Suppose you crash your car head-on into a water-soaked sponge that has the same mass as the car. Would this be as destructive as hitting a solid wall, or perhaps another car? Or would the sponge cushion the impact?
What you you rather be hit with? A wet sponge, or the same weight of steel?
Hitting a wet sponge would do little to no damage to a car.
I disagree. Hitting a really big sponge would do a lot of damage to a car, albeit not as much as hitting a wall would. A car driving at normal speed has a lot of momentum - that energy has to go somewhere when it collides to a stop. Some of it would be absorbed by the sponge but some would be absorbed by the car.
Two big things that affect the severity of a crash are: Where is the energy going? and How long does it take to disperse the energy?
In the case of said wet sponge, some of the energy goes into crushing the sponge and squeezing out the water. There are even highway barriers made of barrels with water or sand in them. Car hits barrel, the water goes flying, taking away some of the energy.
The longer it takes for a crash to finish, the less damaging it is to the person and car. Like fast vs. slow braking from the occupants’ point of view. If the big wet sponge can make the crash take even twice as long, that can help.
On a related note, I’ve seen video of explosions where they covered the “bomb” with a big pile of foam before setting it off. It reduces the blast force to practically nothing. The foam has lots of little cells that absorb and disperse the energy. So a sponge would have a similar effect. But perhaps a somewhat damp one is better than a fully waterlogged one. The air spaces are a key here.
This sounds interesting, I haven’t heard of it before. Would you happen to have a link?
Most of the damage control I’ve been involved with has been attached to the structure, because you don’t know exactly where an unexpected attack is going to come from (by definition). On the other hand, for a suspicious package, this seems pretty cool.
To expound on this point, imagine getting hit in the face with one of those large natural sponges used to was your car. It might not hurt, but it’s not going to feel good.
Again, imagine that large sponge. Now imagine that large sponge weighing two tons.
Why? It’s not as if it’s being dropped on the car.
ftg has it right == it all depends upon how long it takes to stop the car. If a 2 ton sponge stops me in a long enough period of time that I don’t experience any damage, I’ve got no prob;em with hitting a two ton sponge.
I didn’t say anything about anything falling 
The OP specifies that the sponge weighs as much as the car. It doesn’t matter if the sponge is moving towards the car or if the car is moving towards the sponge, from an impact analysis perspective.
ftg does have the first part right, it does matter how long it takes the sponge to compress. But, remember, the sponge weighs two tons. It is not going to be moving anywhere fast, and assuming 60 mph (25m/s); even if it takes a full second to stop the car (let’s assume 2,000 kg) we’re still talking about 50kN of force*, or about 11,000 pounds-force.
- F = m * a
m = 2000 kg
a = 25 m/s / 1 s
F = 50,000 N
Some motorway crash barriers are sort of giant wet sponges - that is, they’re clusters of barrels filled with water - when a vehicle crashes into them, the force is dissipated as the water is squeezed out of the tops.
It’s admittedly a little bit of a stretch to think of a bunch of barrels as a sponge, but there are similarities in concept.
11,000 lbs? The front end of a car is greater than 10 square feet, or 1440 square inches.
11,000 lbs / 1440 square inches is only 7.5 psi.
It might not even break the windshield.
The cars don’t suffer any damage when they hit those barrels? :dubious:
7.5 psi doesn’t sound like a lot, but if the car hits something that is slowing it down in .1 second, that number is going to be 75 psi. A car probably slows down in .1 second when it hits a concrete barrier. The psi number is a bit misleading in this case, for two reasons. First is because we (more than?) double it when talking about impact loads, as opposed to static loads. Also, while that’s the psi at the bumper, there are still going to be points on the car that are experiencing a much, much higher psi than that.
Also, how many accidents have you seen where there was severe damage to the body of the car, but no windows were broken? Although glass breaks at much lower psi than most people think; the ranges I’ve seen given for peak overpressure are between .5 and 5 psi, depending on the thickness and the area.
Good Lord, do I have to really explain this?
The OP specifies that the sponge weighs as much as the car. It doesn’t matter if the sponge is moving towards the car or if the car is moving towards the sponge, from an impact analysis perspective.
