# Crashing a car into a giant wet sponge **URL:** \ **Category:** Factual Questions\ **Created:** [November 19, 2008, 5:07pm UTC](https://boards.straightdope.com/t/crashing-a-car-into-a-giant-wet-sponge/473470 "2008-11-19T17:07:59Z")\ **Posts on this page:** 7\ **Page:** 3
**Author:** ![hobscrk777](https://avatars.discourse-cdn.com/v4/letter/h/a6a055/32.png) [@hobscrk777](https://boards.straightdope.com/u/hobscrk777)\ **Post date:** [November 23, 2008, 5:53pm UTC](https://boards.straightdope.com/t/crashing-a-car-into-a-giant-wet-sponge/473470/41 "2008-11-23T17:53:38Z")
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 = m_v = 53,600 kg_m/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. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [November 23, 2008, 6:18pm UTC](https://boards.straightdope.com/t/crashing-a-car-into-a-giant-wet-sponge/473470/42 "2008-11-23T18:18:17Z")
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. ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [November 23, 2008, 6:58pm UTC](https://boards.straightdope.com/t/crashing-a-car-into-a-giant-wet-sponge/473470/43 "2008-11-23T18:58:49Z")
> [@beowulff](#): > > 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? ---
**Author:** ![jackdavinci](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackdavinci/32/3259_2.png) [@jackdavinci](https://boards.straightdope.com/u/jackdavinci)\ **Post date:** [November 24, 2008, 4:41am UTC](https://boards.straightdope.com/t/crashing-a-car-into-a-giant-wet-sponge/473470/44 "2008-11-24T04:41:04Z")
> [@Santo\_Rugger](#): > > 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. 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. ---
**Author:** ![matt\_mcl](https://avatars.discourse-cdn.com/v4/letter/m/7ab992/32.png) [@matt\_mcl](https://boards.straightdope.com/u/matt_mcl)\ **Post date:** [November 24, 2008, 8:31am UTC](https://boards.straightdope.com/t/crashing-a-car-into-a-giant-wet-sponge/473470/45 "2008-11-24T08:31:53Z")
> [@beowulff](#): > > No facts, all anecdote. 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. ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [November 24, 2008, 12:49pm UTC](https://boards.straightdope.com/t/crashing-a-car-into-a-giant-wet-sponge/473470/46 "2008-11-24T12:49:40Z")
> [@jackdavinci](#): > > 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. 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. ---
**Author:** ![WF\_Tomba](https://avatars.discourse-cdn.com/v4/letter/w/d9b06d/32.png) [@WF\_Tomba](https://boards.straightdope.com/u/WF_Tomba)\ **Post date:** [November 26, 2008, 7:30pm UTC](https://boards.straightdope.com/t/crashing-a-car-into-a-giant-wet-sponge/473470/47 "2008-11-26T19:30:15Z")
> [@Sapo](#): > > 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? 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. > [@Jackmannii](#): > > If you jump off a bridge and hit water as opposed to concrete, it won’t help. The impact is likely to kill you either way. Hit another car at high speed or an equal weight of wet sponge, and you will be severely damaged or killed in either instance. 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. [Previous page](https://boards.straightdope.com/t/crashing-a-car-into-a-giant-wet-sponge/473470.md?page=2)