How Lawn Mower Blades Cut Grass (at 50,000 FRAMES PER SECOND) - Smarter Every Day 196

Hey, it's 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 airflow 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 airflow.

• 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.

Destin - So is that the tab on the top of the lawn mower?

• Yes.

Destin - That's to create airflow 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 dulled up the airflow because we cut that front section off, so the air doesn't flow like it's supposed to, so it actually pushes the grass down. I think if we get high-speed video of my big mower, 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 a handheld 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 airflow.

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 if I'm just talking about a sponsor to get money for Smarter Every Day or if it really means something to my family, which Hello Fresh does. Hello Fresh is a meal kit delivery service that will send meals directly to your house. You can go to the website and you can pick whatever works for you. There's three different types you can choose from: Classic, Veggie, Family— they will get the ingredients together and send them to your house.

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Aren't you supposed to use tongs or something?

• Do you want me to go back and use tongs on your salad?

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.