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This is true, but it all really depends upon how long it takes to slow the car down. The OP doesn’t say how big the sponge is, and it doesn’t say how fast the car is going. If the car is slowed down over a long enough distance, the car may not experience any damage at all. That’s especially easy to imagine with a car traveling at moderate speeds (say, only 20 mph). Even if it hit a saturated sponge, the most massive possibility, I’ll bet there wouldn’t be any damage. Damned right I’d rather hit that than a concrete wall.
On the contrary - I expect they’re often damaged beyond economic repair, but the purpose is to attenuate the impact in such a way as to preserve the lives of the vehicle’s occupants (and I believe they achieve this better than the same volume of concrete blocks).
I don’t think I stated that cars suffered no damage, so I’m not sure what it is you’re finding dubious…
I always thought those barrels were meant to protect the structure rather than the car. The idea being that every stupid drunk doesn’t turn into a massive repair project.
Well, yes, because I still think you’re wrong.
The OP doesn’t specify how big the sponge is, but it does specify a mass. With your logic, a car traveling .01 mph wouldn’t have any damage when it hits a completely rigid, completely immovable object. There are some assumptions that must be made. Mine was that the car was traveling 60 mph. What fun is a collision that happens at 20 mph*?
But, more to the point. Let’s imagine a quasi static example. When you put a sponge on a flat surface, and push down on it, the resistance will be very little until the material is completely compressed. At that point, the resistance will be the same as that of the object it’s sitting on.
I wish I had access to a FEA program, it’d give these impact threads we’ve been having lately a lot more definitive.
Sorry, I was using your post and point to help reinforce mine. The “:dubious:” was not directed at you.
*By fun, I mean to analyze and watch a test of, not to actually be involved or witness a real wreck. I’ve seen dozens of cars blow up (500 pounds of ANFO), it’s fun stuff. The 1,000 pound shots are even cooler.
They do both. By dissipating energy themselves, they’re protecting both the car, the driver, and the structure.
FWIW, whenever nuclear facilities are built, the structure must undergo an analysis of projectiles, including, but not limited to vehicles, objects thrown by a tornado, and airplanes crashing into the facility.
The difference between hitting a wet sponge and hitting a brick wall is pretty much the idea behind most of the safety innovations built into cars nowadays: the longer you can stretch out the time of impact, the more likely the occupants will survive. Crumple zones throughout the frame, collapsing steering columns and brake pedals, airbags, and seatbelt force-limiters; all of these make the 60-to-0 deceleration less sudden (in addition to dissipating some of the collision energy). The water barrels along the side of the road serve the same purpose (they also protect the concrete walls, though I don’t know how much of a priority that is).
With the sponge, you’re taking the same idea of protecting the driver and applying it to the entire car. It’s likely the car would still suffer some damage, just as drivers still get injured, but the results will be a lot better than the same collision with a rigid wall.
Sorry if it looks like I’m saying the damage would be equivalent to crashing into a rigid wall. I’m just saying it wouldn’t be fun to be in the car. While I’d rather be in a new car than an old car if I’m going to hit an old car or a sponge, I’d still rather be on the side drinking a beer. Of course the damage will be less, I didn’t mean to come across as arguing otherwise.
Let’s assume the sponge measures 1m in height, 1m in the direction of the car’s motion, and 2m in width. It’s fully saturated, and a 2000 kg car hits it at 70 km/h. Do we have enough information to know how long it takes the car to stop?
Assuming a perfectly rigid car and sponge backing structure, something less than
(1m)/(19.4m/s)= 0.05 seconds
But this calculation is assuming that the “sponge backing structure” is doing all the actual stopping, isn’t it?
A more reasonable setup would have an arbitrarily large sponge, and see how long that took to stop the car.
I was actually thinking there would be no backing structure. It occurs to me that in that case the friction between the sponge and the road would also be important.
The arbitrarily large sponge is also a good idea–say 2m wide, 1m tall and arbitrarily deep. What do we need to know about the physical properties of the sponge to solve the problem in this case?