Two options spring to mind.
1.) Take the gold coins over one at a time. No-one said you couldn’t do that.
2.) Cast a spell to make the gold weightless. He’s a goddamn magician, after all.
While it flies on the face of the nature of a “riddle,” why specify that it’s a magician if it doesn’t figure in to the answer?
When someone says “Knock knock” to you, do you respond “Ring the bell, please” ? 
Because if you say he’s a juggler, it sort of gives away the joke. If you say he’s a magician, it implies that he has slight sleight of hand skills, i.e., jugglering.
Where does it say he cannot cross the bridge twice, once with 2 and once with 1?:smack:
Already a thread here.
Closed as a duplicate.
MODERATOR NOTE: Post #4 above by admin@mostlycocoa.com was started as a separate thread; this was reported by cochrane and the thread closed by Colibri. I’ve overruled Colibri and am simply merging into the existing thread. – CKDH
Since he’s a magician, he should ask for a volunteer from the audience (who also weighs 68kg), ask them to pick a card and put it in their pocket then walk across the bridge and wait for the magician to tell them what the card is. After the volunteer walks across, the magician follows carrying two bricks and then pulls out the card and says “Is this your card?”. The stunned volunteer reaches into their own pocket to retrieve the card and instead finds the third gold brick.
It’s a sad commentary on our educational system that people fail to realize the force added to the system when the juggler tosses or catches a coin.
What kind of bridge are we talking about?
Seems like if we have a straight forward bridge supported at both ends it would have a weak point in the exact middle. Wouldn’t it have to support more then that minimum weight on either side of that point? If the magician has the extra coin in the air as he walks over that point he should be golden. If the bridge is small enough he may be able to simply step over that spot carrying all three pieces, but that would be less dramatic.
Can we make a special bridge designed to defeat him?
How about a suspension bridge composed of individually suspended boards with cables set to snap if too much weight is on the board and the boards are far enough apart that he can’t keep his weight distributed on two boards at all times.
I don’t think even this bridge is hopeless if you follow these steps.
- Set a gold piece down at your feet.
- Take a step.
- Set a gold piece down on the next board.
- pickup the piece from behind you.
Goto step 1 and repeat until the bridge is crossed.
I’d call this a kind of juggling of the gold pieces. Actually, normal juggling violates the rules of the joke which states you can’t throw the gold pieces. If you are throwing them in the air, that’s still throwing. My method may not be what is expected from juggling, but magicians are tricky, don’t you know?
How little sense am I making here?
Actually, you are making a whole lot of sense in a general sort of way; aside from the curious specifics of your strange bridge designs. The entire problem doesn’t make a whole lot of sense, and there’s some difficulty even with rephrasing it so it would make sense.
First, as others have noted, the problem states that the traveler is a magician. And as others have noted, well duh, you can’t just come out and state up front that he’s a juggler. The fact remains, that describing him as a magician outrightly poisons the problem statement right from the start. If we take at face value that the traveler has the faculty of working magic, then by definition, we are immediately steered in the direction of seeking a supernatural solution. Which could be anything. If he can do telekinesis, he can float the coins across while he stands at one end, then cross over himself. Or he could transport the coins Star Trek style. Or he could levitate himself, coins and all, across the chasm. Or impart extra strength to the bridge by the power of his mind.
WWGD? (What Would Gandalf Do?)
Imagine that he has the power to levitate the coins over his head. As Cecil himself suggested, even if he could do this, he would have to support the floating coins somehow. So suppose the magician can create a force-field over his head to hold the coins up. The weight of the coins against that force-field would still be transferred, through the traveler’s body, to the bridge. That still isn’t quite magic. That’s just advanced technology that to us is indistinguishable from magic. Unless, of course, is isn’t, if the magician really does have command of a supernatural power to prevent that. That would be magic.
In any case, this problem remains, and I put it to The Straight Dope community to solve it: How, then, should the problem be stated in the first place?
It’s a sad commentary on The Straight Dope that Cecil failed to understand this, as evidence by his failure to mention it in his response. He merely agrees with the OP that the magician must be supporting the third coin “somehow, yes?”
Lame response by Cecil. Really really lame.
If I may now digress and ramble a bit:
Cecil mentions what else this reminds him of: That remark that half the people are below the “median”
Another respondent, ALLADDSUP, is reminded of the similar problem of the birds in the truck.
For lame responses, I am reminded of that column about the weight of a cloud vs the weight of a Boeing 747, which got hopelessly derailed. Cecil got the basics right (a big cloud weighs more – lots more), but weasled a bit on weight vs mass, leading to confusion over a cloud’s weight because clouds, y’know, float.
That led to a very prolonged discussion among Cecil, Dave Morgan Ph.D., and the Teeming Masses, which Cecil summarized in the column as linked below. Originally, IIRC, the entire discussion with Dave Morgan (or much of it) was published over a period of several weeks. It went on and on and on, never really discussing or disputing that clouds are very very heavy, but instead arguing endlessly over the finer points of weight, mass, and buoyancy of clouds in the air. What a cloudwreck.
Column: Can a cloud weigh as much as a 747? (Originally published March 13, 1998.)
Thread by the Teeming Masses: Cloud weight weasling (Started March 12, 2001 by rhinobird.)
The magician takes out a set of African sparrows from his coat (which he uses in his magic act), and has them carry one of the gold coins across the bridge while he walks across with the other two coins. Problem solved.
Or is he suppose to take the chicken across first, then the wolf… bring the chicken back across before he picks up the sack of grain?
Or he just bangs on the side of the truck to keep the chicken flying while he [del]drives[/del] levitates the truck across.
I think you mean either African or European swallows.
A quick comment using some math, but keeping it simple:
While the magician is juggling, the force on the bridge is oscillating slightly. That is, it goes up and down slightly around the average. When the magician applies force to a coil to throw it upward, momentarily an equal force is applied downward to the bridge.
It is obvious that the average force on the bridge is 71 kg (normalized to gravity). This means the minimum force on the bridge may very well be around 70 kg. But the maximum force on the bridge will thus be above 71 kg. And that, my dear readers, is when the bridge will break.
Scream at the bridge, make someone else carry his gold across, fail to use any actual magic, then just have a giant eagle fly him to the other side?
Quite. And just where do coconuts fit into the whole magician-coins-bridge equation, anyway?
Out of curiosity, has anyone ever juggled heavy objects while standing on a scale to demonstrate the puzzle?
<pendant moment=true>
As a civil engineer, I would just like to point out that if the bridge has a “capacity” of 68 kg, then its actual capacity would be be at least 204 kg (factor of safety).
</pendant>
Just being pedantic but you wouldn’t mean “Pedant” would you?
first thing that came to my mind was the old riddle about the farmer, the fox, the goose and the cabbage having to cross the river in a boat that won’t hold them all…(and by the way the fact that the guy was a farmer didn’t have any bearing on the solution)…
so take two of the gold pieces over and then go back and get the third…not quite as complicated as the farmer version as the gold wasn’t likely to eat the other gold…but not really requiring higher mathematics either…