MythBusters Recap: Mower Mayhem

by Alan Eggleston / August 2, 2015

Ah, the smell of freshly cut grass… the hum of lawnmowers across the neighborhood… and the sounds of ambulances rushing down the street?

This week’s new episode of MythBusters may be perfect for summer, because a big part of the episode tests the premise that the power of rocks thrown from a lawnmower can be equivalent to the power of a bullet fired from a .357 pistol. No wonder MythBusters frequently flashed the “DO NOT TRY THIS AT HOME” caution!

The other part of the episode tested the premise that a sheet of glass falling from a high-rise building can slice an unsuspecting bystander in half.

Lethal Lawnmowers

First, Adam Savage and Jamie Hyneman set up their lethal “laboratory” at the Chabot Gun Range, where they’ve been firing projectiles for over 10 years. Laying down AstroTurf on a flat surface next to a wall of Styrofoam adorned with blue human figures, Adam tosses down a path of rock, of various sizes and weights.

Adam revs up a standard push mower, removing the safety guard — remember, “Do Not Try This At Home.”

It’s a rough, miserable push, and Adam stops within a few feet from his start. Debris has flown everywhere, including into and through the Styrofoam wall.

Then Jamie ramps up the experiment with a premium rider mower with four times the power of Adam’s push mower.

Debris flies, including rocks, AstroTurf, and anything else not screwed down. The Styrofoam wall is pummeled.

While Adam and Jamie have demonstrated that mowing through debris can be dangerous, they still haven’t proved that the power of the mower equals the power of a .357 pistol. To do that they return to the workshop, where they measure the speed of rocks thrown by a lawnmower blade and the speed of a bullet fired from a .357 pistol. But that doesn’t compare the amount of power between the two.

Adam comes up with a pretty sweet pendulum rig to compare the kinetic energy between the two devices.

They fire off two rounds from the .357 pistol and it consistently swings the pendulum 60 degrees.

Jamie replaces the lawnmower with a pneumatic launcher, which is easier for aiming the stones at the small pendulum target, though it throws at the same speed.

Using the pneumatic launcher to shoot stones twice at the pendulum target, it consistently swings the pendulum 66 degrees.

“That is more energy from the rock!” says an excited Adam. “I totally didn’t expect that result.”

  • Conclusion: Confirmed

Glass Guillotine

How do you find an unsuspecting bystander who will let you test a sheet of glass falling from a high-rise building onto them? You build a human analog and you drop the glass from the Fire Training Center in Santa Rosa, CA. The human analog was made of ballistic gel containing tubes of fake blood.

High-rise buildings use thick tempered glass with squared edges as a safety precaution, and that is what Adam and Jamie use. The question is, will the glass cut cleanly through a body like a blade through a water melon, or will it shatter on impact?

Dropping the 4 foot by 6 foot sheet of glass from 75 feet off the top of the building onto their human analog, it drops vertically until about two-thirds of the way down, then it begins to flip and lands almost horizontally, missing the bystander and shattering into thousands of pieces.

Adam and Jamie decide on a Plan B: a welded steel rack attached to the top of the building with guide wires mounted to the ground, and riders mounted to the glass to keep it aligned with the guide wires so it can’t flip.

Winching the glass to the top, they let it drop and it hits the bystander perfectly, but knocking it over. There’s blood and the bystander would no doubt be dead, but there is no slicing. The well designed square edge of the glass has prevented it.

To “ramp up” the test, Adam and Jamie change from tempered glass to plate glass, which is thinner and doesn’t have the squared edge. They let the plate glass drop — it lands off center, but it slices right through the bystander — dead on.

“So, the right kind (of glass) is busted, but the wrong kind (of glass) is plausible,” says Jamie. Adam calls it slightly different:

  • Conclusion: Plausible

One More Ramp Up

Not satisfied that they had sufficiently ramped up the lawnmower test, Jamie decides to build “The Lawnmower from Hell.”

Refitting his rider mower, Jamie uses four 50 horse power electric motors and an 1800 amp lithium battery, a front-spinning 50-pound steel blade rotating at 5,000 RPM, and a steel-armor protective shield. It looks like a mini-vehicle out of Mad Max.

Adam sets up a test course of wood pallets, split wood logs, watermelons, and glass bottles. Now that’s ramping up a test!

At first making a test run through a bucket containing blue water, everything runs fine and the mower obliterates the bucket. But when Jamie moves to the test course, the axle rotating the blade breaks, the blade slicing through the steel front housing and spinning dangerously across the ground.

  • Conclusion: Dangerous – “Don’t Try This At Home

“You can learn as much from a failed test as from a successful one,” says Adam. “What we learned on this test was, Jamie has built something far too dangerous to turn it on again.”

Next week’s new MythBusters episode is titled “Dangerous Driving,” testing hands-free technology while driving. What could possibly go wrong? Find out next week at 8 pm (ET) on Discovery Channel!


Alan Eggleston A writer from the boomer generation, I was among the first Americans to grow up with television and even got my bachelors degree in broadcasting. My first professional job was working in a television station, working camera and then writing copy and promotions. A few years later I turned to writing for print and then adapted to the Internet. I love writing and I love good television and film - I hope it shows in my reviews.