Science Questions

What happens when a bomb explodes underwater?

Sun, 8th Mar 2009

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Question

Michael Perry asked:

What happens when a bomb explodes underwater?

Answer

Dave - Well when any bomb explodes the first thing you're going to get is a lot of high-pressure gas because you've taken a load of solid and turned it into gas. It wants to expand. Water isn't going to move away nearly as quickly as air does. The pressure's going to remain very high, pushing water away. The fastest the water can move away is roughly the speed of sound in water. That's 1400m/s. You're going to form a bubble. As that water is pushed away very fast you're going to get a second powerful sound wave or pressure wave moving away from it. If the water doesn't compress that's going to have a very high pressure and do a lot of damage which is why depth charges can destroy strong things like submarines, even 10-20m away. Apparently it's way that there's a theory of how you might be able to blow up safes. If you fill a safe with water an drop a small charge in then because the pressure change is so much greater it might blow the door off.

Chris - That's because all the pressure is being exerted on the safe. If you were to just stack a load or dynamite at the front of the safe, some of it would hit the safe but a lot of that pressure would go out.

Dave - The gas that's produced, all that extra volume that's produced is pushed on the side of the safe. Normally you can just compress the air inside the room.

Kat - It would be more explosive if you fart in the bath as well.

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Michael Perry asked the Naked Scientists: Naked scientists, I'm wondering, if a bomb or a similar explosion went off underwater, would the water amplify or dampen the blast? What do you think? mike2niner4, Sat, 7th Mar 2009

I didn't find any good references on this, but my guess is that the concussion would amplify in the inelastic water so that the blast would be felt stronger to objects in the water. But the water would be somewhat of a shield for things outside the water.

Vern, Sun, 8th Mar 2009

I remember reading somewhere that if you want to blow up a concrete dam it's best to place the bomb underwater as it has more effect. DoctorBeaver, Sun, 8th Mar 2009

Wouldn't the effect be twofold?
At its first microsecond when the blast goes of the force should get more concentrated as the waters higher density will 'resist' it like putting a firecracker into a can giving the force a chance to resonance and dtrengthen its overall effect?

Then you have the fact that sound in water travel faster than on land, depending on salinity, temperature and depth around 1400m/s "We can hardly push things through air (at sea level) at those speeds. The fastest bullets reportedly have an initial velocity about 1500m/s." The reason why is also its density :) When you pack molecules close (as in water as compared to air) they will react and pass the 'sound waves' on so much faster, between four to five times the speed of sound in air. And after all the 'blasting force' is just that, molecules 'flying away' in 'shock waves' through whatever medium they travel.

So how about outer space then, there you have nothing, well almost nothing:) Will a blast there give any effect at all, I'll leave that one to you :)
yor_on, Sun, 8th Mar 2009

I suspect that in space you would be subject only to the ejecta from the explosive device. Vern, Sun, 8th Mar 2009

Yep,  that's what I think too :) Whatever molecules there might be will probably belong to the device exploding, and they will disperse quite near the explosion without finding anything to propagate it further with. yor_on, Mon, 9th Mar 2009

wouldnt

wouldnt that just b-because u would want an explosion to happen at the bass of the dam so the rest will crumble?? tony6789, Mon, 9th Mar 2009

since water cant be easily compressed the force of an explosion underwater transfers far more of the energy of a greater distance tony6789, Mon, 9th Mar 2009

Tony that was mighty mystical :)
It's a truth with a disclaimer I think.

If you mean that waters density helps it transfer the shockwaves better than air then that is a truth, But it won't work better if embedded in a diamond, then the blast will be highly contained and not able to travel long at all. What works for fluids is not the same as for solids. Do you agree? yor_on, Mon, 9th Mar 2009

ok not completely understanding wat ur saying...are u asking if a bomb was encased in a large diamond?? if so (assuming the bomb is powerful enough to explode the diamond) then yea it will likely weaken the bombs power. so yea i agree to an extent tony6789, Tue, 10th Mar 2009

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