PROBLEM: modelling collisions

[jeicher]

Was wondering if some of you physics kids could help me with a small problem...

 

I need to know how to model the impact of a human body (or any projectile for that matter) on a reinforced concrete wall to be able to answer questions like: How much energy must be put into the action to generate enough force to push/throw something through a wall? How much damage will the wall/person take? Where does tensile strength enter the equation? etc.etc.etc.etc.

 

I'd appreciate any help, thanks.

 

-J

 

Ps. Please bear in mind the fact that I'm a biochemist when you answer

[RyanJ]

Firstly welcome to SFN!

 

How much energy must be put into the action to generate enough force to push/throw something through a wall?

 

I can't help here much because I don't know the exact equations but it would be a lot, I'm not even shure if a human body could be - it would probably just splat.

 

How much damage will the wall/person take? Where does tensile strength enter the equation? etc.etc.etc.etc.

 

Umm, I don't think it would be possible without the person becoming a smear on the wall. The human body is not really that tough, car impacts show just how fragile the body actually is.

 

Cheers,

 

Ryan Jones

[insane_alien]

Firstly welcome to SNF!

 

wow you messed up the Welcome

 

Human Vs. Concrete wall: Concrete wall suddenly turns red and sticky with a few lumps of bone.

[RyanJ]

wow you messed up the Welcome

 

Human Vs. Concrete wall: Concrete wall suddenly turns red and sticky with a few lumps of bone.

 

So would you if you had been up all night tying to solve a problem

 

Cheers,

 

Ryan Jones

[5614]

Model the human as a flat board of a certain height and width to represent the human.

 

Then model the wall as a similar board.

 

You can't model a human/wall as a particle because in this case the force acting are important and the area of two things colliding directly effects the force applied on them.

 

RyanJ the body won't splat if it is going through a wall of paper. Indeed I was watching an episode of Bottom (comedy) earlier and the guy realised he had to rush somewhere so ran straight through the wall in a building... it looked quite funny and worked because what looked like a concrete wall had been replaced with just wallpaper or some such. [edit] oops, it does specify that it is a concrete wall, still it might be very thin or something.

 

Does that help? What exactly do you want to use this for?

[RyanJ]

RyanJ the body won't splat if it is going through a wall of paper...[edit] oops' date=' it does specify that it is a concrete wall, still it might be very thin or something.

[/quote']

 

Human vs concrete = splat... human vs anything solid = splat. Why would you use a thin concrete wall - or a reinforced one for that matter? I'm guessing it probaby means your average one.

 

The force needed to do this if it were possible would probably crush a person anyway...

 

Cheers,

 

Ryan Jones

[5614]

Get someone to hold a piece of paper in front of you and punch it. Your hand will go through the paper... paper is a solid. So your hand just went through a solid.

 

Concrete is a lot harder than paper, but concrete which is one atom thick is not at all strong.

 

You can't just say "oh well the human will splat" because you know little about the concrete (esp. its thickness).

 

=====

 

In brittle materials (e.g. concrete) tensile strength is very small (compared to the compressive strength) and can be simplified to 0 in some models.

[RyanJ]

Get someone to hold a piece of paper in front of you and punch it. Your hand will go through the paper... paper is a solid. So your hand just went through a solid.

 

Concrete is a lot harder than paper' date=' but concrete which is one atom thick is not at all strong.

 

You can't just say "oh well the human will splat" because you know little about the concrete (esp. its thickness).

 

=====

 

In brittle materials (e.g. concrete) tensile strength is very small (compared to the compressive strength) and can be simplified to 0 in some models.[/quote']

 

 

Yes I understand that, but a 1 atom thich concrete wall is not practical for anything, seeing as this is a modeling quesion I presumed it would be about something practical - like they do when they test car impacts, they don't work out things with a 1mm thich piece of concrete, they use standard blocks.

 

I answered this question on a paractiacal base, if the author intended for something a bit more unusual I appologise

 

Cheers,

 

Ryan Jones

[jeicher]

Thanks for the warm welcome guys.

eeek, looks like I'll have to elaborate a little to clear things up...

I'm trying to model an object (assume for now that it has infinite compressive and tensile strength if that helps) colliding with a 1m thick reinforced concrete wall. i know the tensile strength of concrete without reinforcements is tiny (~3 MPa which is ~3E6 N/m2, I think) but with the steel it goes up significantly. This is obviously tricky to calculate because tensile strength is measured according to surface area and reinforced concrete has not only rods of steel but also a steel-mesh buried within it.

Once that has been solved I'd like to start getting more specific about the projectile (eg. lead anvil --> human body).

I just don't know where to start to include all these parameters in a mathematical model so any direction helps.

Hope that helps a little...

[s pepperchin]

you need to look at section of a limb as an object which hasa center of mass in the middle somewhere and which experiences torques at points of impacts and at joints, an example would be that an arm has a center of mass and something hits the attached hand. There are torques on the arm, one where the hand is struck and one where the arm is attached to the rest of the body.