math help ...

[Charlie Stone]

I know there have to be some serious math people lurking around.
In the past year, 3 houses, a street light, and a fire hydrant have been hit by cars along my block.
My wife and I figure it has to be our turn soon, so we want to put a bolder in as a barricade in our yard. How heavy of a rock would we need to stop a 5000 lb vehicle (average idiot not paying attention) traveling at 35 mph?

[John Coloccia]

Are you trying to kill somebody? Maybe petition the town for a guard rail. That will serve the same purpose without killing the driver of the car. I suspect if a car actually did hit your boulder that you might very well have a lawsuit on your hands.

For what it's worth, though, I have a very steep drop off at the end of my driveway (my property drops about 50 feet straight down onto the main part of the property). There are some boulders at the end that are about 1 ton. The old owner hit them with his pickup, albeit slowly, but they kept him from going over. A LOT of energy is dissipated into crushing the the crumple zones over a relatively long period of time. Still, I suspect you'll need a sizeable rock anyway, and after the car hits there's no telling where it will go. It can launch it, or very well send it into your own house.

[Charlie Stone]

I am trying to keep the cars out of my living room. The drunk drivers are trying to kill someone. In my mind, better them than my daughter playing tea party in the living room. I don't see them hitting a bolder in my yard vs. my brick house, or my neighbors retaining wall would set me up any more for a lawsuit any more than when someone drives in to a bolder left there by nature along the side of a highway.

[Brian Kent]

I'm glad you're not taking your safety for granite, Mr. Stone.

Is there landscaping you can do where the car hits bushes, then a split rail fence, then the boulder, as a crush zone to save their lives?

[Matt Meiser]

Nature didn't post on a public forum that it was intentionally placing the boulder in a spot where it could be hit, and that if it killed someone then so be it. Nature therefore doesn't have to worry about a lawyer finding the post and bring into court.

In a quick google search, I think your target weight is about 20% high. Also I think any boulder big enough dimension-wise to provide an effective barrier will probably weigh more than enough. Guard rail might be cheaper than moving such a beast. Maybe you can disguise the guardrail with landscaping and/or fencing.

[John Coloccia]

i'm glad you're not taking your safety for granite, mr. Stone.

rofl.

[Charlie Stone]

there is a curb, a 10' blvd (with trees and a lamp post depending on the line of travel, sidewalk, and 10 feet of yard before they hit my house. The accident which happened last week across the street, the guy swerved though an empty corner lot, back on to the street, spun the van 90 degrees went up a 1-2' concrete porch, though the front door and corner of a house, across an alley and though the covered porch of another house until he hit the concrete stair case where he put a huge V in the front of his van.

[David Weaver]

You're looking at a physics question rather than math, but speaking from experience...

... when I was little, people clipped off our mailbox at regular intervals. usually drunk people, but sometimes in the winter people skidded into it, even though it was along a straight of road and there was a telephone pole a fair way beyond it which you would obviously want to avoid.

One year, my father had enough of people clipping off the mailboxes and put a 6x6 oak post in a very large concrete footer.

A drunk driver hit it later that year and almost killed himself. His car literally severed into two parts, and it made my dad sick (this was probably around 1980). We went back to tolerating that the mailbox was broken from time to time.

I can still remember being there at age 4 or 5 having no clue what was going on other than seeing a car in two parts in the middle of the night and standing in the yard with the flashing lights everywhere. I have pictures of that car in my picture album, taken with a 110 camera back then. Not something I'd want to be responsible for.

It sounds like a township issue, and I'd agree with matt, a guard rail would maybe not put you in a position where someone could at a minimum force you to go into court and fight off a frivolous suit.

[Charlie Stone]

f-150 (very common) is about 7000 GVW.

[Charlie Reals]

f-150 (very common) is about 7000 GVW.

2005 f-150 gvw 5396, goooood truck

[John Coloccia]

Just to give an idea of the energies involved, a 5000lb truck doing 35MPH has about the same energy as a loaded 18 wheeler doing 10MPH.

[Charlie Stone]

Ya know, you are all right about this. Maybe I am not taking the drunks safety into account.

Allow me to rephrase the question. In physics, How many tons of feathers would it take to stop a 18 wheeler doing 10 miles per hour.

[Dan Friedrichs]

I'm not sure you actually have any liability issue to worry about (IANAL) - our HOA has had its front entrance landscape/fence/sign hit several times by vehicles. This last time, the insurance company wouldn't pay to fix it UNLESS we installed several large boulders in front of it! I don't know what they weigh, but they're about 24" diameter granite. I believe the insurance co specified the size and they were installed by a professional landscaping company.

[David Weaver]

Ya know, you are all right about this. Maybe I am not taking the drunks safety into account.

You might feel a little different after the fact. Especially if the person has kids, went through hard times out of their control, etc. But you'd find that out after the fact - maybe by meeting their kids or their spouse.

What you should do if you want a boulder, though, is go at it with the overbuild philosophy, like 2 or 3 times what you think it'd actually take.

So that the same drunks don't find a bigger truck than you're thinking (like a loaded shop truck or something) and bulldoze the rock right through your house.

The guy at the corner in my neighborhood, and the lady at the other corner - both have rocks - like 10 of them, and probably on the order of about 1000 pounds each. I've never seen anyone in their yards, and they're in prime spots for someone to cut a corner short or miss a turn on our unlighted suburban streets.

[Eric DeSilva]

This isn't a math question. In physics, you see problems like this, but the solutions always rely on the collisions being inelastic. The problem here is that the collision between a car and a boulder is anything but inelastic. There are too many factors you just can't assess--how deep is boulder buried? How dense is the soil? What is the shape of the boulder? You might as well just take a wild guess.

That said, and in deference to some of the other comments, you might think about placing the boulders/guards such that any car that lost control might be redirected away from your house, as opposed to being stopped dead.