Let’s assume the car has a mass of 2000 kg and is moving at a velocity of 26.8 m/s (60 mph). This gives the car momentum equal to p = mv = 53,600 kgm/s. Regardless of whether the car hits a steel plate or a super-elastic sponge, this momentum is going to be have to reduced to zero for the car to come to a stop. Since impulse = momentum, the wall would have to deliver a change in impulse equal to the momentum of the car. The difference is that the steel plate, being more rigid, might stop the car in 1 ms. Since impulse J = Favg * t, the average force delivered by the steel plate to stop the car is 53.6 MN (>12 million pounds-force). Suppose the super-elastic sponge stops the car in 5 seconds. The sponge would then deliver an average force of 10.72 kN (only 2400 pounds-force) to the car.
Here’s another issue, which I was loathe to bring up before: I doubt a sponge of dimensions comparable to a car could retain even a small fraction of the water in its fully-saturated state.
You mean it would squeeze itself under its own weight?
Anyways, there will always be some size of sponge that rests with a weight equal to our car, right? Does it matter that it be saturated?
True but the relative direction of gravity does matter. The weight of the sponge won’t matter in a side impact because the car will only interact with a small portion of the total mass. But if the sponge is coming from above, the car will interact with all of the mass that is above it, and in addition to the impact force there will be a crushing force.
I don’t think the care will be any more damaged than it would be if it were very carefully set down on it’s front end. That is - can the car’s structural integrity handle it’s own weight?
I wonder what would happen if a car were dropped onto one of those huge inflatable things they use so stunt men can jump off buildings.
Although I love the fact that a Doper actually has crashed his car into a giant sponge.
Next we’ll find out that someone on the Dope actually has taken off in a plane from a giant treadmill.
They did that in Fear Factor with some regularity. They didn’t exactly use an inflatable stunt-a-stop ™, but a massive block of empty cardboard boxes. Oftentimes, the car didn’t even reach the floor.
It matters that it have a lot of water in it, since a wet sponge will not behave the same way as a dry sponge.
But when you fall into a body of water, you’re hitting something much bigger than you. The force you exert on the area of water that you hit is actually pressing against the combined weight of all the water in the lake, right?
Imagine you’re a skydiver falling through the air, and right below you a rocket releases a ball of water at the top of its trajectory. You hit the water before it has had time to disperse or fall more than a foot or so. The ball of water has the same mass as you. Does it kill you?
I see the sponge as sort of a similar situation. The water is suspended in front of you, but it can be displaced much more easily than water in a lake.