Tuesday, April 07, 2015

The Projectile Incident

I was working in my study. Eli 13.7 was practicing the diabolo in the living room, and I heard the steady whir as it spun.

Then I heard a bang, or something.

Eli walked in a few seconds later. "Dad, I think the lawn guys just broke our door," he said.

"What?" I asked.

"Come look," he said.

There are about a thousand services in Austin that mow lawns, and we use one. They come by every two weeks in a big riding lawn mower.

We reached the entryway and Eli pointed (bottom right of the glass):

"You're right," I said. "Let's find what came through the glass." We looked around for a few seconds and saw a rock. This rock:

That's Gloria's hand, by the way.

The rock had only stopped when it hit a wall. That got my attention.

Here are the numbers, even though I've long forgotten how to calculate the speed of the projectile (I bet a bunch of you haven't, though):
Approximate distance from door when rock was ejected from mower: 25 feet.
Approximate mower blade RPM: 3,500
Rock height when it hit door: 26"
Rock height when it hit wall: 21"
Distance from door to wall: 15'

I don't know the exact speed, but that rock was moving.

The lawn service had the glass panel replaced in one day. Impressive.

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