that the tab on
the top of the lawn mower?
- Yes.
- That's to create air
flow to suck the grass up.
- That's exactly right.
- All right here's the
deal, I'm 36 years old
and I've studied aerodynamics
and I've spent the better part
of my life on a lawn mower.
But my barber just taught me that that tab
on the back of the lawn
mower blade is for lift
and the purpose of that is
to pull the grass blades up
so that it can cut.
This all makes perfect sense
And I'm ashamed that I
just now learned this.
So, this is my bigger mower.
It's still got the lift tab.
I think what we did on the other one
is we kind of duced up the air flow
because we cut that front section off
so the air doesn't flow
like its supposed to
so it actually pushes the grass down.
I think if we get high
speed video of my big mower
and we look into that grass discharge deck
we should be able to see
it pulling the grass up
and cutting it. Maybe. Let's try that.
(lawn mower buzzes)
Okay this is hand held lawn mower.
This is dumb, here we
go, we're gonna try it.
Holy moly! Do it!
Go! Go fast!
I don't think I did a good job.
If you look close, you
can see that the grass
is pulled up because of the blade's lift.
It's easier to see if you focus
on a single blade of grass.
(slow motion lawn mower sounds)
You know what this reminds me of?
It reminds me of the Macaws
in the Amazon Rainforest.
- Oh, with feathers on
the back of their backs?
- Yeah, when they would push down,
the feathers would lift up
and that's exactly what
the grass is doing.
- I wonder if you had a longer tab
if you'd have a better cut.
- Yes, but it would also take
more energy to turn the blade
because you have more resistance
because lift and drag
are proportional, right?
So the shape of the deck matters.
So it's controlling the
air flow. Look at that!
The air's flowing in around
the bottom of the deck
and it's flowing out
around the discharge chute.
There's a lot of physics
here. For example,
when the blade starts
to spin, as it speeds up
and goes through different flow regimes
like you've got the
spaghetti grass blade effect
over the blade until it rips it.
And then there's this
other thing that happens
when the back lift tab starts
to straighten the blades up
and it cuts it, that's
a whole different thing.
- 3, 2, 1
(lawn mower sounds)
- Here's the deal, the
reason I wanted to look
at my lawn mower is there's
a guy named Rodney who's
going to every state and
he's mowing yards for people
who can't do it: the elderly, veterans,
it's a good thing, he's a good
dude, and I interviewed him
and everybody's interviewing him right now
but it's on the second
channel. Go check that out.
Also, I want to do a whole
series on lawn tools,
if you have any questions
you want answered
I may be able to do it with my tools here.
If you have questions,
leave comments below
and let's just look in more detail
and let's do like the
Amazon series and stuff,
let's do a whole series on lawn equipment.
Yeah. That's awesome. Let's do that.
Lawn tool series: I have no idea why
I didn't think of this before.
I am excited about that.
OK, you're smart, you know
how internet videos work.
This video is sponsored by Hello Fresh
but you're also smart enough to know
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Aren't you supposed to
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- Do you want me to go back
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Destin - (laughs) No I don't.
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Straight up no lie, I'm
sitting here shooting this ad
for Hello Fresh and my wife
walks in with Hello Fresh.
Did that just happen?
- That did just happen. (laughs)
Destin - What are you laughing at?
- I'm trying to hide.
- Do you like Hello Fresh?
- I do like Hello Fresh.
- OK. Is it real? Like
everything I probably said
that you didn't hear? You think it's real?
- Probably.
- Yeah, OK, cool. Thank you.
- All right, I hope you
enjoyed this episode
of Smarter Every Day and it
earned your subscription.
That was the point. We have
started the Lawn Tool Series.
So feel free to subscribe
to Smarter Every Day
if you wanna see more stuff
and leave comments below
for your suggestions.
Anyway, I'm Destin,
getting smarter every day.
Have a good one.
I got grass in... aw man
we have a grass issue.
Don't get in the camera! There we go.
