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        riddles >> what happened >> Cutting down trees with dynamite
        
(Message started by: maryl on Jun 11^th, 2003, 12:49pm)

Title: Cutting down trees with dynamite
Post by maryl on Jun 11^th, 2003, 12:49pm

Dynamite is a tool that can be used to cut down trees. To cut a tree 18 inches in diameter requires five sticks of dynamite: one on the north, south, east and west sides of the tree, and the fifth stick on the side to which the tree should fall. During construction of a dam, a tree 18 inches in diameter was completely covered by water. Since the treetop was fouling the boat's propellers, it had to be cut down. A diver went down and put a stick of dynamite on the four sides of the tree. Since the current is flowing south at 2 knots per hour, on which side of the tree would you instruct the diver to put the fifth stick of dynamite if you wanted the tree to fall north?

Title: Re: Cutting down trees with dynamite
Post by James Fingas on Jun 11^th, 2003, 1:24pm

<Edit: hidden>
[hide]It took me a while to see what the complication of this riddle is. The problem is that submerged trees don't fall: they rise to the surface. We now have to figure out what we mean when we say "fall to the North". Probably it means that we want the trunk of the tree to point South.

Without a fifth stick, as the dynamite is detonated, the tree will become severed from its stump, but likely it will gain very little kinetic energy from the explosion itself. Assuming that the tree isn't waterlogged, it will start to rise quickly to the surface (slowed by its foliage or smaller branches). Initially, it will start to lean slightly to the South because the water was pushing it that way to start with, and because the water exerts a larger force on the top of the tree, and as it bobs up out of the water for the first time, it will tend to fall over more strongly to the South. [/hide]

But with the fifth stick of dynamite, hmm ... without knowing more about the dynamics of an underwater explosion, it's really hard to say. My guess is that no matter where you put the dynamite, the tree with "fall" South, because the current in a body of water is larger at the top than the bottom, and the tree has more area at the top than the bottom, so there will be a larger force pushing South at the top of the tree regardless of the dynamite.

But I bet that's not the answer you were looking for...

Title: Re: Cutting down trees with dynamite
Post by maryl on Jun 11^th, 2003, 2:06pm

Ah James, you had it at the beginning of your first paragraph; but all that other stuff is good to know too. ;)

Title: Re: Cutting down trees with dynamite
Post by James Fingas on Jun 12^th, 2003, 6:27am

I guess I'll hide my answer so other people can enjoy it ... guess I was just looking too deep :-[

Title: Re: Cutting down trees with dynamite
Post by Chewdogscp on Jun 26^th, 2003, 12:52pm

Quote:

Assuming that the tree isn't waterlogged, it will start to rise quickly to the surface (slowed by its foliage or smaller branches).

You were almost there... I'm thinking that [Hide]the "slowed by it's foliage or smaller branches" is the key part. With the fifth stick of dynamite u can blow away those excess branches cuaseing the tree to be slowed more to one side. So if you want the top of the tree to point north, you should put the fifth stick of dynamite in the southern branches so that the watter slows down the norther side cuasing the tree to tilt towards the north. When it is detached from the stump, it will be slowed more on the northern side, tilt and the tree will wind up floating south with it's stump facing south and it's top facing north! [/Hide] Aights i think that's it unless i'm way off....... sorry if i am but i think i got it here.

Title: Re: Cutting down trees with dynamite
Post by Squireking on Sep 17^th, 2003, 6:39pm

I didn't know you could use dynamite under water

Title: Re: Cutting down trees with dynamite
Post by Icarus on Sep 17^th, 2003, 8:37pm

Well, Squireking - glad we were able to impart new information to you - because - yes, you can! I'm not sure whether it takes a special type, but underwater demolition predates the sophisiticated explosives more commonly used today.

Title: Re: Cutting down trees with dynamite
Post by towr on Sep 17^th, 2003, 11:05pm

You can even use gunpowder under water, though it's a lot harder. Explosive contain their own oxygen, so they don't need air to explode.
The two things you need to be worry about is that gunpowder and nitroglycerin (the explosive compound in dynamite) won't work if they're too wet, and you need to light them somehow. Dynamite is a bit more tolerant in that respect. To set them off you can just use an electric spark. So basicly, put a plastic bag around it to keep it dry.