My friend claims that in an explosion most of the force will be directed against place that offers the most resistance. For example if an explosion happened up against a concrete wall, the wall would take most of the force, while a person would take less force because it has all gone into the wall. This doesn’t seem to make sense to me. In fact I’d say it was the other way around. So who is right?
Jockstrap
PS I hope I’ve explained this well enough. If not please tell me.
The explosion would probably direct it’s force outwards sphericaly, assuming that it’s not a shaped charge. So it would transfer it to the air in an expanding bubble, forcing the wall as it passes and reflects. Then it would expand past you and force you, exploding outwards. In order for more of an explosion to be directed against a wall, it would have to be buried into that wall somewhat so that more of it’s energy would be transfered into it.
Your friend’s actually right on this one; the wall will take more force. Newton’s Third Law of Motion states that whenever A exerts a force on B, B will also exert a force on A, which is exactly equal in magnitude, and opposite in direction (often stated as “For every action, there is an equal and opposite reaction”, but that’s often misinterpreted). The wall is capable of exerting a much greater force on the explosion, so it’ll take a much greater force from it, as well.
Another way to look at it: Let’s consider an explosion to be a very strong wind. If you’re standing on the ground in, say, a 60 MPH wind, it’s going to be exerting a significant force on you, right? Now suppose that you’re tooling down the highway at 60 MPH, in the same direction. If you stick your hand out the window, you won’t feel any force at all. The body moving with the wind feels less force than the unmoving body. Similarly, the body moving with the explosion (yours) feels less force from the explosion.
All of this is not to say, of course, that the brick wall will necessarily take more damage.
I think you friend may have deceptivley worded his statement to make it seem like he’s wrong and egg you into an argument (that’s if I understood what you wrote correctly). As Chronos pointed out, the wall will TAKE more force, simply because it has much more surface area and mass than you; think of first standing in the 60 mph wind naked (snicker), and then standing in the wind holding a sheet of plywood in front of you… you might get blown over in the later case. The wall wouldn’t magnetically ATTRACT more force than was naturally headed it’s way though. If you stood opposite a wall with a bomb going off between you and the wall, you’d still get thrown back and burt a nice golden brown. The part of the sphere of expansion that’s coming towards you wouldn’t decide to go the opposite direction instead because there’s a big wall over there. Ask your friend if he thinks he’d be safe from the blast from a pound of C4 if he happened to be standing a few feet from a parked car :D.
Thanks for your help everyone. This has made me think of something else. If explosion hit the concrete wall and doesn’t knock it down, will it then ‘bounce’ back and exert some of its force in the other direction?
Your friend is nuts. If he were right, the farther away the explosives were placed, the more damage they would do. Open air does not concentrate explosive energy, it dissipates it.
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When commercial blasting is done on vertical surfaces, they place the explosives deep inside a long, thin hole. For horizontal surfaces, if it is not possible to drill a proper hole, they place the explosives on the horizontal surface and then cover it with a pile of dirt or mud. -Elsewise it makes a lot of noise but hardly blows up nuthin’. - MC
I think someone is missing out on the Brissance effect here.
The Brissance effect relates to the rate of expansion of the gases released, the higher the velocity the greater the rate of change when it hits an immobile object and the greater the shattering power, in other words it is the shockwave that precedes the gas.
Since this deals with the rate of expansion, which is at its greatest closest to the explosion and drops off rapidly with volume it follows that the greatest Brissnace effect will be upon an object close to the explosion, in this case the wall.
This is not the total energy of the explosion, the majority dissapears as heat or as gas expansion across three dimensions.
The are high and low velocity explosives with differant applications, but a high velocity explosive does not necessarily mean a higher energy content.
Let me clarify: I was not referring to the larger surface area of the wall, although that would also have an effect. What I was referring to was just the fact that the wall doesn’t move: If you used a full-sized stone statue of a human, you’d have the same effect. Something that moves with the explosion to some degree will feel less force than something that does not move.