How a Lawn Mower Cuts Grass

How a Lawn Mower Cuts Grass

It seems like I’ve spent an inordinate amount of time cutting grass lately. I’m not prone to wondering about physics, engineering, lift force, or any of the other techy, nerd science that goes into many of the tools we use every day. I’m mostly just happy that my lawn equipment works as advertised. Occasionally, my brain does settle into these generally uncharted waters. And, what with so much of my free time spent this summer mowing my unruly lawn, I caught myself wondering about the physics behind how a lawn mower cuts grass. I also wanted to see a lawnmower cutting grass in slow motion.

Thankfully, Destin, over at SmarterEveryday.com, has one of those “engineering-type” brains, along with the willingness to teach others. He recently tackled this subject matter on his Youtube channel,  where he filmed himself cutting grass in slow motion. It’s awesome. And, here’s what I’ve learned…

How a Lawn Mower Cuts Grass

Rather than using two opposing blades to supply sheer cutting force localized at one location, like how a pair of scissors works, a lawnmower blade swings a blade unidirectionally at an incredibly fast rate. However, if you’ve ever tried to cut something really pliable without something else providing some sort of resistance, you’re probably aware that the best-case scenario is a tear from the torsional force. Imagine trying to cut a string with your pocket knife, but without holding onto both ends of the string.

How A Lawn mower Cuts Grass

So, for a lawn mower to cut effectively, you need to apply resistance to both ends of the grass. The ground holds one end of a grass blade fast, but without any other external force, the other end is free to flop around. This would limit the actual cutting potential of your mower.

Have you ever wondered why your lawn mower blade has the strange-looking design that it does? It’s all physics, baby. Quite a few years ago, engineers realized that if they could add a lifted tab to the back end of the mower blade, the spinning blade could then generate lifting force. It works similarly to the flaps on an airplane wing.

how a lawn mower cuts grass

As these wonky-looking blades spin inside your mower deck, they create enough upward force to pull air up from under the deck, creating a sort of vacuum effect. That air actually pulls the blades of grass upward as well, holding them in place and providing the necessary resistance for the business end of the mower blade to make an even, clean cut.

Watch a Lawn Mower Cutting Grass in Slow Motion

For a better how-to explanation than I just provided, check out this video of a lawn mower cutting grass in slow motion:


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