I think you've not provided enough information. How much movement is allowed for the concrete blocks? What is the elasticity of the strap?

Have you considered using earth anchors and a spring or shock absorber?

Concrete blocks are not allowed to move--and staps are not flexible. Spring or shock absorbers cannot be used.

GA emc

if we add some springs at the end of straps (i.e between hooks and straps), then what will the specification for the same. Also if u can provide a better solution for stopping the car, i will be thankful.

zahid

Bungees.

Dashpots.

Tailhooking a chain with weights dispersed along the length of the chain.

Cows (in a pinch lawyers will do, but they are not as tasty)

"But with an inelastic strap, you are essentially saying there is no deceleration until contact, and then deceleration is instantaneous, which means infinite force."

As many times noticed cars have, in case of contacts with stops,a tendency to deform so that the acceleration in not infinite but determined by the car stiffness and deformation of its structure!

No question, but a reasonable design using a strap for passively stopping a car would not deform it. If you just want to stop the car, that's what the concrete blocks are for. The straps imply a gradual deceleration, if you allow for some elasticity. That was what I was trying to explain to the OP.

The answer to his secondary question, which is what sort of springs or elasticity to add, would be to look at the impact forces at a collision that provides a known level of destruction. For instance, in the USA, cars are tested at a 35 mph impact and the car can be totalled, but the occupants are supposed to be safe (per my recollection). There used to be a 5 mph requirement of minimal damage that basically drove bumper design, but I believe that is no longer in force.

The idea here is that depending on whether you are looking only at occupant safety, or looking at minimal vehicle damage, you would design the strap elasticity to provide the same degree of deceleration.

Better be someone you don't like

You are not a mechanical engineer ? aren't you?

ya

Will the car be allowed to deform?

If so please give details.

If not - on whose side are you.

yeah the car is allowed to deform

I think we've done his HOMEWORK.

I suspect this probably is homework (or something like that), but you do know there is no such thing as a strap that doesn't stretch. I wouldn't have asked if it weren't a real question.

There is a method similar to this that is used to stop autos. As the cable/strap is impacted the blocks move to a point in contact with side of the auto trapping the person/people inside and immobilizing it. This is one method that is used in some security checkpoints.

On a side note if the cable did not impact with something solid like the engine block or frame ends the auto could skip over it or worst case cause a shearing the top off.

Charles

metalSmith's

This is really very similar to a classic tailhook design for aircraft (except we don't normally deform them permanently). You really face the same problem here as in the tailhook scenario, i.e. don't kill the human. The limiting factor will be brain acceleration which should be less than 10g. We can assume all sorts of cushioning, air bags, etc, but we need to start with this criterion.

Yeah it has to work in similar way. So if u can suggest me how we can stop in the same way as it is done by a tail hook in case of aircrafts. I am thinking i will go for the tail hook. So can you guide me how to make it

What you're looking for is Aircraft Arresting Gear (or sometimes called AAG). Here is the Wiki link

http://en.wikipedia.org/wiki/Arrestor_wires

These, of course, are fairly complex, but you can get the basic idea. The simple idea is that you want to apply a controlled acceleration and you want the plane/auto to stop within a certain distance. Most systems use some sort of adjustable control that gets changed for each type aircraft. Modern electric motor systems can presumably use a back emf signal to control this automatically, but none of this helps you in your application which is, I think, much, much simpler.

You might look at the designs used for tractor pull weight sleds. This link shows the use

http://tractorpull.calpoly.edu/thesleds.html

Here's a patent link for the design

http://www.freepatentsonline.com/4296840.html

You still have the acceleration problem in these designs as you would if you used, for example, a cam-shaped pulley holding a weight. But, you ought to be able to keep acceleration within a reasonable range.

I think you could do this with some fancy hydraulic circuitry, but I haven't taken the time to work that out.

Cheers.

Thanks for your valuable suggestions. What you think if we allow the car to strike with some type of flat board/plate which inturn will be connected with an oil filled cylinder and the oil is allowed to release (which will be having high pressure due to impact of car) at a controlled rate so that this piston will act as a shocker as shown in picture. Here i have given a large clearance to piston, but we can add to an ordinary piston-cylinder assembly a hose to the cylinder of suitable dia to release the pressure at controlled rate.

If you want occupants to survive, lets follow MOBI's advice, rely on the Navy and go with something like this:

Something like that might work (I'd have gone pneumatic but hydraulic might be OK). I'd look at having lots of exhaust port area at the start and gradually decreasing so that you taper up the force.

Sorry!!!!

Have I missed something are you going to try for a job in the Fleet Air Arm........you know........... landing on an aircraft carrier..........or something!!!!!